asce 7-05 code master

CodeMasterSEISMIC DESIGN

This CodeMaster identifies the 11 steps involved in designing a typical one- to three-story building for seismic loads in accordance with the 2006 International BuildingCode (IBC), ASCE 7-05 Minimum Design Loads for Buildings and Other Structures,and the 2003 NEHRP Recommended Provisions for Seismic Regulations for NewBuildings and Other Structures (known as 2003 NEHRP Provisions or FEMA 450-1*).

Information will be presented on how these three documents work together. TheNEHRP Provisions feed directly into the ASCE 7 development process; ASCE 7 inturn serves as a primary referenced standard in the IBC. The seismic designprovisions of the 2006 IBC are based on those of ASCE 7-05 and make extensivereference to that standard. In fact, almost all of the seismic design provisions areadopted through reference to ASCE 7-05. Beginning with Step 4, only referencesto ASCE 7-05 and the 2003 NEHRP Provisions are made. The only seismicprovisions included in the text of the 2006 IBC are related to ground motion, soilparameters, and determination of Seismic Design Category (SDC), as well asdefinitions of terms actually used within those provisions and the four exceptionsunder the scoping provisions. It is important to note that where this CodeMasterprovides section references from the documents, the corresponding requirementsoften differ from one another. In some cases, these differences are subtle and anexplanation of these differences is beyond the scope of this CodeMaster.

* The 2003 NEHRP Provisions (FEMA 450-1) is a resource document funded andpublished by the Federal Emergency Management Agency (FEMA). It is intendedto capture research results and lessons learned and may contain informationbeyond that found in ASCE 7-05 or the IBC. The accompanying Commentary(FEMA 450-2) may assist the user in understanding the basis for coderequirements. Copies of the 2003 NEHRP Provisions and the accompanyingCommentary may be viewed or downloaded on the Building Seismic SafetyCouncil's (BSSC) website: www.bssconline.org. The 2003 NEHRP Provisions alsoincludes a CD that contains the two documents as well as the seismic design mapsand a program to determine the mapped seismic design values. Hard copies orthe CD may be obtained free-of-charge by contacting the FEMA PublicationDistribution Facility at 1-800-480-2520.

The following seismic base shear equation is given in ASCE 7-05 Section12.8.1[2003 NEHRP Provisions Section 5.2.1]:

V = CsW where Cs is the seismic response coefficientW is the weight of the building plus that of any contents that could, with ahigh degree of probability, be attached to the structure at the time of theearthquake. In addition to the obvious dead load of the structure, ASCE 7-05 Section 12.7.2 [2003 NEHRP Provisions Section 5.2.1] requires that thefollowing loads be included in the effective seismic weight, W:

ASCE 7-05 Section 12.8.3 [2003 NEHRP Provisions Section 5.2.3]describes how the seismic base shear is distributed over the height of thestructure. The story forces are computed as follows:

Fx = Cvx V

Where:

For structures with T < 0.5 sec, k=1For structures with T > 2.5 sec, k = 2For structures with 0.5 sec < T < 2.5 sec, k can be 2 or can bedetermined by linear interpolation between 1 and 2.

An example of this distribution is shown in the figure below. A k exponentlarger than 1 places a greater proportion of the base shear in the upperstories, compared with a linear distribution produced by a k value of 1, toaccount for higher modes of vibration in structures having fundamentalperiods exceeding 0.5 seconds. For a one- to three-story building, theperiod is less than 0.5 second; therefore, the distribution of seismic forceswill be linear.

The structural effects of the earthquake forces, meaning the bending moments,shear forces and axial forces caused by them, must be combined with the effects ofgravity (bending moments, shear forces, axial forces caused by the dead, live, snowloads, etc.) using the design load combinations set forth in 2006 IBC Section 1605[ASCE 7-05 Section 2.0; no corresponding section in the 2003 NEHRP Provisions].For strength design, the two load combinations applicable in seismic design are:

1.2D + 1.0E + f1L + f2S (2006 IBC Eq. 16-5 – Additive)0.9D + 1.0E + 1.6H (2006 IBC Eq. 16-7 – Counteractive)

CLOSING COMMENTS

The special seismic load combinations set forth in IBC Section 1605.4 are requiredfor such elements as collectors; columns or other elements supporting reactionsfrom discontinuous shear walls or frames; and batter piles and their connections.

The interstory drift expected to be caused by the design earthquake is limited bythe code. Some reasons for limiting drift are: 1) to control member inelastic strain,2) to minimize differential movement demand on the seismic safety elements, and3) to limit damage to nonstructural elements.

ASCE 7-05 Section 12.12.1 contains drift control requirements [2003 NEHRPProvisions Section 4.5.1]. Drift determination is addressed in ASCE 7-05 Section12.8.6 [2003 NEHRP Provisions Section 5.2.6]. The first step is to determine δxe,the elastically computed lateral deflection at floor level x under code-prescribedseismic forces (the design base shear, V, distributed along the height of thestructure in the manner prescribed by the code). Next, the deflections, δxe, aremultiplied by the deflection amplification factor, Cd, (because the actual lateraldeflections will be greater under the design earthquake excitation) and divided byI in accordance with the following equation:

δx = Cd δxe/ ICd is set forth in ASCE 7-05 Table 12.2-1 (2003 NEHRP Provisions Table 4.3-1). Iis in the denominator of the equation to eliminate I from the drift computation(remember that the code-prescribed seismic forces that produced δxe wereoriginally augmented by I). It is important and necessary to do this because thedrift limits of ASCE 7-05 and the 2003 NEHRP Provisions are a function of theoccupancy of a structure. The drift limit for a hospital is half that for an officebuilding on the same site.

CodeMaster developed by:

SSttrruuccttuurreess && CCooddeess IInnssttiittuutteeA subsidiary of S.K. Ghosh Associates Inc.www.skghoshassociates.comISBN 978-0-9793084-1-3

Tel: (847) 991-2700Fax: (847) 991-2702

[email protected]

SCI

ASCE7-005

SEISMICDESIGN

obtaining seismic design parameters using the same data that was used toprepare the ground motion maps published in the 2006 IBC, ASCE 7-05,and the 2003 NEHRP Provisions. By inputting the longitude and latitude ofthe building location, this method provides for a more accurate and reliabledetermination of Ss and S1. The FEMA 450 CD also contains thiscalculation tool.

2006 IBC Section 1613.1 allows the following four exceptions from compliancewith the 2006 IBC seismic design requirement:

STEP 7 DETERMINE SEISMIC BASE SHEAR, V

The redundancy coefficient reflects the multiple load path concept – that ofproviding more than one alternate path for every load to travel from its pointof application to the ultimate point of resistance. Just as regular structureshave proven themselves to outperform irregular structures in earthquakes,structures with redundant seismic force-resisting systems have performedbetter than those with little or no redundancy. The redundancy coefficient isapplied as necessary to increase the effect of the horizontal earthquakeground motion to compensate for the lack of structural redundancy in theseismic force-resisting system.

ASCE 7-05 Section 12.3.4 [2003 NEHRP Provisions Section 4.3.3]describes how to determine the redundancy coefficient, ρ. The redundancycoefficient does not apply (meaning that it may be taken equal to 1) inSDCs A, B, and C; seismic design forces for structures assigned to theseseismic design categories are therefore unaffected by the redundancy ofthe seismic force-resisting system.

For structures assigned to SDC D, E or F, the value of the redundancycoefficient equals 1.3, unless it can be shown that one of two describedconditions is met. The first condition involves showing that the removal of anindividual seismic force-resisting element will not cause: (1) the remainingstructure to suffer a reduction in story strength of more than 33 percent, or(2) create an extreme torsional irregularity. The second condition appliesonly to a structure that is regular in plan at all levels and requires that theseismic force-resisting system consists of at least two bays of seismic force-resisting perimeter framing on each side of the structure in each orthogonaldirection at each story resisting more than 35 percent of the base shear.

