Seismic Design ASCE7 Part 1 by Ryan Freund November 3, 2012 5 Comments

ASCE 7 Seismic Design Part 1 We are going to break down and review seismic design in regards to ASCE7-05. We are going to cover the

basics and some commentary. Hopefully I will be able to elaborate sometime in the future and include some

discussion.

Based ASCE7-05

1) Exceptions

a) Detached 1 and 2 family dwellings with a Ss

o Evaluate the Maximum Considered Earthquake (MCE) spectral response acceleration for short

periods and 1 sec periods.

Sms=Fa*Ss Sm1=FvS1

4. Design Spectral Acceleration Parameters (these are the values used in design).

o Sds=2/3*Sms (short period)

o Sd1=2/3*Sm1 (1 s period)

5. Determine Occupancy Category from Table 1-1

6. Determine Importance Factor from Table 11.5-1

7. Determine Seismic Design Category (SDC) A,B,C ,D, E or F. (E is reserved for S1>0.75 and F is

reserved for Occupancy Category IV w/ S1>0.75) based occupancy category and period response

acceleration parameter.

o Table 11.6-1 SDC based on Sds

o Table 11.6-2 SDC based Sd1

o Use the most severe case. It is permitted to use Table 11.6-1 if S1 < 0.75 and all of the following

apply:

1. Unless Ta

o Omega: The Overstrength factor increases the required seismic forces and is applied in specific cases or in the design of certain parts of the structure. 0 is intended to reflect the upper bound lateral strength of the structure and estimates the maximum forces in elements that

are to remain non-yielding during the design basis ground motion. In summary, R reduces the

required seismic forces realizing the some yielding of the structure will help dissipate energy. To

force a more ductile response some brittle members are designed to resist higher forces so that

they stay in the elastic range during the seismic event.

o (Cd) Deflection Amplification Factor: Realizing that the structure is intended to yield (ductile

response) deflection will be greater than that found from an elastic analysis. Cd amplifies the

deflection of the structure based on an elastic analysis.

o

Response Factor, Deflection Amplification Factor and Overstrength Factor

2. Different systems may be used in the same structure. If the systems are in orthogonal directions the R,

Omega, Cd shall be applied to each system. Systems used in combination to resist lateral forces in the

same direction are referred to as dual systems. Some dual systems are listed in Table 12.2-1. For other

systems the more stringent system limitation shall apply.

o If R, C and vary over the height of the structure; the story below shall meet the most stringent of the stories above (avoid weak story) for systems in the same direction.

There are multiple exceptions see (12.2.3.1) o If R, C and vary within the same story (Horizontal Combinations). R shall be the lowest of the

different systems for that story. R may vary for different lines of LFRS if the building category is 1

or 2, two stories or less and the use of flexible diagrams. However the diaphragm shall use the

lowest R value. Cd and Omega in the direction under consideration under consideration at any

story shall not be less than the largest value of this factor for the R factor used in the same

direction being considered.

o Further restrictions and direction is given in 12.2.5 for specific system requirements.

3. Irregularities Irregularities are covered in chapter 12.3. They are specific to certain geometries and

mass distributions.

o Vertical Irregularities Differences from story to story including

Variable stiffness, variable weight distribution, offset of vertical elements.

o Horizontal Irregularities Reentrant corners, torsional, discontinuous diaphragms,

non parallel systems.

4. Redundancy Factor, equal to 1 for the following: o Structures assigned to SDC B or C

o Calculating drift and P-Delta effects.

o Design of nonbuilding structures

o Design of collector elements, splices and connections when using the overstrength factor.

o Diaphragm loads using Eq 12.10-1

o Structures with damping systems (Section 18).

5. Redundancy Factor, equal to 1.3 for SDC D,E and F. Unless the exceptions of 12.3.4.2 are met and comply with table 12.3-3.

6. Diaphragm Flexibility Rigid, Flexible and Semi-Rigid. All diaphragms are semi-rigid, meaning that load

is distributed to from the diaphragm to the vertical elements depends on the stiffness of the diaphragm

and stiffness of the vertical elements.

1. Rigid When concrete is used, span-to-width is

Helpful Links for Determining Minimum Design Loads by Ryan Freund November 28, 2012 0 Comments

Helpful Links for Determining Minimum Design Loads Hopefully these links can save you some time and help get you more accurate design loads. A quick heads-up

you will usually need to search the town/county/state to see if the Authority Having Jurisdiction (AHJ) has a

specific requirement.

