ASCE 7-05 Load Combinations 1

[Pete23]

I am designing a 2 story wood frame residence that meets the requirements of ASCE 7-05 Section 12.14 Simplified Alternative Structural Design Criteria for Simple Bearing Wall or Building Frame Systems. Section 12.14.2 says "The structure shall include complete lateral and vertical force resisting systems with adequate strength to resist the design seismic forces, specified in this section, in combination with other loads" But in the very next sentence says "in accordance with the procedures of Section 12.14.8" Section 12.14.8 says "The lateral forces applied in each direction shall sum to a total seismic base shear given by Section 12.14.8.1" I think that means "just distribute V vertically and don't use the combination equations." So, if the combination equations are not to be used then why spend so much space in section 12.14 defining them?

In my first pass through the design I used the combination equations and got shear loads so great that I could not resist them with wood frame construction. (This is a simple house - that was obviously the wrong answer). I reviewed a set of calculations for another house by another engineer. He did not use the combinations. I feel like I am missing something in ASCE 7-05. Can somebody help?

 

[JAE]

I don't have 7-05 in front of me but generally the seismic demand is calculated by the applicable procedure (in your case you state 12.14.8).

This equates to a total base shear at the bottom of the structure and is the same total force in either direction (X or Y).

The total base shear is then typically distributed to each story based on story heights and the mass at each story, per the formula given for that vertical distribution.

Once you have the story forces in each direction, you then have a "seismic basic load case". This is combined with other basic load cases (such as Dead Load, Live Load, etc.) to form load combinations that are specified also in 7-05.

Typically, everything in ASCE 7-05 ends up in a load combination so ignoring the load combinations is not an option.

 

[StructuralCA]

Pete23
For shear wall design, the load combination that matters is 0.7QE (for ASD.) This is case 8 in Sec. 12.14.3.1.3.

QE is the horizontal shear in wall (based on horizontal distribution of floor seismic design shear using horizontal diaphragm. Floor seismic shear is obtained from vertical distribution of Vx using 12.14-13.

M.S. Genidy
Structural Soft, LLC

 

[swivel63]

distribution of base shear is E in those combos. it's like wind.

 

[Pete23]

Thanks. This was very helpful.