How to determine design story drifts using ASCE 7-10

[adarpodracir]

Hi,

I am having a hard time trying to understand how to determine the design story drifts of a RC building using ASCE 7-10. The building belongs to Seismic Design Category C. Section 12.8.6 (ASCE 7-10) deals with story drift determination and states that:

For determining compliance with the story drift limits of Section 12.12.1, it is permitted to determine the elastic drifts, (δxe), using seismic design forces based on the computed fundamental period of the structure without the upper limit (CuTa) specified in Section 12.8.2.

For me, seismic design forces are those forces computed using 100% of the forces for one direction plus 30% of the forces for the perpendicular direction, according to Section 12.5.3.a (ASCE 7-10). However, I am not quite sure whether I should use the orthogonal combination procedure to determine design story drifts in my building.

Any comments will be greatly appreciated.

Thank you!

 

[Hokie93]

I believe you need to consider four loading cases in determining the design story drift per Chapter 12.8.6 of ASCE 7-10. Load case number 1 is 100% of the seismic design force in the X-direction. Load case number 2 is 100% of the seismic design force in the Y-direction. Load case number 3 is 100% of the seismic design force in the X-direction coupled with 30% of the seismic design force in the Y-direction. Load case number 4 is 100% of the seismic design force in the Y-direction coupled with 30% of the seismic design force in the X-direction. Although load cases 3 and 4 may very well control, you need to consider load cases 1 and 2 since you must satisfy the loading requirements for a Seismic Design Category B structure in addition to the requirements for SDC C. Note that you do NOT need to consider accidental torsion in both directions, at the same time, when applying load concurrently in two orthogonal directions (see Chapter 12.8.4.2).

I take it that a Type 5 horizontal irregularity is driving the need to consider the 100% plus 30% load requirement?

 

[adarpodracir]

Thank you very much for the reply.

My building has an "extreme torsional irregularity" (type 1b) and a "reentrant corner irregularity" (type 2). The reentrant corner irregularity is making the building to twist drastically along the CR axis. This is the reason why the building is having an extreme torsional irregularity. So because of that, I think I need to consider torsional and orthogonal effects on my building at the same time. I know that orthogonal effects should only be included when the building has a "nonparallel system irregularity", as prescribed in Section 12.5.3. However, I think orthogonal effects are as equally important as torsional effects in my building. In any case, if my building belonged to Category D, I would have to include orthogonal effects in my building to conform with the minimum requirements of Section 12.5.4, wouldn't I?

Thank you!

 

[CELinOttawa]

Sad.... This isn't baking....

If the code's intent is this hard to understand and the commentary is insufficient, how do you keep from having unintended misinterpretations by regular practising engineers?

Just frustrated about which way things are going in our profession..

 

[AELLC]

AELLC's Law #17:

Because the ASCE (and most other major Associations) want you to spend extra money going to their over-priced seminars. Then, and only then, can a normal person ever understand their convoluted codes.

 

[Hokie93]

Per ASCE 7-10 Chapter 12.5.4, you are only required to consider concurrent loading in two orthogonal directions in Seismic Design Category D if you have a Type 5 horizontal irregularity (this is a carryover from the SDC C requirements) or if you have a column or wall that forms part of two (or more) intersecting seismic force-resisting systems and has axial seismic force exceeding 20% of column or wall axial design strength. Note that the axial seismic load in the column or wall in question is determined initially from lateral loads applied independently in the two principal orthogonal directions (e.g., 100% of the load applied in the X-direction and 100% of the load applied, separately, in the Y-direction). Put another way, just because a structure is assigned to SDC D does not mean, per se, that you must include concurrent loading in two orthogonal directions.

Given the horizontal structural irregularities you describe, it may very well be prudent for you to exceed the code requirements as far as direction of loading is concerned.

 

[adarpodracir]

Ok, thank you very much.