I’m looking for some help interpreting and ASCE 7 code equation.
I’m currently designing a storage rack system, installed above grade. The supporting structure is a non-building, 8-ft tall laterally braced steel platform. I have attached a sketch of what the system looks like, along with the relevant code section.
My understanding is that any component installed above grade will experience amplification of the seismic forces. This is addressed in ASCE7-22, section 13.3.1 - Seismic Demands on Nonstructural components.
My question is about the factor “Hf”, in section 13.3.1.1. “Hf” is the amplification with height. Hf is a function of the fundamental period Ta of the supporting structure and also, a function of the ratio z/h where:
z = Height above the base of the supporting structure to the point of attachment of the component.
h = Average roof height of supporting structure with respect to the base
Basically if the non-structural component (the storage rack) is supported at grade, Hf becomes 1 because the ratio z/h = 0. In my case, the structure is located on top of the platform so z/h = 1 and Hf is basically maximized – I’m getting Hf = 3.5, assuming the fundamental period of the supporting structure Ta = 0.1 seconds. I studied the variation of Hf vs Ta (see attachement), and found that the minimum value of Hf is something around 2, but that's for Ta = 10 seconds, which doesn't seem practical.
Are there any circumstances in which Hf = 1 can be justified, even if the storage rack is located on top of the platform? I would think that having a theoretical “infinitely rigid” platform, i.e a solid concrete block of height “h” would satisfy this. Is there maybe a requirement for the lateral rigidity of the platform that I can calculate so I can safely assume Hf = 1 for the storage rack?
Thank you so much!
I’m currently designing a storage rack system, installed above grade. The supporting structure is a non-building, 8-ft tall laterally braced steel platform. I have attached a sketch of what the system looks like, along with the relevant code section.
My understanding is that any component installed above grade will experience amplification of the seismic forces. This is addressed in ASCE7-22, section 13.3.1 - Seismic Demands on Nonstructural components.
My question is about the factor “Hf”, in section 13.3.1.1. “Hf” is the amplification with height. Hf is a function of the fundamental period Ta of the supporting structure and also, a function of the ratio z/h where:
z = Height above the base of the supporting structure to the point of attachment of the component.
h = Average roof height of supporting structure with respect to the base
Basically if the non-structural component (the storage rack) is supported at grade, Hf becomes 1 because the ratio z/h = 0. In my case, the structure is located on top of the platform so z/h = 1 and Hf is basically maximized – I’m getting Hf = 3.5, assuming the fundamental period of the supporting structure Ta = 0.1 seconds. I studied the variation of Hf vs Ta (see attachement), and found that the minimum value of Hf is something around 2, but that's for Ta = 10 seconds, which doesn't seem practical.
Are there any circumstances in which Hf = 1 can be justified, even if the storage rack is located on top of the platform? I would think that having a theoretical “infinitely rigid” platform, i.e a solid concrete block of height “h” would satisfy this. Is there maybe a requirement for the lateral rigidity of the platform that I can calculate so I can safely assume Hf = 1 for the storage rack?
Thank you so much!