DETERMINE SS AND S1STEP 1The first step in seismic design is determining the mapped maximum consideredearthquake (MCE) spectral response accelerations at short periods, Ss, and at 1-second period, S1. These values can be determined using one of two methods:

1. 2006 IBC Figures 1613.5(1) through 1613.5(14) [ASCE 7-05 Figures 22-1through 22-20; 2003 NEHRP Provisions Figures 3.3-1 through 3.3-14], or

2. USGS website at http://earthquake.usgs.gov/research/hazmaps/. The U.S.Geological Survey (USGS) has prepared an Internet calculation tool for This CodeMaster has presented the step-by-step process required to complete

seismic design as it relates to the seismic design demands. Many other coderequirements need to be addressed when completing the entire seismic design of abuilding. These other code requirements cover: direction of loading, deformationcompatibility, P-Δ effects, detailing, structural component load effects, nonstructuralcomponents, inspections, foundations, and material specific requirements.

2003NNEEHHRRPP

2006IBC

2003 NEHRPProvisions

ASCE 7–05 2006 IBC

DETERMINE IF STRUCTURE IS EXEMPT FROM

SEISMIC REQUIREMENTSSTEP 2

ExceptionNo. 1

Detached one- and two- family dwellings in SDC A, B, or C orlocated where Ss is less than 0.4g.

Areas of U.S. with Ss < 0.4 g (Shown in green)For areas outside the conterminous United States, visit

www.skghoshassociates.com/CMSDC

At this stage, the SDC has not been determined; however, Ss hasbeen determined in Step 1. After Step 3 is completed, this exceptionmay be revisited.

ExceptionNo. 2

Conventional light-frame wood construction complying with 2006IBC Section 2308 (see definition for "conventional light-frame woodconstruction" in 2006 IBC Section 2302).

ExceptionNo. 3

Agricultural storage structures intended for incidental humanoccupancy only (see definition for "agricultural building" in 2006 IBCSection 202).

ExceptionNo. 4

Vehicular bridges, electrical transmission towers, hydraulicstructures, buried utility lines and their appurtenances, nuclearreactors and other similarly described structures in the code.

2006 IBCSection1613.5.1

Structures located in areas with Ss < 0.15g and S1 < 0.04g needonly comply with SDC A requirements.

Areas of U.S. with Ss < 0.15g and S1 < 0.04g(shown in green)

For areas outside the conterminous United States, visitwww.skghoshassociates.com/CMSDC

Similar exceptions are found in ASCE 7-05 Sections 11.1.2 and 11.4.1 and 2003NEHRP Provisions Section 1.1.2.1.

Description Include in Seismic WeightAreas of storage (other thanpublic garages and openparking garages)

25 percent of floor live load

Building with partitions 10 psf or actual weight, whichever is greaterBuildings with roofsdesigned for snow

Where flat roof snow loads are greater than30 psf, 20 percent of the design snow loadneeds to be included, regardless of actualroof slope.

Permanent equipment 100 percent of operating weight

Cs is calculated according to one of three equations depending on theperiod of the structure as illustrated in the following figure (there are alsominimum base shear requirements for long-period structures):

The period TL is given in ASCE 7-05 Figures 22-15 through 22-20 [2003NEHRP Provisions Figures 3.3-16 through 3.3-21]. The building site needsto be located on the applicable map to determine TL, which ranges between4 and 16 seconds, depending upon the location. The following map is theTL map for the conterminous United States:

(For areas outside the conterminous United States, visitwww.skghoshassociates.com/CMSDC)

The typical one- to three-story building addressed in this CodeMaster willqualify as a short-period building and, therefore, the seismic base shear isdetermined by the following equation:

V = SDS is determined in Steps 1 and 3; R is determined in Step 5; I isdetermined in Step 6; and W is the seismic weight of the building asdescribed in this step.

SDS

R/I W

STEP 8 DISTRIBUTE V OVER THE HEIGHT OF THE BUILDING

∑=

= n

1i

kii

kxx

vx

hw

hwC

Fn

Hn

Fi

Hi

Level i Wi

VBuilding, n stories high

Distribution of Seismic Forces

STEP 9 DETERMINE REDUNDANCY COEFFICIENT, ρ

DETERMINE SEISMIC LOAD EFFECTS, E AND EMSTEP 10ASCE 7-05 Sections 12.4.2 and 12.4.3 [2003 NEHRP Provisions Sections 4.2.2.1and 4.2.2.2] address the determination of E and Em.

What is E? E is the combined effect of horizontal and vertical earthquake-inducedforces and is quantified by the following equation:

{ 43421

DSQE DSE 2.0±= ρ

ρ: Determined in Step 9SDS: Determined in Steps 1 and 3

D: Design Dead Load

Effect of horizontal earthquakeground motion

Effect of vertical earthquakeground motion

2006 IBC Eq. 16-5 is the additive load combination in which gravity effects add toearthquake effects. 2006 IBC Eq. 16-7 is the counteractive load combination inwhich gravity effects counteract earthquake effects (the plus sign includes theminus and the minus sign governs). With incorporation of the expression for E, theabove load combinations become:

(1.2 + 0.2SDS)D + f1L + f2S + ρQE (2006 IBC Eq. 16-5 – Additive)(0.9 - 0.2SDS)D - ρQE + 1.6H (2006 IBC Eq. 16-7 – Counteractive)

In other words, the consideration of vertical earthquake ground motion increasesthe dead load factor in the additive load combination and decreases it in thecounteractive load combination.

For example, consider a fully redundant structure (ρ = 1.0) located where SDS = 1.0with a bearing wall system consisting of shear walls used for the seismic force-resisting system and f1 =1.0. If the bending moments in a shear wall cross-sectiondue to dead loads, live loads, snow loads and horizontal earthquake forces are 200ft-kips, 60 ft-kips, 0 ft-kips and 150 ft-kips, respectively, the design moments(required flexural strengths) by the strength design load combinations (IBCEquations 16-5 and 16-7) are:Mu = [(1.2) + (0.2)(1.0)]( 200) + 60 + (1)(150) = 490 ft-kipsMu = [(0.9) - (0.2)(1.0)](200) - (1)(150) = -10 ft-kipsThe shear wall needs to be reinforced to carry these bending moments at thecross-section in question.

What is Em? Em is the maximum seismic load effect and is required for the designof certain elements critical to the stability of the structure. This maximum loadeffect generated in a building can be much greater than those due to the design-level force.

Em= Ω0QE ± 0.2SDSD

Ωo is the overstrength factor and increases the design-level internal forces torepresent the actual forces that may be experienced by an element as a result ofthe design-level ground motion. Ωo is obtained from ASCE 7-05 Table 12.2-1 [2003NEHRP Provisions Table 4.3-1]. Em is determined using the same procedure asfor determining E. Em is used in the additive and the counteractive loadcombinations the same way as E, except that the factored snow load effect, f2S, istypically not included in the additive combination.

Because Em is a strength-level force effect, adjustments need to be made ifallowable stress design is used. The allowable stresses may be increased by afactor of 1.2 in accordance with ASCE 7-05 Section12.4.3.3.

CHECK DRIFT CONTROL REQUIREMENTSSTEP 11

The design story drift, Δ, is computed as the difference of the deflections δx at thecenters of mass of the diaphragms at the top and the bottom of the story underconsideration. For structures assigned to SDC C and higher, with horizontalirregularities 1a or 1b, the design story drift, Δ, is computed as the largest differenceof the deflections along any of the edges of the diaphragms at the top and thebottom of the story under consideration. This accounts for torsional effects.

Once the drift is computed, it is checked against the allowable story drift set forthin ASCE 7-05 Table 12.12-1 [2003 NEHRP Provisions Table 4.5-1]. The first andthe last rows of the table apply to buildings other than masonry shear wallbuildings. If such buildings are more than four stories tall, the last row applies. If,however, such buildings are four stories or less in height, the designer has a choicebetween two drift limits: (1) where nonstructural elements have been designed toaccommodate the story drift (less stringent) and (2) all other structures (morestringent). This is consistent with the intent of the drift limit, which is to limit damageto drift-sensitive nonstructural elements.