Wind Load

A favorite for Wind Loads in accordance w/ ASCE 7

http://www.atcouncil.org/windspeed/index.php

Seismic

A favorite for determining your base acceleration coefficients:

http://earthquake.usgs.gov/hazards/designmaps/

Snow

This site is no longer free but when I used it, it was useful:

http://www.groundsnowbyzip.com/

This is a little dated and really not that useful but Ill mention it anyway:

http://www.fs.fed.us/t-d/snow_load/states.htm

Others

This is a pay-for site but some may use it:

http://www.groundsnowbyzip.com/

Wind Load ASCE 7-05 VS ASCE 7-10 by Ryan Freund April 27, 2012 2 Comments

In comparing the 2010 edition to the 2005 edition of the ASCE 7 we see that there are significant changes to the

layout, format, load factors used for wind and basic wind speed maps. These changes affect how you determine

wind design wind pressures.

References

ASCE 7-10 Minimum Design Loads for Buildings and Other Structures. Found here

ASCE 7-05 Minimum Design Loads for Buildings and Other Structures. Found here

The Basics

ASCE 7-05 uses a single basic wind speed map. For each building risk category an importance factor is applied.

Note that these importance factors only depend on the type of building, not where the building is located. The

wind-load factor is then applied to determine the design wind pressure. For this edition (05), the ASD wind-load

factor is 1.0 and the strength design wind-load factor is 1.6.

ASCE 7-10 uses three different basic wind speed maps for different categories of building occupancies. These

maps provide basic wind speeds that are directly applicable for determining pressures for strength design.

Consequently, the strength design wind-load factor was changed to 1.0 in this version. Simply put, ASCE 7-10

uses three maps based on strength design in conjunction with a wind-load factor of 1.0 for strength design

(LRFD) and 0.6 for service level loads (ASD), while ASCE 7-05 uses a single map with an importance factor and

wind-load factor of 1.6 for strength design (LRFD) and 1.0 for service level loads (ASD).

Why the Change?

The commentary in ASCE 7-10 (section states 26.5.1) a few reasons for basic wind speed changes:

1. A strength design wind speed map brings the design approach used for wind in-line with that used for

seismic loads.

2. Multiple maps remove the inconsistencies inherent the importance factor approach. With multiple maps

a distinction may be made based on location (i.e. hurricane prone vs non-hurricane prone which also

changes the recurrence interval).

3. New maps establish a more uniform return period for the design-basis winds.

4. The maps more clearly inform owners and their consultants (thats you) about the storm intensities for

which designs are preformed.

5. We have justify our pay check somehow

Summary:

ASCE 7-10: 3 wind speed map based on 3-sec gust at 33ft above ground. The different maps are calibrated to

strength level design (LRFD LF=1.0) and also include building classification and location.

ASCE 7-05: (1) wind speed map based on 3-sec gust at 33ft above ground. Importance factors and Load

Factors are used to increase design pressures.

(Bonus Info)

EIA-TIA-222 Rev G: Wind speeds are similar to 7-05 with different definitions of classification of structures and

gust effect factors.

EIA-TIA-222 Rev F: Wind speed maps based on fastest mile. These are not directly comparable to ASCE 7-05

or 10, as the ASCE 7 uses 3-sec gust. The 3-sec gust represents the peak gust wind speed where as the

fastest-mile wind speed represents the average wind speed over the time required for one mile of wind to pass

the site. The design pressures are derived using different adjustments for height/exposure and gust effects than

that of Rev G and/or the ASCE 7 standard

How ASCE 7-10 Wind speed were developed return periods:

Risk Cat I which is based on 25-yr return period equates to 300yr return period

Risk Cat II: 700yrs or 0.0014 annual exceedance probability

Risk Cat III and IV which are based on a 100-yr return period (thus there importance factor was greater in -05):

1,700yrs or 0.000588 annual exceedance probability

Note

Interestingly enough new research gathered since 2005 indicated that design wind speeds should be reduced

(they also note that the overall rate of intense storms increased). Therefore it is likely that you will noticed

reduced wind pressures along coastal regions.

For most of the US of A the wind load remains basically unchanged. A quick look at the basics

ASCE 7-10 (eqn 27.3-1) or ASCE 7-05 (eqn 6-15) wind pressure:

Assuming that and V = 90 mph then we have

ASCE 7-05 => (ASD)

ASCE 7-10 => (ASD)

A nice paper by AWC (American Wood Council)

http://www.awc.org/pdf/ASCE7-10WindChanges.pdf