ASCE 7-10 15.7.10.4 1

[HDStructural]

Hello All,

I am working on a design for an elevated vessel and am designing a support structure for it. The structure will look something similar to the attached image. The seismic base shear from the equipment is about 45% of the wind base shear, so wind controls but there are seismic requirements that need to be followed.



The structure is SDC D and the state uses ASCE 7-10. My question has to do with one of the requirements in 15.7.10.4.

This section states "The connection of the post or leg with the foundation shall be designed to resist both the vertical and lateral resultant from the yield load in the bracing assuming the direction of the lateral load is oriented to produce the maximum lateral shear at the post to foundation interface."

Is this essentially saying that I need to design my anchor bolts for the "yield" load in the braces, which would be the tensile capacity.
It can't mean compressive capacity since it would state buckling load or something.

I expected that I would have to design these connections considering the seismic load with the overstrength factor applied, but I am not seeing that. Designing for the yield load would result in brace forces of 100k, compared to a 30k C or T brace force from wind and a 14k brace force from seismic (35k with overstrength). Designing for 100k seems excessive.

 

[XR250]

I don't know the answer to your seismic question, but don't you need transverse X-bracing on intermediate posts? Or, does the vessel act to transfer all of the side-to-side load to the end frames?

 

[HDStructural]

The vessel has a diaphragm of it's own. All lateral forces come in at the corners.

 

[haynewp]

I don't see a way around it in the ASCE section you cited, unless it is storing gas vapors only. For your case, I would make the braces as small as possible to meet wind and then design the connection for yielding in the tension brace. If buckling is controlling the brace sizes under wind, then I would put in vertical stacks of braces separated by that orange or brown (I'm color blind) horizontal member instead of using two tall braces as you currently show. You should be able to reduce the brace sizes then.

 

[HDStructural]

Stacking the braces is a good idea to reduce the force in each brace, and lower the unbraced length.

I could also make the braces tension only, then the yield load would be much much closer to the design load.

 

[JAE]

I think the idea here is to ensure that the overall system fails in ductile yielding - so this section requires you to ensure the strength of the anchorage to at least what would be generated by the yielding of the braces. The end connections under seismic design are higher capacity typically as you state.

So what haynewp suggests - make the braces as small as possible to ensure a ductile failure.

 

[PMR06]

For what its worth, ASCE 7-16 changed the verbiage of 15.7.10.1, better differentiating between tanks with integral support towers, and tanks that are just supported by a another tower structure. It sounds like your tank may be the latter condition. (The commentary in 7-16 also has some photo examples of the different arrangements.)

If your arrangement is a tank supported by another structure, then maybe you could get the AHJ/Owner on board with the intent of ASCE 7-16 (and later). In that case, the requirements of 15.7.10.4 would not be applicable to your structure. You would design per sections 15.3 and 15.5.5.

 

[HDStructural]

Thank you for pointing that out.