ASCE 7-05 TABLE 12.12-1 ALLOWABLE STORY DRIFT, Δaa, b

StructureOccupancy Category

I or II III IV

Structures, other than masonry shear wall structures, 4 stories or lesswith interior walls, partitions, ceilings and exterior wall systems thathave been designed to accommodate the story drifts.

0.025hsxc 0.020hsx 0.015hsx

Masonry cantilever shear wall structuresd 0.010hsx 0.010hsx 0.010hsx

Other masonry shear wall structures 0.007hsx 0.007hsx 0.007hsx

All other structures 0.020hsx 0.015hsx 0.010hsx

a,b,c,d See ASCE 7-05 Table 12.12-1 for footnotes.

V =SDSWR/I

V =SD1W(R/I)T

V =0.5S1W

R/I, where S1 > 0.6g

T1 = SD1/SDS Period, T TL

Des

ign

Bas

e Sh

ear,

V

V =SD1TLW

(R/I)T2

V = 0.01W

CMSeismicNoBullets.qxp 3/19/2008 8:55 AM

SDC if certain conditions are met. The conditions that a structure must satisfy forthis relaxation to be applicable are:

• S1 < 0.75g at site of structure.• Ta < 0.8Ts where Ta is the approximate fundamental period of the structure

and Ts = SD1/SDS.• Upper-bound design base shear is used in design.• T used to calculate story drift < Ts.• Diaphragms are rigid, or for diaphragms that are flexible, vertical elements of

seismic force-resisting system spaced at < 40 ft.

Areas of U.S. with S1 > 0.75g (shown in red)For areas outside the conterminous United States, visit

www.skghoshassociates.com/CMSDCOnce the SDC is determined, it is important to understand the impact such aclassification has on the seismic design of the building. If a building is assignedSDC A, this means that the building has a minimal seismic vulnerability. All of thedesign requirements applicable to such a building are found in ASCE 7-05 Section11.7 [2003 NEHRP Provisions Section 1.5].Each subsequent SDC letter assignment (B through F) means an increase inseismic performance requirements. Among other code requirements, the SDCestablishes permissible structural systems, height limits, restrictions on irregularbuildings, permitted analysis procedures, detailing requirements, andrequirements for nonstructural components.

STEP 6 DETERMINE SEISMIC IMPORTANCE FACTOR, I

STEP 5 DETERMINE R, RESPONSE MODIFICATION COEFFICIENT

The SDC assigned to a building is a classification based on its occupancy or useand the level of expected soil-modified seismic ground motion at its site. In orderto determine the SDC, the following items first need to be determined:1. Soil Classification. The soil needs to be classified as Site Class A, B, C, D,

E, or F in accordance with 2006 IBC Section 1613.5.2 and Table 1613.5.2[ASCE 7-05 Sections 11.4.2, 20.1, 20.3 and 20.4; 2003 NEHRP ProvisionsSection 3.5]. Site class definitions are dependent on soil parameters such asshear wave velocity, standard penetration resistance, undrained shearstrength, and soil profile descriptions.

STEP 4 DETERMINE ANALYSIS PROCEDURES

The seismic importance factor represents an attempt to control the seismicperformance capabilities of buildings in different occupancy categories.The importance factor modifies the minimum base shear forces andreflects the relative importance assigned to the occupancy during andfollowing an earthquake. The seismic importance factor is related to theOccupancy Category. An Occupancy Category I or II structure is assignedI = 1.0; an Occupancy Category III structure is assigned I = 1.25; and anOccupancy Category IV structure is assigned I = 1.5. As will be seen inStep 7, I = 1.25 results in increasing the design seismic force by 25 percentand I = 1.50 results in increasing the design seismic force by 50 percent.(See ASCE 7-05 Table 11.5-1 and 2003 NEHRP Provisions Table 1.3-1 forimportance factor assignments).

The R-value represents a relative rating of the ability of a structural systemto resist severe earthquake ground motion without collapse. It is also thereduction in seismic force demand in proportion to the perceived ductility ofa given structural system (ductility is the ability of a structure to continue tocarry gravity loads as it deforms laterally beyond the stage of elasticresponse). The following table illustrates the different types of seismicforce-resisting systems addressed in ASCE 7-05 Table 12.2-1, which setsforth the R-values [2003 NEHRP Provisions Table 4.3-1].

Three types of analysis procedures can be used in the seismic design of a buildingaccording to ASCE 7-05: 1) simplified design procedure, 2) equivalent lateral forceprocedure, and 3) dynamic analysis procedure.The simplified design procedure is in stand-alone ASCE 7-05 Section 12.14 (2003NEHRP Provisions Alternative Simplified Chapter 4). It is a conservative methodof determining design forces for certain simple buildings. It is optional for thesesimple buildings, but it should be kept in mind that the design forces will be higherthan those calculated using one of the other two methods. The procedure is limitedin its applicability to simple and redundant Occupancy Category I and II structuresnot exceeding 3 stories where the seismic force-resisting elements are arranged ina torsion-resistant, regular layout. Furthermore, only bearing wall and buildingframe systems qualify to use the procedure. See ASCE 7-05 Section 12.14.1.1 for12 limitations that must be met in order for the simplified design procedure to beused [2003 NEHRP Provisions Section Alt. 4.1.1].Permissible analysis procedures for buildings not qualifying for the simplified designprocedure are set forth in ASCE 7-05 Section 12.6 [2003 NEHRP Provisions Section4.4.1]. The following table summarizes the permissible analysis procedures. In orderto use this table, a building's fundamental period needs to be determined, as doesTs, and whether or not it is regular or irregular – all of which are explained below.

ASCE 7-05 Table 12.6-1 (Summarized)

STEP 3 DETERMINE SEISMIC DESIGN REQUIREMENTS (SDC)

SECRETS OF THE CODEMASTER: 2006 IBC Section 1613.5.2 [ASCE 7-05Section 20.1; 2003 NEHRP Provisions Section 3.5] makes the following allowancefor situations where soil properties are not known:

When the soil properties are not known in sufficient detail to determine the siteclass, Site Class D can be used unless the building official determines that SiteClass E or F soil is likely to be present at the site.

ASCE 7-05 Section 20.1 includes the following statement:Where site-specific data are not available to a depth of 100 feet, appropriate soilproperties are permitted to be estimated by the registered design professionalpreparing the soils report based on known geologic conditions.

2. SDS and SD1. SDS is the 5-percent-damped design spectral responseacceleration at short periods and is calculated as follows: SDS = (2/3)(Fa)(Ss).The Fa value is obtained from 2006 IBC Table 1613.5.3(1) [ASCE 7-05 Table11.4-1; 2003 NEHRP Provisions Table 3.3-1] and is a function of the site classand Ss.SD1 is the 5-percent-damped design spectral response acceleration at 1-second period and is calculated as follows: SD1 = (2/3)(Fv)(S1). The Fv value isobtained from 2006 IBC Table 1613.5.3(2) [ASCE 7-05 Table 11.4-2; 2003NEHRP Provisions Table 3.3-2] and is a function of the site class and S1.

3. Occupancy Category. Occupancy Category is a term used to describe thecategory of structures based on occupancy or use. Use 2006 IBC Table1604.5 to determine the Occupancy Category [ASCE 7-05 Table 1-1; 2003NEHRP Provisions uses Seismic Use Group in accordance with Section 1.2].The following table summarizes Occupancy Category assignments:

OccupancyCategory Nature of Category

IOccupancy Category I is assigned to agricultural facilities,temporary facilities and minor storage facilities that represent alow hazard to human life in the event of failure.

IIOccupancy Category II is assigned to most buildings; it isassigned to buildings not otherwise classified as OccupancyCategory I, III, or IV.

III

Occupancy Category III is for buildings with large numbers ofpersons such as:

• Schools with more than 250 students,• Assembly uses with more than 300 people, and • Buildings with total occupancy greater than 5000 people.

