1Earthquake ResistantDesign According To 1997

UBCMajor Changes from UBC 1994

(1) Soil Profile Types:The four Site Coefficients S1 to S4 of the UBC 1994, which are independent of the level of ground shaking, were expanded to sixsoil profile types, which are dependent on the seismic zone factors, in the 1997 UBC (SA to SF) based on previous earthquake records.The new soil profile types were based on soil characteristics forthe top 30 m of the soil. The shear wave velocity, standardpenetration test and undrained shear strength are used to identifythe soil profile types.

(2) Structural FramingSystems:

In addition to the four basic framing systems (bearing wall,buildingframe, moment-resisting frame, and dual), two new structuralsystemclassifications were introduced: cantilevered column systems and shear wall-frame interaction systems.

(3) LoadCombinations:

The 1997 UBC seismic design provisions are based on strength-leveldesign rather than service-leveldesign.

2(4) Earthquake Loads:In the 1997 UBC, the earthquake load (E) is a function of boththehorizontal and vertical components of the groundmotion.

(5) Design BaseShear:

The design base shear in the 1997 UBC varies in inverseproportion to the period T, rather than T2/3 prescribed previously.Also, the minimum design base shear limitation for SeismicZone 4 wasintroduced as a result of the ground motion that was observed at sites near the fault rupture in 1994 Northridge earthquake.

(6) Simplified Design BaseShear:

In the 1997 UBC, a simplified method for determining the design base shear (V) was introduced for buildings not more than threestories in height (excludingbasements).

(7) Displacement and Drift:In the 1997 UBC, displacements are determined for thestrength-level earthquakeforces.

(8) Lateral Forces on Elements ofStructures:

New equations for determining the seismic forces (Fp) for elements of structures, nonstructural components and equipment are given.

The Static Lateral Force

Procedure Applicability:

The static lateral force procedure may be used for thefollowingstructures:

A.All structures, regular or irregular (Table A-1), in SeismicZone no. 1 (Table A-2) and in Occupancy Categories 4 and 5(Table A-3) in Seismic Zone 2.

B. Regular structures under 73 m in height with lateral forceresistance provided by systems given in Table (A-4) exceptfor structures located in soil profile type SF, that have a periodgreater than 0.70 sec. (see Table A-5 for soil profiles).

C. Irregular structures not more than five stories or 20 m in height.

D.S tructures having a flexible upper portion supported on arigid lower portion where both portions of the structureconsidered separately can be classified as being regular, theaverage story stiffness of the lower portion is at least ten timesthe average stiffness of the upper portion and the period of theentire structure is not greater than 1.10 times the period of theupper portion considered as a separate structure fixed at the base.

RegularStructures:Regular structures are structures having no significant physicaldiscontinuities in plan or vertical configuration or in theirlateral force resisting systems.

Irregular Structures:Irregular structures are structures having significant physicaldiscontinuities in configuration or in their lateral force resistingsystems (See Table A-1.a and A-1.b for detailed description ofsuch structures).

Design BaseShear:The total design base shear in a given direction is to bedetermined from the following formula.

V Cv I

W

(A-1)

R TThe total design base shear need not exceed the following:

V 2.5 Ca I

WR

(A-2)

The total design base shear shall not be less than the following:

V 0.11 Ca I W

(A-3)

In addition, for Seismic Zone 4, the total base shear shall not be less than the following:

V 0.8 Z Nv I

WR

(A-4)

The minimum design base shear limitation for Seismic Zone 4

was introduced as a result of the ground motion effects observed atsites near fault rupture in 1994 Northridge earthquake.

Where

V = total design lateral force or shear at the base.

W = total seismic dead load- In storage and warehouse occupancies, a minimum of 25 %

of floor live load is to be considered.- Total weight of permanent equipment is to be included.- Where a partition load is used in floor design, a load of not

less than 50 kg/m2 is to be included.

I = Building importance factor given in Table (A-3).

Z = Seismic Zone factor, shown in Table (A-2).

R = response modification factor for lateral force resisting system, shown in Table (A-4).

Ca = acceleration-dependent seismic coefficient, shown in Table (A-6).

Cv = velocity-dependent seismic coefficient, shown in Table (A-7).

N a = near source factor used in determination of CaZone 4, shown in Table (A-8).

N v = near source factor used in determination ofCvZone 4, shown in Table (A-9).

in

Seismic in

Seismic

T = elastic fundamental period of vibration, in seconds, of thestructure in the direction under consideration evaluatedfrom the following equations:

For reinforced concrete moment-resisting frames,T 0.073hn 3/ 4

(A-5)

For other buildings,

T 0.0488hn 3 / 4Alternatively, for shear walls,

h 3 / 4T 0.0743Ac

(A-6)

(A-7)

Where

hn = total height of building in meters

Ac = combined effective area, in m2, of the shear walls in the first

story of the structure, given by D 2 Ac Ai

0.2

e De / hn0.9

(A-8)

Where

hn

De is the length, in meters, of each shear wall in the first story in thedirection parallel to the applied forces.

