7/21/2019 2006 IBC Section 160521 Seismic Strength Design Load Combinations

http://slidepdf.com/reader/full/2006-ibc-section-160521-seismic-strength-design-load-combinations 1/2

2006 IBC Section 1605.2.1: Seismic Strength Design

Load Combinations

 cenews.com /article/5742/2006-ibc-section-1605-2-1-seismic-strength-design-load-combinations

 April 2008 » Columns

Discussing a common question about the seismic load effect, E, used in the 2006 International Building

Code Section 1605 load combinations and defined in Section 12.4.2 of ASCE 7-05.

S.K. Ghosh, Ph.D., and Susan Dowty, S.E.

This "Code Simple" addresses a common question asked about the seismic load effect, E, used in the

2006 International Building Code (IBC) Section 1605 load combinations and defined in Section 12.4.2 of 

 ASCE 7-05. The question is: How does one combine a horizontal component, due to the base shear, V ,

with a vertical component, due to the dead load?

In a literal sense, loads are never combined. Gravity loads (which act in a vertical direction) and wind

forces (which act horizontally) are simply not combinable. What is combined through the so-called load

combinations are load "effects." The word effects is very important. Gravity loads cause bending moments,

shear forces, and axial forces at critical sections of structural members and these are the effects of gravity

loads. Horizontal wind forces do exactly the same thing, the resulting bending moments and so on are the

effects of wind forces. These bending moments, shear forces, and axial forces are combinable, as

reflected in the building codes.

The earthquake effect, E , is more complicated in the sense that it has both a horizontal and a vertical

component to it. First, a close look at the two equations that include seismic loads in the strength load

combinations of the 2006 IBC Section 1605.2.1 will be helpful. They are Equations 16-5 and 16-7. For the

purpose of discussion, let it be assumed that there is not an H  load (load due to lateral earth pressure,

ground water pressure, or pressure of bulk materials), f 1 = 0.5 and f 2  = 0.2.

Equation 16-5 can be rewritten as follows and is considered an additive load combination because gravity

and seismic forces are causing bending moments, shear forces, and axial forces in the same direction:

= 1.2D + 0.5L + 0.2S + 1.0E ,

= (1.2 + 0.2SDS)D + 0.5L + 0.2S + ρQE,

because in this case E  = E h + E v  = ρQE  + 0.2SDSD per ASCE 7-05 Section 12.4.2.

Equation 16-7 can be rewritten as follows and is considered a counteractive load combination because

gravity and seismic forces cause bending moments, shear forces, and axial forces in opposite directions:

= 0.9D + 1.0E,

= (0.9—0.2SDS)D + ρQE,

because in this case E  = E h - E v  = ρQE  - 0.2SDSD per ASCE 7-05 Section 12.4.2

In both the above equations, the horizontal earthquake effect, QE , can be positive or negative. In the first

equation, the positive sign governs, in the second equation the negative sign governs. The vertical load

effect, ±0.2SDSD, increases or decreases the dead load factor in the seismic strength design load

combinations. In the additive load combination, the dead load factor is increased by considering vertically

7/21/2019 2006 IBC Section 160521 Seismic Strength Design Load Combinations

http://slidepdf.com/reader/full/2006-ibc-section-160521-seismic-strength-design-load-combinations 2/2

downward earthquake effect; in the counteractive load combination, the dead load factor is decreased by

considering vertically upward earthquake effect. In each case, this is the conservative approach.

For example, consider a fully redundant structure (ρ = 1.0) located where SDS = 1.0; a bearing wall

system consisting of reinforced concrete shear walls is used for the seismic force-resisting system. If the

bending moments in a shear wall cross-section due to dead loads, live loads, snow loads, and horizontal

earthquake forces are 200 foot-kips, 60 foot-kips, 0 foot-kips, and 150 foot-kips, respectively, the design

moments (required flexural strengths) by the strength design load combinations (IBC Equations 16-5 and

16-7) are:

M u = [(1.2) + (0.2)(1.0)]( 200) + (0.5)(60) + (1)(150) = 460 foot-kips

M u = [(0.9)—(0.2)(1.0)](200)—(1)(150) = -10 foot-kips

The shear wall needs to be reinforced to carry these bending moments at the cross-section in question.

Answers to FAQs:

Q: If designing an element that only resists seismic load (such as a diaphragm) using allowable stress

design (ASD), do you design for E  or (0.7)(E )? Is E  always E /1.4 for ASD and 1.0E  for LRFD?

A: Load combinations are always used in design, except that sometimes the effects of one or more loads

may be zero. For instance, the horizontal shear in a shear wall or a diaphragm would often be caused by

earthquake forces alone. It is not that gravity effects are disregarded. It is just that gravity simply does not

contribute to the particular load effect being considered. Then, if ASD is used, the element is designed for 

0.7E , and if LRFD is used, the element is designed for 1.0E . In these cases, E is ρQE  because D is zero.

S.K. Ghosh Associates Inc., is a seismic and building code consulting firm located in Palatine, Ill., and 

Laguna Niguel, Calif. President S.K. Ghosh, Ph.D., and Susan Dowty, S.E., are active in the

development and interpretation of national structural code provisions. They can be contacted at 

skghosh@aol.com and dowtyskga@cox.net , respectively, or at www.skghoshassociates.com.