Occupancy Category III is also assigned to:• Nonessential utility facilities, and• Jails and detention facilities.

IV

Occupancy Category IV includes hospitals and acute carefacilities; fire, police and emergency response stations;structures containing highly toxic materials; aviation controltowers; and utilities required for essential facilities.

Once SDS, SD1 and the Occupancy Category have been determined, 2006 IBC Tables1613.5.6(1) and 1613.5.6(2) should be used for the SDC determination [ASCE 7-05Tables 11.6-1 and 11.6-2; 2003 NEHRP Provisions Tables 1.4-1 and 1.4-2], unless thestructure is located where S1 > 0.75g. If that is the case, Occupancy Category I, II orIII structures are assigned to SDC E, and Occupancy Category IV structures areassigned to SDC F. Although 2006 IBC Section 1613.5.6 [ASCE 7-05 Section 11.6and 2003 NEHRP Provisions Section 1.4.1] indicates that the building is to beassigned the more severe SDC in accordance with the two tables, there is anexception in this section that allows only Table 1613.5.6(1) to be used to determine

SDC Structural CharacteristicsEquivalent

Lateral ForceProcedure

DynamicAnalysis

ProcedureB, C All structures P P

D, E, F

Regular structures with T < 3.5 Ts and allstructures of light-frame construction P P

Irregular structures with T < 3.5Ts and havinghorizontal irregularities Type 2, 3, 4, or 5 ofASCE 7-05 Table 12.3-1 or vertical irregularitiesType 4 or 5 of ASCE 7-05 Table 12.3-2.

P P

All other structures NP P

How toDetermine theFundamentalPeriod, T, of aBuilding

The fundamental period, T, of a building may be taken equal toTa, as given in ASCE 7-05 Section 12.8.2.1 [2003 NEHRPProvisions Section 5.2.2.1]: Ta = Cthx

where hn is the height in feet above the base to the highest levelof the structure and the parameters Ct and x are determinedfrom ASCE 7-05 Table 12.8-2 [2003 NEHRP Provisions Table5.2-2].

Note: For a three-story building with hn equal to 30 feet,depending on the structural system, the approximate period canvary from 0.26 second to 0.43 second.

Once the design using base shear computed from T=Ta hasprogressed to a certain stage, the value of the fundamentalperiod may be refined through rational analysis. However, therationally computed T is still limited (except in driftcomputations) to no more than CuTa, where Cu is a coefficientgiven in ASCE 7-05 Table 12.8-1 [2003 NEHRP ProvisionsTable 5.2-1].

How toDetermine Ts

Ts is the period at which the flat-top portion of the responsespectrum transitions to the descending (period-dependent)branch. Ts is shown in ASCE 7-05 Figure 11.4-1 [2003 NEHRPProvisions Figure 3.3-15] and is illustrated as follows:

A typical value of Ts is 0.5 second. For any building one to threestories in height, T will always be less than 3.5 Ts. It is not untila building is in the 17- to 20-story height range that T may begreater than 3.5 Ts.

How toDetermine ifBuilding isIrregular?

ASCE 7-05 Tables 12.3-1 and 12.3-2 define the differenthorizontal and vertical structural irregularities [2003 NEHRPProvisions Tables 4.3-2 and 4.3-3].

What is important to note is that if a building is SDC D, E or Fand has a T > 3.5 Ts, it must be designed using a dynamicanalysis procedure. Also, if a building meets all of the followingconditions, it must be designed using a dynamic analysisprocedure:

• SDC D, E or F, and

• Not of light-frame construction, and

• Contains one of the following irregularities: horizontalirregularity type 1a or 1b or vertical irregularity type 1a, 1b,2 or 3.

n

P indicates permitted; NP indicates not permitted.

HORIZONTAL STRUCTURAL IRREGULARITIES

HorizontalIrregularityType 1a:

TORSIONALIRREGULARITY

HorizontalIrregularityType 1b:

EXTREMETORSIONALIRREGULARITY

HorizontalIrregularityType 2:

REENTRANTCORNER


DIAPHRAGMDISCONTINUITY


OUT-OF-PLANEOFFSETS


NONPARALLELSYSTEMS

• Torsional irregularityexists when

• Torsional irregularityis to be considered onlywhen diaphragms arenot flexible.

⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛ +> ΔΔΔ2

2.1 212

Δ2 Δ1

• Extreme torsionalirregularity exists when

• Extreme torsionalirregularity is to beconsidered only whendiaphragms are notflexible.

⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛ +> ΔΔΔ2

4.1 212

Δ2 Δ1

Re-entrant corner irregularityexists when both projection b >0.15a and projection d > 0.15.c.

ab

d

re-entrant corner

c

Diaphragm discontinuity exists when area of opening > 0.5(a)(b) oreffective diaphragm stiffness changes more than 50% from one story to the next.

openingb

a

Out-of-plane offsetirregularity exists whenthere are discontinuitiesin the vertical elementsof the lateral force-resisting system.

Nonparallel systems irregularity exists wherethe vertical lateral force-resisting elements arenot parallel to or symmetric about the majororthogonal axes of the seismic force-resistingsystem.

VERTICAL STRUCTURAL IRREGULARITIES

VerticalIrregularityType 1a:

SOFT STORY

soft story

Stiff resistingelements

Soft story irregularity exists whensoft story stiffness < 70% storystiffness above or < 80% of theaverage stiffness of 3 stories above.

VerticalIrregularityType 1b:

EXTREMESOFT STORY

VerticalIrregularityType 2:

WEIGHT(MASS)IRREGULARITY


VERTICALGEOMETRICIRREGULARITY


VERTICALDISCONTINUITYIN VERTICALLATERAL-FORCERESISTINGELEMENTS


WEAK STORY


EXTREMEWEAK STORY

Extreme soft story


Extreme soft story irregularityexists when soft story stiffness< 60% story stiffness above or< 70% of the average stiffnessof 3 stories above.

Heavy mass

Weight irregularity exists whenstory mass > 150% adjacentstory mass (a roof that is lighterthan the floor below need notbe considered).

a

b

b > 1.3a

Vertical geometricirregularity exists whenhorizontal dimension of lateral-force-resistingsystem in story > 130%of that in adjacentstory.

a a a


In planeoffset = 2a.Length oflateral-force-resistingelement = a

In-planediscontinuity invertical lateral-force-resistingelements existswhen the in-planeoffset is greaterthan the lengths ofthose elements orthere exists areduction instiffness ofresisting elementsin the story below.


Weak story

Weak story irregularity exists when the story lateral strength < 80%lateral strength of story above.


Weak story

Extreme weak story irregularity exists when the storylateral strength < 65% lateral strength of story above.

Further discussion of the simplified design procedure and discussion of thedynamic analysis procedures are beyond the scope of this CodeMaster.The equivalent lateral force procedure is discussed in the following steps.

Gravity Loads

Lateral Forces

Stiff Resisting Elements(Shear Walls or Braced Frames)

Bearing Wall System

Lateral ForcesGravityLoads


Building Wall System

GravityLoads

LateralForces

Moment-Resisting Frame System

GravityLoads

(supportedby frames)Lateral

Forces


Dual Systems with Moment Frames(Moment frames resist at least 25% of the

design seismic forces)

LateralForces

Gravity LoadsStiff ResistingElements(Shear Walls)

OrdinaryMomentFrame

Shear Wall-Frame Interactive System

Gravity Loads

LateralForces

Cantilevered Column SystemA system in which lateral forces are resisted

entirely by columns acting as cantileversfrom the base

The following table provides sections indicating how to determine R-valuesfor different combinations.

