Non-Structural Component - z/h what is the base? 1

[CQT3]

Hello all, I have an industrial equipment anchored on top of a framed structure that is 4 to 5 feet above the ground. Would this make the z/h=1 for the anchorage of the equipment to the frame? Hypothetically speaking, what if this industrial equipment is anchored on top of a framed structure that itself is anchored on top of a building?

Thanks,

 

[Agent666]

For the first question I believe that's the intent. Height of the attachment is the height of the uppermost mass.

For the second hypothetical question. I don't have a definitive answer as it probably depends on a number of things like the relative stiffnesses and masses and probably a lot more besides. Exercise some judgement and conservatism or head down a more rigorous analysis path if throwing some conservatism in is too onerous.

 

[KootK]

I have an industrial equipment anchored on top of a framed structure that is 4 to 5 feet above the ground. Would this make the z/h=1

I believe so.

Hypothetically speaking, what if this industrial equipment is anchored on top of a framed structure that itself is anchored on top of a building?

Still z/h = 1.0.

I think that the key to understanding how to apply these provisions is to have a fundamental picture in your head of the model that ASCE7 assumes for equipment anchorage. The sketch below shows what my understanding of that model is. Obviously, this is not how most real structures respond to seismic events. The model is a greatly simplified one that makes it such that equipment anchorage can be a relatively simple exercise not needing to be proceeded by a full seismic analysis of the structure.

 

[CQT3]

Would it be fair to say that if the supporting structure (in this case the frame) is really rigid, then z/h=0? If the equipment is going on a really low concrete pedestal, I can't imagine justifying z/h=1.

Thanks,

 

[KootK]

Would it be fair to say that if the supporting structure (in this case the frame) is really rigid, then z/h=0? If the equipment is going on a really low concrete pedestal, I can't imagine justifying z/h=1.

I think that it would indeed be fair were the supporting structure rigid with respect to both shear deformation and flexural / overturning deformation. This assumes, of course, that the pedestal is the supporting structure rather than just an element within the supporting structure.

Looking back at my sketch now, I see that I made an error. I presented the building deformation as the cantilever deformation of a rigid bar. Instead, I think that it would be better to visualize it as the pure shear deformation mode of a framed building. The deformed shape and acceleration would still be linear, however, so my conclusions remain valid I believe.

 

[r13]

Does picture below answers your questions? (z/h <= 1.0)

 

[haynewp]

The attached has some info on the basis of that equation.

 

[HTURKAK]

.....Would this make the z/h=1 for the anchorage of the equipment to the frame? Hypothetically speaking, what if this industrial equipment is anchored on top of a framed structure that itself is anchored on top of a building?

The answer is Yes for both of your questions. That is , z/h=1.0 for both cases. I would like to remind also the calculated force Fp is tha anchor force for the equipment. In order to find the forces for the supporting structure, you should calculate the fundamental period of the supporting structure together with component and folow the procedure at ASCE 7 12.8.1.1 .

The following pictures useful to see the concept for 0.4SDS(1 + 2 z/h) and z/h.

h

https://www.fema.gov/media-library-...850c3050c34da135376baa478a1b/P-752_Unit14.pdf

 

[r13]

I remember h is measured to the average roof height of the structure, and z is to the elevation of anchorage. So, if an equipment is anchored to the roof, z = h, z/h = 1.0. If it is anchored to the ground, z=o, z/h = 0.

 

[KootK]

Thanks to Haynewp I now do know where the 3X factor comes from.