Ai = cross-sectional area of individual shear walls in the direction of loads in m2

Load Combinations:Based on section 1612 of UBC, structures are to resist the most critical effects from the following combinations of factored loads:1.4 D 1.7 L0.75 (1.4 D 1.7 L 1.7 W)0.9 D 1.3 W

(A-9)(A-10)(A-11)

1.32 D 1.1

f1 L 1.1 E

(A-12)

n

0.99 D

1.1 E

(A-13)

Wheref1 = 1.0 for floors in public assembly, live loads in excess of 500

kg/m2 and for garage live loadsf1 = 0.5 for other live loads

Earthquake Loads:Based on UBC 1630.1.1, horizontal earthquake loads to be used in the above-stated load combinations are determined as follows:E p

Eh Ev

(A-14)

Em EhWhere:

(A-15)

E = earthquake load resulting from the combination of the horizontalcomponent Eh

, and the vertical component, Ev

Eh = the earthquake load due to the base shear, V

Em = the estimated maximum earthquake force that can be developed in the structure

Ev = the load effects resulting from the vertical component of the earthquake ground motion and is equal to

the addition of1.50 Ca I

Dto the dead load effects D

seismic force amplification factor as given in Table (A-4), andaccounts for structural over-strengthp redundancy factor, to increase the effects of earthquake loads

on structures with few lateral force resisting elements, given by

p 2 6.10 (A-16)

rmax Ag

Ag the minimum cross-sectional area in any horizontal plane inthe first story of a shear wall in m2

rmax the maximum element-story shear ratioFor a given direction of loading, the element story shear ratio is theratio of design story shear in the most heavily loaded single elementdivided by the total design story shear.

rmax is defined as the largest

of the element story shear ratio, ri

, which occurs in any of the story

levels at or below two-thirds height level of the building. For moment-resisting

frames,ri is taken as the maximum of

the sum of the shears in any two adjacent columns in amoment-resisting frame bay divided by the story shear

For shear walls,

ri is taken as the maximum of the product of

the wall shear multiplied by

3.5 / lw

and divided by the total

story shear, where lw

is the length of the wall in meters.

For dual

p 80 % of the values calculated above.

When calculating drift, or when the structure is located in Seismic Zones 0, 1, or 2, p shall be taken as 1.0.

p can't be smaller than 1.0 and can't be grater than 1.5.

Vertical Distribution of Force:The base shear evaluated from Eqn. (A-17) is distributed over the height of the building according to the following Eqn.

VFx

Ft nwx hx

(A-17)

wi hii 1

Fig. (A-1) Vertical Distribution of Force

Where

Ft 0for

T 0.7 sec.

Ft 0.07T V

0.25V

for

T 0.7 sec.

The shear force at each story is given by Eqn. (A-18)n

Where

Vx Ft

Fi

i x

(A-18)

n = number of stories above the base of the buildingFt = the portion of the base shear, concentrated at the top of

the structure to account for higher mode effectsFi , Fn , Fx = lateral forces applied atlevels hi , hn , hx = height above the baseto levels Vx = design shear in story x

i , n , ori , n , or

x , respectivelyx , respectively

Horizontal Distribution of Force:The design story shear in any direction

Vx , is distributed to the

various elements of the lateral force-resisting system in proportiontotheir rigidities, considering the rigidity of thediaphragm.

Horizontal TorsionalMoment:To account for the uncertainties in locations of loads, the massat each level is assumed to be displaced from the calculated centerof mass in each direction a distance equal to 5 % of the buildingdimension at that level perpendicular to the direction of theforce under consideration. The torsional design moment at a givenstory is given by moment resulting from eccentricities betweenapplied design lateral forces applied through each storys center ofmass at levels above the story and the center of stiffness of thevertical elements of the story, in addition to the accidental torsion.

OverturningMoments:Buildings must be designed to resist the overturning effectscaused by the earthquake forces.

The overturning moment

M x at level x is given by Eqn. (A-19).n

M x Ft

hn hx

Fi hi hx

i x 1

(A-19)

Overturning moments are distributed to the various elements of the vertical lateral force-resisting system in proportion to their rigidities.

Displacement and Drift:The calculated story drifts are computed using the maximuminelastic response displacement drift (m

), which is an estimate of

the displacement that occurs when the structure is subjected to thedesign basis ground motion.According to UBC 1630.9.2,

Where:

m 0.7 R s

(A-20)

s

design level response displacement, which is the total drift or

total story drift that occurs when the structure is subjected to the design seismic forces.