Combination Description ASCE 7-05 2003 NEHRP

Framing Systems in Different Directions Section 12.2.2 Section 4.3.1.2.1

Framing Systems in Same Horizontal Direction Section 12.2.3 Section 4.3.1.2.1

Vertical Combinations of Framing Systems Section 12.2.3.1 Section 4.3.1.2.1

Horizontal Combinations of Framing Systems Section 12.2.3.2 Section 4.3.1.2.2

CMSeismicNoBullets.qxp 3/19/2008 8:55 AM

SDC if certain conditions are met. The conditions that a structure must satisfy forthis relaxation to be applicable are:

• S1 < 0.75g at site of structure.• Ta < 0.8Ts where Ta is the approximate fundamental period of the structure

and Ts = SD1/SDS.• Upper-bound design base shear is used in design.• T used to calculate story drift < Ts.• Diaphragms are rigid, or for diaphragms that are flexible, vertical elements of

seismic force-resisting system spaced at < 40 ft.

Areas of U.S. with S1 > 0.75g (shown in red)For areas outside the conterminous United States, visit

www.skghoshassociates.com/CMSDCOnce the SDC is determined, it is important to understand the impact such aclassification has on the seismic design of the building. If a building is assignedSDC A, this means that the building has a minimal seismic vulnerability. All of thedesign requirements applicable to such a building are found in ASCE 7-05 Section11.7 [2003 NEHRP Provisions Section 1.5].Each subsequent SDC letter assignment (B through F) means an increase inseismic performance requirements. Among other code requirements, the SDCestablishes permissible structural systems, height limits, restrictions on irregularbuildings, permitted analysis procedures, detailing requirements, andrequirements for nonstructural components.

STEP 6 DETERMINE SEISMIC IMPORTANCE FACTOR, I

STEP 5 DETERMINE R, RESPONSE MODIFICATION COEFFICIENT

The SDC assigned to a building is a classification based on its occupancy or useand the level of expected soil-modified seismic ground motion at its site. In orderto determine the SDC, the following items first need to be determined:1. Soil Classification. The soil needs to be classified as Site Class A, B, C, D,

E, or F in accordance with 2006 IBC Section 1613.5.2 and Table 1613.5.2[ASCE 7-05 Sections 11.4.2, 20.1, 20.3 and 20.4; 2003 NEHRP ProvisionsSection 3.5]. Site class definitions are dependent on soil parameters such asshear wave velocity, standard penetration resistance, undrained shearstrength, and soil profile descriptions.

STEP 4 DETERMINE ANALYSIS PROCEDURES

The seismic importance factor represents an attempt to control the seismicperformance capabilities of buildings in different occupancy categories.The importance factor modifies the minimum base shear forces andreflects the relative importance assigned to the occupancy during andfollowing an earthquake. The seismic importance factor is related to theOccupancy Category. An Occupancy Category I or II structure is assignedI = 1.0; an Occupancy Category III structure is assigned I = 1.25; and anOccupancy Category IV structure is assigned I = 1.5. As will be seen inStep 7, I = 1.25 results in increasing the design seismic force by 25 percentand I = 1.50 results in increasing the design seismic force by 50 percent.(See ASCE 7-05 Table 11.5-1 and 2003 NEHRP Provisions Table 1.3-1 forimportance factor assignments).

The R-value represents a relative rating of the ability of a structural systemto resist severe earthquake ground motion without collapse. It is also thereduction in seismic force demand in proportion to the perceived ductility ofa given structural system (ductility is the ability of a structure to continue tocarry gravity loads as it deforms laterally beyond the stage of elasticresponse). The following table illustrates the different types of seismicforce-resisting systems addressed in ASCE 7-05 Table 12.2-1, which setsforth the R-values [2003 NEHRP Provisions Table 4.3-1].

Three types of analysis procedures can be used in the seismic design of a buildingaccording to ASCE 7-05: 1) simplified design procedure, 2) equivalent lateral forceprocedure, and 3) dynamic analysis procedure.The simplified design procedure is in stand-alone ASCE 7-05 Section 12.14 (2003NEHRP Provisions Alternative Simplified Chapter 4). It is a conservative methodof determining design forces for certain simple buildings. It is optional for thesesimple buildings, but it should be kept in mind that the design forces will be higherthan those calculated using one of the other two methods. The procedure is limitedin its applicability to simple and redundant Occupancy Category I and II structuresnot exceeding 3 stories where the seismic force-resisting elements are arranged ina torsion-resistant, regular layout. Furthermore, only bearing wall and buildingframe systems qualify to use the procedure. See ASCE 7-05 Section 12.14.1.1 for12 limitations that must be met in order for the simplified design procedure to beused [2003 NEHRP Provisions Section Alt. 4.1.1].Permissible analysis procedures for buildings not qualifying for the simplified designprocedure are set forth in ASCE 7-05 Section 12.6 [2003 NEHRP Provisions Section4.4.1]. The following table summarizes the permissible analysis procedures. In orderto use this table, a building's fundamental period needs to be determined, as doesTs, and whether or not it is regular or irregular – all of which are explained below.

ASCE 7-05 Table 12.6-1 (Summarized)

STEP 3 DETERMINE SEISMIC DESIGN REQUIREMENTS (SDC)

SECRETS OF THE CODEMASTER: 2006 IBC Section 1613.5.2 [ASCE 7-05Section 20.1; 2003 NEHRP Provisions Section 3.5] makes the following allowancefor situations where soil properties are not known:

When the soil properties are not known in sufficient detail to determine the siteclass, Site Class D can be used unless the building official determines that SiteClass E or F soil is likely to be present at the site.

ASCE 7-05 Section 20.1 includes the following statement:Where site-specific data are not available to a depth of 100 feet, appropriate soilproperties are permitted to be estimated by the registered design professionalpreparing the soils report based on known geologic conditions.

2. SDS and SD1. SDS is the 5-percent-damped design spectral responseacceleration at short periods and is calculated as follows: SDS = (2/3)(Fa)(Ss).The Fa value is obtained from 2006 IBC Table 1613.5.3(1) [ASCE 7-05 Table11.4-1; 2003 NEHRP Provisions Table 3.3-1] and is a function of the site classand Ss.SD1 is the 5-percent-damped design spectral response acceleration at 1-second period and is calculated as follows: SD1 = (2/3)(Fv)(S1). The Fv value isobtained from 2006 IBC Table 1613.5.3(2) [ASCE 7-05 Table 11.4-2; 2003NEHRP Provisions Table 3.3-2] and is a function of the site class and S1.

3. Occupancy Category. Occupancy Category is a term used to describe thecategory of structures based on occupancy or use. Use 2006 IBC Table1604.5 to determine the Occupancy Category [ASCE 7-05 Table 1-1; 2003NEHRP Provisions uses Seismic Use Group in accordance with Section 1.2].The following table summarizes Occupancy Category assignments:

OccupancyCategory Nature of Category

IOccupancy Category I is assigned to agricultural facilities,temporary facilities and minor storage facilities that represent alow hazard to human life in the event of failure.

IIOccupancy Category II is assigned to most buildings; it isassigned to buildings not otherwise classified as OccupancyCategory I, III, or IV.

III

Occupancy Category III is for buildings with large numbers ofpersons such as:

• Schools with more than 250 students,• Assembly uses with more than 300 people, and • Buildings with total occupancy greater than 5000 people.

Occupancy Category III is also assigned to:• Nonessential utility facilities, and• Jails and detention facilities.

IV

Occupancy Category IV includes hospitals and acute carefacilities; fire, police and emergency response stations;structures containing highly toxic materials; aviation controltowers; and utilities required for essential facilities.

Once SDS, SD1 and the Occupancy Category have been determined, 2006 IBC Tables1613.5.6(1) and 1613.5.6(2) should be used for the SDC determination [ASCE 7-05Tables 11.6-1 and 11.6-2; 2003 NEHRP Provisions Tables 1.4-1 and 1.4-2], unless thestructure is located where S1 > 0.75g. If that is the case, Occupancy Category I, II orIII structures are assigned to SDC E, and Occupancy Category IV structures areassigned to SDC F. Although 2006 IBC Section 1613.5.6 [ASCE 7-05 Section 11.6and 2003 NEHRP Provisions Section 1.4.1] indicates that the building is to beassigned the more severe SDC in accordance with the two tables, there is anexception in this section that allows only Table 1613.5.6(1) to be used to determine

SDC Structural CharacteristicsEquivalent

Lateral ForceProcedure

DynamicAnalysis

ProcedureB, C All structures P P

D, E, F

Regular structures with T < 3.5 Ts and allstructures of light-frame construction P P

Irregular structures with T < 3.5Ts and havinghorizontal irregularities Type 2, 3, 4, or 5 ofASCE 7-05 Table 12.3-1 or vertical irregularitiesType 4 or 5 of ASCE 7-05 Table 12.3-2.

