Seismic Design for Nonstructural Components

[gte447f]

I have a question regarding a potential project doing seismic design for nonstructural components. I occasionally get jobs designing or reviewing the designs of others for attching MEP equipment to an existing structure, and I am familiar with the requirements of ASCE 7 Chapter 13. However, a subcontractor has asked me about a military project in which the specs specifically say that he is to supply calculations stamped by an engineer for the seismic bracing and that "verify the capability of structural members to which bracing is attached for carrying the load from the brace". It is this verification of the superstructure that has me concerned. This is new construction, not a retrofit. Is this standard procedure? It doesn't seem right, or fair, to put the liability for the superstructure on the subcontractor and his specialty engineer. If this responsibilty does fall on the specialty engineer, what scope and fee impacts would be expected beyond those for only designing the seismic bracing?

 

[JedClampett]

Well, let me be the devil's advocate:
Let's say you were the EOR for the building. You know there was field located equipment (pipes, HVAC, etc.) supported by your structure, but since you didn't know the exact position or materials, you couldn't do much more than an approximate analysis. And since not every support needed a seismic brace, it was impossible to determine where to beef up your members for bracing loads. So you had it written into the specifications that the component design engineer would check the structure for the seismic bracing loads.
So that's where you come in. I agree, it's very difficult to recreate the design and add these loads to the members. But someone has to do it. Maybe the EOR will be a nice guy or gal and give you his calculations. Otherwise I hope you have sufficient budget to do this.

 

[JoshPlumSE]

I've seen wording like that before. What I take it to mean is that you must check the connections to the existing structure to make sure that they can take the equipment's seismic load. Once that load gets into the main lateral force resisting system your responsibility should end..... Unless, of course, the equipment you're adding significantly increases the seismic weight of the structure and would require an oveall change to the lateral force resisting system. There is certainly some engineering judgment involed in making that distinction. But, I'd calculate the weight of my equipment as a percentage of the overall weight of the structure. If it's less than 2% (as I imagine it would be) then I'm good. If it's over 10% then we tell the owner that the whole structure must be re-evaluated. If it's somewhere in between... then I don't know.

Back in the poste Northridge days there was some fairly well established criteria for how much of a change in stiffness or base shear of the structure would require you to re-evaluated the structure under current codes. Unfortunately, I don't recall the reference.

 

[gte447f]

JedClampett, I understand where you are coming from, but I think that if the seismic bracing of nonstructural equipment isn't handled during the building design process, then some contigency needs to be made, like the way that RTU's are often handled by designating an RTU zone and specifiying the allowable distribution and magnitudes of RTU loads within that zone, for example.

As it stands, I am still in the "bidding" phase, but I'm afraid to include a decent contingency for recreating and verifying the original design will eliminate me from consideration.

JoshPlum, I'm not worried about the Lateral System, but even checking structural members for localized effects of brace reactions is condierably more detailed than stopping at the anchorage.