Calculated story drift m

shall not exceed 0.025 times the story

height for structures having a fundamental period of less than0.70 seconds.

Calculated story drift m

shall not exceed 0.020 times the story

height for structures having a fundamental period equal to orgreater than 0.70 seconds.

P

P 0.1.

Effects:effects are neglected when the ratio given by Eqn. (A-21) is

M sec ondary

M primary

Px Vx

hsx

(A-21)

Px = total unfactored gravity load at and above level x= seismic story drift by design seismic forces ( s )Vx = seismic shear between levels x and

x 1

hs x = story height below level x

In seismic zones no. 3 and 4,

P

need not be considered

when the story drift ( s )

1.2 hs x / R

times the story height.

Simplified Design Base Shear:Applicability:

Buildings of any occupancy and buildings not more thanthree stories in height, excluding basements, in standardoccupancy structures.

Other buildings not more than two stories in height,excluding basements.

Base Shear:The total design base shear in a given direction is determined fromthe following formula:

V 3.0 Ca

WR

(A-22)

When the soil properties are not known in sufficient detail todetermine the soil profile type, type S DZones 3 and 4.

is used in Seismic

When the soil properties are not known in sufficient detail todetermine the soil profile type, type S EZones 1, 2A and 2B.

Vertical Distribution of Force:

is used in Seismic

The forces at each level are calculated from the following formula:

F 3.0 Ca wi x R

(A-23)

Table (A-1.a) Vertical Structural Irregularities

Irregularity Type and D efinition1- Stiffness Irregularity- - -Soft StoryA soft story is on e in w h ich th e lateral stiffn ess in less th an 70 percen t of th an in th e story above or less th an 80 p ercen t of th e averagestiffn ess of th e th ree stories above.2- Mass IrregularityMass irregu larity is con sid ered to exist w h ere th e effective mass of any story is m ore th an 150 p ercen t of th e effective m ass of an ad jacen tstory. A roof th at is ligh ter th an th e floor below n eed n ot be con sid ered.3- Vertical Geometric IrregularityVertical geom etric irregu larity sh all be con sid ered to exist w h ere the h orizon tal d im en sion of th e lateral force-resistin g system in an ystory is more th an 130 p ercen t of th at in an ad jacen t story.On e-story p en th ou ses n eed n ot be con sid ered .4- In-Plane D iscontinuity in Vertical Lateral Force-resisting Element An in -p lan e offset of th e lateral load -resistin g elem en ts greater th an th e len gth of th ese elem en ts.5- D iscontinuity in Capacity-Weak StoryA w eak story is on e in w h ich th e story stren gth is less th an 80 percen t of th at in th e story above. Th e story stren gth is th e total strength of all seism ic-resistin g elemen ts sh arin g th e story sh ear for the d irection u n d er con sid eration .

Table (A-1.b) Plan Structural Irregularities

Irregularity Type and D efinition1- Torsional IrregularityTorsion al irregu larity is to be con sid ered to exist w h en th e m axim um story d rift, com p u ted in clu d in g accid en tal torsion , at on e en d ofth e stru ctu re tran sverse to an axis is more th an 1.2 tim es th e averageof th e story d rifts of th e tw o en d s of th e stru ctu re.2- Re-entrant CornersPlan con figu ration s of a stru ctu re an d its lateral force-resistin gsystem con tain re-en tran t corn ers, w h ere both p rojection s of th e structu re beyon d a re-en tran t corn er are greater th an 15 % of th e p lan dim en sion of th e stru ctu re in th e giv en d irection .3- D iaphragm D iscontinuityDiap h ragm s w ith abru p t d iscon tin u ities or variation s in stiffness, in clu d in g th ose h avin g cu tou t or op en areas greater th an 50 %of th e gross en closed area of th e d iap h ragm , or ch an ges ineffective d iap h ragm stiffn ess of m ore th an 50 % from on e story to th e next.4- Out-of-plane OffsetsDiscon tin u ities in a lateral force p ath , su ch as ou t-of-p lan e offsets of th e vertical elem en ts.5- N onparallel SystemsTh e vertical lateral load -resistin g elem en ts are n ot p arallel to orsymm etric abou t th e m ajor orth ogon al axes of th e lateral force-resistin g system .

Table (A-2) Seismic Zone Factor Z

Zone 1 2A 2B 3 4Z 0.075 0.15 0.20 0.30 0.40

N ote : Th e z on e sh all be d eterm in ed from th e seismic zon e m ap .