P P

All other structures NP P

How toDetermine theFundamentalPeriod, T, of aBuilding

The fundamental period, T, of a building may be taken equal toTa, as given in ASCE 7-05 Section 12.8.2.1 [2003 NEHRPProvisions Section 5.2.2.1]: Ta = Cthx

where hn is the height in feet above the base to the highest levelof the structure and the parameters Ct and x are determinedfrom ASCE 7-05 Table 12.8-2 [2003 NEHRP Provisions Table5.2-2].

Note: For a three-story building with hn equal to 30 feet,depending on the structural system, the approximate period canvary from 0.26 second to 0.43 second.

Once the design using base shear computed from T=Ta hasprogressed to a certain stage, the value of the fundamentalperiod may be refined through rational analysis. However, therationally computed T is still limited (except in driftcomputations) to no more than CuTa, where Cu is a coefficientgiven in ASCE 7-05 Table 12.8-1 [2003 NEHRP ProvisionsTable 5.2-1].

How toDetermine Ts

Ts is the period at which the flat-top portion of the responsespectrum transitions to the descending (period-dependent)branch. Ts is shown in ASCE 7-05 Figure 11.4-1 [2003 NEHRPProvisions Figure 3.3-15] and is illustrated as follows:

A typical value of Ts is 0.5 second. For any building one to threestories in height, T will always be less than 3.5 Ts. It is not untila building is in the 17- to 20-story height range that T may begreater than 3.5 Ts.

How toDetermine ifBuilding isIrregular?

ASCE 7-05 Tables 12.3-1 and 12.3-2 define the differenthorizontal and vertical structural irregularities [2003 NEHRPProvisions Tables 4.3-2 and 4.3-3].

What is important to note is that if a building is SDC D, E or Fand has a T > 3.5 Ts, it must be designed using a dynamicanalysis procedure. Also, if a building meets all of the followingconditions, it must be designed using a dynamic analysisprocedure:

• SDC D, E or F, and

• Not of light-frame construction, and

• Contains one of the following irregularities: horizontalirregularity type 1a or 1b or vertical irregularity type 1a, 1b,2 or 3.

n

P indicates permitted; NP indicates not permitted.

HORIZONTAL STRUCTURAL IRREGULARITIES

HorizontalIrregularityType 1a:

TORSIONALIRREGULARITY

HorizontalIrregularityType 1b:

EXTREMETORSIONALIRREGULARITY


REENTRANTCORNER


DIAPHRAGMDISCONTINUITY


OUT-OF-PLANEOFFSETS


NONPARALLELSYSTEMS

• Torsional irregularityexists when

• Torsional irregularityis to be considered onlywhen diaphragms arenot flexible.

⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛ +> ΔΔΔ2

2.1 212

Δ2 Δ1

• Extreme torsionalirregularity exists when

• Extreme torsionalirregularity is to beconsidered only whendiaphragms are notflexible.

⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛ +> ΔΔΔ2

4.1 212

Δ2 Δ1

Re-entrant corner irregularityexists when both projection b >0.15a and projection d > 0.15.c.

ab

d

re-entrant corner

c

Diaphragm discontinuity exists when area of opening > 0.5(a)(b) oreffective diaphragm stiffness changes more than 50% from one story to the next.

openingb

a

Out-of-plane offsetirregularity exists whenthere are discontinuitiesin the vertical elementsof the lateral force-resisting system.

Nonparallel systems irregularity exists wherethe vertical lateral force-resisting elements arenot parallel to or symmetric about the majororthogonal axes of the seismic force-resistingsystem.

VERTICAL STRUCTURAL IRREGULARITIES


SOFT STORY

soft story


Soft story irregularity exists whensoft story stiffness < 70% storystiffness above or < 80% of theaverage stiffness of 3 stories above.


EXTREMESOFT STORY


WEIGHT(MASS)IRREGULARITY


VERTICALGEOMETRICIRREGULARITY


VERTICALDISCONTINUITYIN VERTICALLATERAL-FORCERESISTINGELEMENTS


WEAK STORY


EXTREMEWEAK STORY

Extreme soft story


Extreme soft story irregularityexists when soft story stiffness< 60% story stiffness above or< 70% of the average stiffnessof 3 stories above.

Heavy mass

Weight irregularity exists whenstory mass > 150% adjacentstory mass (a roof that is lighterthan the floor below need notbe considered).

a

b

b > 1.3a

Vertical geometricirregularity exists whenhorizontal dimension of lateral-force-resistingsystem in story > 130%of that in adjacentstory.

a a a


In planeoffset = 2a.Length oflateral-force-resistingelement = a

In-planediscontinuity invertical lateral-force-resistingelements existswhen the in-planeoffset is greaterthan the lengths ofthose elements orthere exists areduction instiffness ofresisting elementsin the story below.


Weak story

Weak story irregularity exists when the story lateral strength < 80%lateral strength of story above.


Weak story

Extreme weak story irregularity exists when the storylateral strength < 65% lateral strength of story above.

Further discussion of the simplified design procedure and discussion of thedynamic analysis procedures are beyond the scope of this CodeMaster.The equivalent lateral force procedure is discussed in the following steps.

Gravity Loads

Lateral Forces


Bearing Wall System

Lateral ForcesGravityLoads


Building Wall System

GravityLoads

LateralForces

Moment-Resisting Frame System

GravityLoads

(supportedby frames)Lateral

Forces


Dual Systems with Moment Frames(Moment frames resist at least 25% of the

design seismic forces)

LateralForces

Gravity LoadsStiff ResistingElements(Shear Walls)

OrdinaryMomentFrame

Shear Wall-Frame Interactive System

Gravity Loads

LateralForces

Cantilevered Column SystemA system in which lateral forces are resisted

entirely by columns acting as cantileversfrom the base

The following table provides sections indicating how to determine R-valuesfor different combinations.

Combination Description ASCE 7-05 2003 NEHRP

Framing Systems in Different Directions Section 12.2.2 Section 4.3.1.2.1

Framing Systems in Same Horizontal Direction Section 12.2.3 Section 4.3.1.2.1

Vertical Combinations of Framing Systems Section 12.2.3.1 Section 4.3.1.2.1

Horizontal Combinations of Framing Systems Section 12.2.3.2 Section 4.3.1.2.2

CMSeismicNoBullets.qxp 3/19/2008 8:55 AM Page 2

CodeMasterSEISMIC DESIGN

This CodeMaster identifies the 11 steps involved in designing a typical one- to three-story building for seismic loads in accordance with the 2006 International BuildingCode (IBC), ASCE 7-05 Minimum Design Loads for Buildings and Other Structures,and the 2003 NEHRP Recommended Provisions for Seismic Regulations for NewBuildings and Other Structures (known as 2003 NEHRP Provisions or FEMA 450-1*).

Information will be presented on how these three documents work together. TheNEHRP Provisions feed directly into the ASCE 7 development process; ASCE 7 inturn serves as a primary referenced standard in the IBC. The seismic designprovisions of the 2006 IBC are based on those of ASCE 7-05 and make extensivereference to that standard. In fact, almost all of the seismic design provisions areadopted through reference to ASCE 7-05. Beginning with Step 4, only referencesto ASCE 7-05 and the 2003 NEHRP Provisions are made. The only seismicprovisions included in the text of the 2006 IBC are related to ground motion, soilparameters, and determination of Seismic Design Category (SDC), as well asdefinitions of terms actually used within those provisions and the four exceptionsunder the scoping provisions. It is important to note that where this CodeMasterprovides section references from the documents, the corresponding requirementsoften differ from one another. In some cases, these differences are subtle and anexplanation of these differences is beyond the scope of this CodeMaster.

