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IBC Chapter 23 and 2005 NDS — ASD/LRFD dual format standard, ICC, Sept. 2008

ASCE 7-05 Section 12.8.6 — Story Drift Determination, SKGA, August 2008

ASCE 7-05 Supplement No. 2 , SKGA, June 2008

2006 IBC Section 1605.2.1: Seismic Strength Design Load Combinations, SKGA,April 2008

2006 IBC Section 2308: Conventional light-frame construction, ICC, March 2008

2006 IBC Section 1605.4: Special Seismic Load Combinations, SKGA, Feb. 2008

2006 IBC Section 1805: Footings and foundations, ICC, Jan. 2008

1997 UBC vs. 2006 IBC Concrete Anchor Design, SKGA, Dec. 2007

2006 IBC Section 2300: Wood design methods, ICC, Nov. 2007

2006 IBC Section 1802: Foundation and soils investigations, SKGA, Oct. 2007

2006 IBC Section 1609: Exposure D, SKGA, Aug. 2007

2006 IBC Section 1613.5.1: Seismic Design Category A Requirements, ICC, July 2007

2006 IBC Section 1605.3: Load Combinations Using Allowable Stress Design,SKGA, June 2007

ASCE 7-05 seismic provisions errata, SKGA, April 2007

ASCE 7-05 Table 12.2-1 Bearing Wall System vs. Building Frame System, SKGA,

Feb. 20072006 IBC Section 2308.4: Design of elements, ICC, Jan. 2007

ASCE 7-05 Figure 11.4-1 Design Response Spectrum, SKGA, Dec. 2006

2006 IBC Section 1801.2.1: Foundation design for seismic overturning, SKGA, Oct. 2006

Section 1604.5: Importance Factors, ICC, Sept. 2006

Section 1602.1: Denitions (Diaphragm, exible) and 1617.5.3:Horizontal Distribution, SKGA, Aug. 2006

Section 1620.4.5: Building Separations, SKGA, June 2006

Section 1617.6: Seismic-Force-Resisting Systems, SKGA, April 2006

Section 1607.9: Passenger vehicle parking garage live loads, SKGA, Feb. 2006

The Code Simple column published in August 2008explained what role the Importance Factor, I , has inthe determination of drift. In this Code Simple, we

would like to take a closer look at Section 12.12.3Building Separation and at Section 12.8.7 P-Delta Effects. The question we would like to answer is, does it make sense touse in these requirements aE x where I has been divided out?

Let’s say, for example, that an Occupancy Category IIIstructure is being designed and I has been established equalto 1.25. Therefore, the seismic design base shear is multi-plied by a factor of 1.25 to account for the higher OccupancyCategory. In the case of drift where Table 12.12-1 sets forthdrift limitations in accordance with Occupancy Category, itmakes sense to divide the drift value by a factor of I . How-ever, it does not make sense to divide the calculated drift by I in the case of building separations or P-delta effects. Bydoing so, you are violating the intent of the code, which is

to improve the ability of the structure to function after anearthquake and limit nonstructural damage.

This also becomes an issue when calculating seismic rela-tive displacements for nonstructural components (see ASCE7-05 Section 13.3.2). Equation 13.3-5 is based onE x, whichdivides out I , so therefore, I is not taken into account whendetermining the seismic relative displacements for nonstruc-tural components.

Table 1 (at right) provides a historical perspective on thedetermination ofE x and R. Before studying the table, the readermust be aware of the following two fundamental points:1. As it is used in the IBC and ASCE 7, E xe is the elastically

computed deection at oor level x under code-prescribedseismic forces. The corresponding term in the UBC is%s.

2. As it is used in the IBC and ASCE 7, E x reects theamplied deection (which takes into account the inelas-tic response of the structural system). The corresponding

CODE SIMPLE By S. K. Ghosh, Ph.D., and Susan Dowty, S.E.

Section 12.12.3 Building Separationand Section 12.8.7 P-Delta Effects

The building codes and referenced standards that governmuch of how structural engineers work are regularly updated

and enhanced. Keeping abreast of the changes to these regula-tions is a constant challenge.

In response, Structural Engineer publishes articles aboutcodes and standards in two formats: Bi-monthly “Code Series”articles are contributed by the associations that write the codesand standards, while the monthly “Code Simple” column iscontributed by industry experts to explain provision language

and rationale behind changes to specic sections.The following is a partial list of “Code Simple” columns pre-

viously published in Structural Engineer . To access the fulllist online, go to www.gostructural.com/codes.

Articles referenced with “SKGA” were contributed by S.K.Ghosh, Ph.D., and Susan Dowty, S.E., of S.K. Ghosh AssociatesInc. Articles referenced with “ICC” were contributed by John R.Henry, P.E., of the International Code Council.

Focus On: Codes

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term in the UBC is %m. The difference between the twoterms is that I is taken out of E x, but not out of %m.

A lot of confusion would have been avoided if ASCE7-05 Eq. 12.8-15 would have dened E x as being equal toCd E xe (without the division by I ); % could remain as denedcurrently (E x – E x–1); and Section 12.12.1 could have then

required that%

/ I not exceed%

a as given in Table 12.12-1. Webelieve this to have been the intent of ASCE 7 all along.

S.K. Ghosh Associates Inc., is a structural seismic, and code consult-ing rm located in Palatine, Ill., and Laguna Niguel, Calif. PresidentS.K. Ghosh, Ph.D. , and Susan Dowty, S.E. , are active inthe development and interpretation of national structural code provi-sions. They can be contacted at skghosh@aol.com and dowtyskga@cox.net,respectively, or at www.skghoshassociates.com.

Table 1: A historical perspective on the determination of Ex and R.

Code / Standard Design Earthquake Displacement, Design Drift P – % Effects

1997 UBC Equation 30-17: % M

(0.7R) Section 1630.1.3: P – % effects need not beconsidered if

Note that UBC did not make drift limitations P x % s

V x

hsx

P x % M

0.7 R

V x

hsx

b 0.10

dependent on occupancy; therefore,

I is not divided out.

In Seismic Zones 3 and 4, P – % effects need not be considered if

% s hsx % M 0.7 R hsx b 0.2 R

Note I is not divided out of %S , %

M

1997 NEHRP Provisions Section 5.3.7.1* Section 5.3.7.2**

ASCE 7-98 Equation 9.5.3.7.1* Section 9.5.3.7.2**

ASCE 7-02 Equation 9.5.5.7.1* Section 9.5.5.7.2**

ASCE 7-05 Equation 12.8-15*: Section 12.8.7**:

** P – % effects need not be considered when

* Ex C d Exe

I P x %

V x hsx C d

b 0.10 % E

x – Ex – 1

Here I is divided out because Here I is divided out, which means considerationdrift limits are a function of I . of P – % effects is independent of the importance

of the structure.

2000 IBC Equation 16-46: Section 1617.4.6.2:

Ex C d Exe

I P – % effects need not be considered when

% E

x – Ex – 1

P x %

V x hsx C d

b 0.10

2003 IBC References ASCE 7-02 References ASCE 7-02

2006 IBC References ASCE 7-05 References ASCE 7-05

Here I is divided out because drift limits Here I is divided out, which means consideration ofare a function of I . P – % effects is independent of the importance of the

structure.