Table (A-3) Occupancy Importance Factors

Occupancy Category Seismic Importance Factor, I1-Essen tial facilities

1.252-H azard ou s facilities

1.253-Sp ecial occu p an cy stru ctu res 1.004-Stan d ard occu p an cy stru ctu res 1.005-Miscellan eou s stru ctu res

1.00

Table (A-4) Structural Systems

Basic Structural System

Lateral- force resisting systemdescription

R Height limitZones 3 &4.

(meters)Bearing Wall Concrete shear walls 4.5 2.8 48Building Frame Concrete shear walls 5.5 2.8 73Moment- SMRF 8.5 2.8 N.LResisting Frame IMRF 5.5 2.8 ----

OMRF 3.5 2.8 ----Dual Shear wall + SMRF 8.5 2.8 N.L

Shear wall + IMRF 6.5 2.8 48

CantileveredColumn Building

Cantilevered column elements 2.2 2.0 10

Shear-wall FrameInteraction

5.5 2.8 48

Table (A-5) Spoil Profile Types

SoilProfileType

Soil ProfileN ame/GenericD escription

Average Soil Properties For Top 30 m Of SoilProfileShear WaveVelocity,vs m/s

Standard Penetration Test, N (blow s/foot)

Undrained Shear Strength, Su kPa

S A H ard Rock > 1,500 --- ---S B Rock 760 to 1,500SC Very Den se Soil an

d Soft Rock360 to 760 > 50 > 100

S D Stiff Soil Profile 180 to 360 15 to 50 50 to 100S E Soft Soil Profile < 180 < 15 < 50S F Soil Requ irin g Site-sp ecific Eva lu a tion

Table (A-6) Seismic Coefficient CaSoil Profile Type Se ismic Zone Factor, Z

Z =0.075 Z = 0.15 Z = 0.2 Z = 0.3 Z = 0.4

S A 0.06 0.12 0.16 0.24 0.32 N aS B 0.08 0.15 0.20 0.30 0.40 N aSC 0.09 0.18 0.24 0.33 0.40 N aS D 0.12 0.22 0.28 0.36 0.44 N aS E 0.19 0.30 0.34 0.36 0.36 N aS F See Footnote

Footnote: Site-sp ecific geotechn ical in vestigation an d d yn a m ic resp on se an a lysis sh all be p erform ed to d etermin e seism ic coefficien ts for soil Profile Typ e S F .

Table (A-7) Seismic Coefficient CvSoil Profile Type Se ismic Zone Factor, Z

Z =0.075 Z = 0.15 Z = 0.2 Z = 0.3 Z = 0.4S A 0.06 0.12 0.16 0.24 0.32 N aS B 0.08 0.15 0.20 0.30 0.40 N aSC 0.13 0.25 0.33 0.45 0.56 N aS D 0.18 0.32 0.40 0.54 0.64 N aS E 0.26 0.50 0.64 0.84 0.96 N aS F See Footnote

Footnote : Site-sp ecific geotech n ical in vestiga tion an d d yn am ic resp on se an alysis sh all be p erform ed to d etermin e seism ic coefficien ts for soil Profile Typ e S F .

Table (A-8) N ear-Source Factor N aSeismic Source Type Closest D istance to Know n Seismic Source

2 km 5 km 10 kmA 1.5 1.2 1.0B 1.3 1.0 1.0C 1.0 1.0 1.0

Table (A-9) N ear-Source Factor N vSeismic Source Type Closest D istance to Know n Seismic Source

2 km 5 km 10 km 15 kmA 2.0 1.6 1.2 1.0B 1.6 1.2 1.0 1.0C 1.0 1.0 1.0 1.0

Example (A-1):

Using UBC 97, evaluate the seismic base shear acting on aregular twelve-story building frame system with reinforcedconcrete shear walls in the principal directions, as the mainlateral force-resisting system. The building which is located inGaza City is 31.2 m by 19 m in plan and 32.8 m in height(Standard Occupancy). It is constructed on a sandy soil profile withSPT values ranging from 20 to 50 blows/foot.

So l u t i on:From Table A-2 and for Zone 1, Z = 0.075

From Table A-3 and for Standard Occupancy, I =1.0

From Table A-5, Soil Profile Type is S DFrom Table A-4, R = 5.5

From Table A-6,

Ca From Table A-7,

Cv From Eqn. (A-

6),

= 0.12

= 0.18

T 0.048838.283/ 40.75 sec.

From Eq. (A-1), the total base shear is

V Cv I W

0.18 W

0.0436

R T 5.50.75 W

From Eq. (A-2), the total base is not to exceed

2.5 C V I W

2.50.12W

0.0545 W

O.K

R 5.5

From Eq. (A-3), the total design base is not to be less than

V 0.11 Ca

I W 0.11(0.12)W

0.0132 W

O.K

a