* The 2003 NEHRP Provisions (FEMA 450-1) is a resource document funded andpublished by the Federal Emergency Management Agency (FEMA). It is intendedto capture research results and lessons learned and may contain informationbeyond that found in ASCE 7-05 or the IBC. The accompanying Commentary(FEMA 450-2) may assist the user in understanding the basis for coderequirements. Copies of the 2003 NEHRP Provisions and the accompanyingCommentary may be viewed or downloaded on the Building Seismic SafetyCouncil's (BSSC) website: www.bssconline.org. The 2003 NEHRP Provisions alsoincludes a CD that contains the two documents as well as the seismic design mapsand a program to determine the mapped seismic design values. Hard copies orthe CD may be obtained free-of-charge by contacting the FEMA PublicationDistribution Facility at 1-800-480-2520.

The following seismic base shear equation is given in ASCE 7-05 Section12.8.1[2003 NEHRP Provisions Section 5.2.1]:

V = CsW where Cs is the seismic response coefficientW is the weight of the building plus that of any contents that could, with ahigh degree of probability, be attached to the structure at the time of theearthquake. In addition to the obvious dead load of the structure, ASCE 7-05 Section 12.7.2 [2003 NEHRP Provisions Section 5.2.1] requires that thefollowing loads be included in the effective seismic weight, W:

ASCE 7-05 Section 12.8.3 [2003 NEHRP Provisions Section 5.2.3]describes how the seismic base shear is distributed over the height of thestructure. The story forces are computed as follows:

Fx = Cvx V

Where:

For structures with T < 0.5 sec, k=1For structures with T > 2.5 sec, k = 2For structures with 0.5 sec < T < 2.5 sec, k can be 2 or can bedetermined by linear interpolation between 1 and 2.

An example of this distribution is shown in the figure below. A k exponentlarger than 1 places a greater proportion of the base shear in the upperstories, compared with a linear distribution produced by a k value of 1, toaccount for higher modes of vibration in structures having fundamentalperiods exceeding 0.5 seconds. For a one- to three-story building, theperiod is less than 0.5 second; therefore, the distribution of seismic forceswill be linear.

The structural effects of the earthquake forces, meaning the bending moments,shear forces and axial forces caused by them, must be combined with the effects ofgravity (bending moments, shear forces, axial forces caused by the dead, live, snowloads, etc.) using the design load combinations set forth in 2006 IBC Section 1605[ASCE 7-05 Section 2.0; no corresponding section in the 2003 NEHRP Provisions].For strength design, the two load combinations applicable in seismic design are:

1.2D + 1.0E + f1L + f2S (2006 IBC Eq. 16-5 – Additive)0.9D + 1.0E + 1.6H (2006 IBC Eq. 16-7 – Counteractive)

CLOSING COMMENTS

The special seismic load combinations set forth in IBC Section 1605.4 are requiredfor such elements as collectors; columns or other elements supporting reactionsfrom discontinuous shear walls or frames; and batter piles and their connections.

The interstory drift expected to be caused by the design earthquake is limited bythe code. Some reasons for limiting drift are: 1) to control member inelastic strain,2) to minimize differential movement demand on the seismic safety elements, and3) to limit damage to nonstructural elements.

ASCE 7-05 Section 12.12.1 contains drift control requirements [2003 NEHRPProvisions Section 4.5.1]. Drift determination is addressed in ASCE 7-05 Section12.8.6 [2003 NEHRP Provisions Section 5.2.6]. The first step is to determine δxe,the elastically computed lateral deflection at floor level x under code-prescribedseismic forces (the design base shear, V, distributed along the height of thestructure in the manner prescribed by the code). Next, the deflections, δxe, aremultiplied by the deflection amplification factor, Cd, (because the actual lateraldeflections will be greater under the design earthquake excitation) and divided byI in accordance with the following equation:

δx = Cd δxe/ ICd is set forth in ASCE 7-05 Table 12.2-1 (2003 NEHRP Provisions Table 4.3-1). Iis in the denominator of the equation to eliminate I from the drift computation(remember that the code-prescribed seismic forces that produced δxe wereoriginally augmented by I). It is important and necessary to do this because thedrift limits of ASCE 7-05 and the 2003 NEHRP Provisions are a function of theoccupancy of a structure. The drift limit for a hospital is half that for an officebuilding on the same site.

CodeMaster developed by:

SSttrruuccttuurreess && CCooddeess IInnssttiittuutteeA subsidiary of S.K. Ghosh Associates Inc.www.skghoshassociates.comISBN 978-0-9793084-1-3

Tel: (847) 991-2700Fax: (847) 991-2702

[email protected]

SCI

ASCE7-005

SEISMICDESIGN

obtaining seismic design parameters using the same data that was used toprepare the ground motion maps published in the 2006 IBC, ASCE 7-05,and the 2003 NEHRP Provisions. By inputting the longitude and latitude ofthe building location, this method provides for a more accurate and reliabledetermination of Ss and S1. The FEMA 450 CD also contains thiscalculation tool.

2006 IBC Section 1613.1 allows the following four exceptions from compliancewith the 2006 IBC seismic design requirement:

STEP 7 DETERMINE SEISMIC BASE SHEAR, V

The redundancy coefficient reflects the multiple load path concept – that ofproviding more than one alternate path for every load to travel from its pointof application to the ultimate point of resistance. Just as regular structureshave proven themselves to outperform irregular structures in earthquakes,structures with redundant seismic force-resisting systems have performedbetter than those with little or no redundancy. The redundancy coefficient isapplied as necessary to increase the effect of the horizontal earthquakeground motion to compensate for the lack of structural redundancy in theseismic force-resisting system.

ASCE 7-05 Section 12.3.4 [2003 NEHRP Provisions Section 4.3.3]describes how to determine the redundancy coefficient, ρ. The redundancycoefficient does not apply (meaning that it may be taken equal to 1) inSDCs A, B, and C; seismic design forces for structures assigned to theseseismic design categories are therefore unaffected by the redundancy ofthe seismic force-resisting system.

For structures assigned to SDC D, E or F, the value of the redundancycoefficient equals 1.3, unless it can be shown that one of two describedconditions is met. The first condition involves showing that the removal of anindividual seismic force-resisting element will not cause: (1) the remainingstructure to suffer a reduction in story strength of more than 33 percent, or(2) create an extreme torsional irregularity. The second condition appliesonly to a structure that is regular in plan at all levels and requires that theseismic force-resisting system consists of at least two bays of seismic force-resisting perimeter framing on each side of the structure in each orthogonaldirection at each story resisting more than 35 percent of the base shear.

DETERMINE SS AND S1STEP 1The first step in seismic design is determining the mapped maximum consideredearthquake (MCE) spectral response accelerations at short periods, Ss, and at 1-second period, S1. These values can be determined using one of two methods:

1. 2006 IBC Figures 1613.5(1) through 1613.5(14) [ASCE 7-05 Figures 22-1through 22-20; 2003 NEHRP Provisions Figures 3.3-1 through 3.3-14], or

2. USGS website at http://earthquake.usgs.gov/research/hazmaps/. The U.S.Geological Survey (USGS) has prepared an Internet calculation tool for This CodeMaster has presented the step-by-step process required to complete

seismic design as it relates to the seismic design demands. Many other coderequirements need to be addressed when completing the entire seismic design of abuilding. These other code requirements cover: direction of loading, deformationcompatibility, P-Δ effects, detailing, structural component load effects, nonstructuralcomponents, inspections, foundations, and material specific requirements.

2003NNEEHHRRPP

2006IBC

2003 NEHRPProvisions

ASCE 7–05 2006 IBC

DETERMINE IF STRUCTURE IS EXEMPT FROM

SEISMIC REQUIREMENTSSTEP 2

ExceptionNo. 1

Detached one- and two- family dwellings in SDC A, B, or C orlocated where Ss is less than 0.4g.

Areas of U.S. with Ss < 0.4 g (Shown in green)For areas outside the conterminous United States, visit

www.skghoshassociates.com/CMSDC

At this stage, the SDC has not been determined; however, Ss hasbeen determined in Step 1. After Step 3 is completed, this exceptionmay be revisited.

ExceptionNo. 2

Conventional light-frame wood construction complying with 2006IBC Section 2308 (see definition for "conventional light-frame woodconstruction" in 2006 IBC Section 2302).

ExceptionNo. 3

Agricultural storage structures intended for incidental humanoccupancy only (see definition for "agricultural building" in 2006 IBCSection 202).

ExceptionNo. 4

Vehicular bridges, electrical transmission towers, hydraulicstructures, buried utility lines and their appurtenances, nuclearreactors and other similarly described structures in the code.

2006 IBCSection1613.5.1

Structures located in areas with Ss < 0.15g and S1 < 0.04g needonly comply with SDC A requirements.

Areas of U.S. with Ss < 0.15g and S1 < 0.04g(shown in green)

For areas outside the conterminous United States, visitwww.skghoshassociates.com/CMSDC

Similar exceptions are found in ASCE 7-05 Sections 11.1.2 and 11.4.1 and 2003NEHRP Provisions Section 1.1.2.1.

Description Include in Seismic WeightAreas of storage (other thanpublic garages and openparking garages)

25 percent of floor live load

Building with partitions 10 psf or actual weight, whichever is greaterBuildings with roofsdesigned for snow

Where flat roof snow loads are greater than30 psf, 20 percent of the design snow loadneeds to be included, regardless of actualroof slope.

Permanent equipment 100 percent of operating weight

Cs is calculated according to one of three equations depending on theperiod of the structure as illustrated in the following figure (there are alsominimum base shear requirements for long-period structures):

The period TL is given in ASCE 7-05 Figures 22-15 through 22-20 [2003NEHRP Provisions Figures 3.3-16 through 3.3-21]. The building site needsto be located on the applicable map to determine TL, which ranges between4 and 16 seconds, depending upon the location. The following map is theTL map for the conterminous United States:

(For areas outside the conterminous United States, visitwww.skghoshassociates.com/CMSDC)

The typical one- to three-story building addressed in this CodeMaster willqualify as a short-period building and, therefore, the seismic base shear isdetermined by the following equation:

V = SDS is determined in Steps 1 and 3; R is determined in Step 5; I isdetermined in Step 6; and W is the seismic weight of the building asdescribed in this step.

SDS

R/I W

STEP 8 DISTRIBUTE V OVER THE HEIGHT OF THE BUILDING

∑=

= n

1i

kii

kxx

vx

hw

hwC

Fn

Hn

Fi

Hi

Level i Wi

VBuilding, n stories high

Distribution of Seismic Forces

STEP 9 DETERMINE REDUNDANCY COEFFICIENT, ρ

DETERMINE SEISMIC LOAD EFFECTS, E AND EMSTEP 10ASCE 7-05 Sections 12.4.2 and 12.4.3 [2003 NEHRP Provisions Sections 4.2.2.1and 4.2.2.2] address the determination of E and Em.

What is E? E is the combined effect of horizontal and vertical earthquake-inducedforces and is quantified by the following equation:

{ 43421

DSQE DSE 2.0±= ρ

ρ: Determined in Step 9SDS: Determined in Steps 1 and 3

D: Design Dead Load

Effect of horizontal earthquakeground motion

Effect of vertical earthquakeground motion

2006 IBC Eq. 16-5 is the additive load combination in which gravity effects add toearthquake effects. 2006 IBC Eq. 16-7 is the counteractive load combination inwhich gravity effects counteract earthquake effects (the plus sign includes theminus and the minus sign governs). With incorporation of the expression for E, theabove load combinations become:

(1.2 + 0.2SDS)D + f1L + f2S + ρQE (2006 IBC Eq. 16-5 – Additive)(0.9 - 0.2SDS)D - ρQE + 1.6H (2006 IBC Eq. 16-7 – Counteractive)

In other words, the consideration of vertical earthquake ground motion increasesthe dead load factor in the additive load combination and decreases it in thecounteractive load combination.

For example, consider a fully redundant structure (ρ = 1.0) located where SDS = 1.0with a bearing wall system consisting of shear walls used for the seismic force-resisting system and f1 =1.0. If the bending moments in a shear wall cross-sectiondue to dead loads, live loads, snow loads and horizontal earthquake forces are 200ft-kips, 60 ft-kips, 0 ft-kips and 150 ft-kips, respectively, the design moments(required flexural strengths) by the strength design load combinations (IBCEquations 16-5 and 16-7) are:Mu = [(1.2) + (0.2)(1.0)]( 200) + 60 + (1)(150) = 490 ft-kipsMu = [(0.9) - (0.2)(1.0)](200) - (1)(150) = -10 ft-kipsThe shear wall needs to be reinforced to carry these bending moments at thecross-section in question.

What is Em? Em is the maximum seismic load effect and is required for the designof certain elements critical to the stability of the structure. This maximum loadeffect generated in a building can be much greater than those due to the design-level force.

Em= Ω0QE ± 0.2SDSD

Ωo is the overstrength factor and increases the design-level internal forces torepresent the actual forces that may be experienced by an element as a result ofthe design-level ground motion. Ωo is obtained from ASCE 7-05 Table 12.2-1 [2003NEHRP Provisions Table 4.3-1]. Em is determined using the same procedure asfor determining E. Em is used in the additive and the counteractive loadcombinations the same way as E, except that the factored snow load effect, f2S, istypically not included in the additive combination.

Because Em is a strength-level force effect, adjustments need to be made ifallowable stress design is used. The allowable stresses may be increased by afactor of 1.2 in accordance with ASCE 7-05 Section12.4.3.3.

CHECK DRIFT CONTROL REQUIREMENTSSTEP 11

The design story drift, Δ, is computed as the difference of the deflections δx at thecenters of mass of the diaphragms at the top and the bottom of the story underconsideration. For structures assigned to SDC C and higher, with horizontalirregularities 1a or 1b, the design story drift, Δ, is computed as the largest differenceof the deflections along any of the edges of the diaphragms at the top and thebottom of the story under consideration. This accounts for torsional effects.

Once the drift is computed, it is checked against the allowable story drift set forthin ASCE 7-05 Table 12.12-1 [2003 NEHRP Provisions Table 4.5-1]. The first andthe last rows of the table apply to buildings other than masonry shear wallbuildings. If such buildings are more than four stories tall, the last row applies. If,however, such buildings are four stories or less in height, the designer has a choicebetween two drift limits: (1) where nonstructural elements have been designed toaccommodate the story drift (less stringent) and (2) all other structures (morestringent). This is consistent with the intent of the drift limit, which is to limit damageto drift-sensitive nonstructural elements.

ASCE 7-05 TABLE 12.12-1 ALLOWABLE STORY DRIFT, Δaa, b

StructureOccupancy Category

I or II III IV

Structures, other than masonry shear wall structures, 4 stories or lesswith interior walls, partitions, ceilings and exterior wall systems thathave been designed to accommodate the story drifts.

0.025hsxc 0.020hsx 0.015hsx

Masonry cantilever shear wall structuresd 0.010hsx 0.010hsx 0.010hsx

Other masonry shear wall structures 0.007hsx 0.007hsx 0.007hsx

All other structures 0.020hsx 0.015hsx 0.010hsx

a,b,c,d See ASCE 7-05 Table 12.12-1 for footnotes.

V =SDSWR/I

V =SD1W(R/I)T

V =0.5S1W

R/I, where S1 > 0.6g

T1 = SD1/SDS Period, T TL

Des

ign

Bas

e Sh

ear,

V

V =SD1TLW

(R/I)T2

V = 0.01W

CMSeismicNoBullets.qxp 3/19/2008 8:55 AM Page 1

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