Vertical Acceleration and Live Load

[EngineerRam]

Hello,

In ASCE 7, a minimum of 25% of the floor live load in storage facilities shall be used for the effective seismic weight. However, vertical acceleration is only a function of the dead load. Does anyone know why this is? It is rational to me that if the live load contributes to the horizontal effect, it should contribute to the vertical effect (unless unrestrained, then it should only contribute to the downward vertical effect).

Thanks

 

[HTURKAK]

I have copy and pasted the commentary for Effective Seismic Weight.

C12.7.2 Effective Seismic Weight. During an earthquake, the structure accelerates laterally, and these accelerations of the
structural mass produce inertial forces. These inertial forces,accumulated over the height of the structure, produce the
seismic base shear.
When a building vibrates during an earthquake, only that portion of the mass or weight that is physically tied to the
structure needs to be considered as effective. Hence, live loads
(e.g., loose furniture, loose equipment, and human occupants)need not be included. However, certain types of live loads, such as storage loads, may develop inertial forces, particularly where they are densely packed.


And definition of Effective Seismic Weight;
12.7.2 Effective Seismic Weight. The effective seismic weight,W, of a structure shall include the dead load, as defined in Section 3.1, above the base and other loads above the base as listed below:
1. In areas used for storage, a minimum of 25% of the floor
live load shall be included.

My opinion is , at least 25% of the floor live load in storage facilities and rigidly connected mech. eq.etc. shall be added for the effective seismic weight calculation..










Don't underestimate a nail. A nail saves a horseshoe, a horseshoe saves a horse, a horse saves a commander, a commander saves an army, an army saves a whole country.. GENGHIS KHAN

 

[KootK]

It is rational to me that if the live load contributes to the horizontal effect, it should contribute to the vertical effect (unless unrestrained, then it should only contribute to the downward vertical effect).

I agree and it's an interesting point that I'd not thought of before. If anything, I would think a much greater percentage of the storage load would have to be included in the design for vertical seismic acceleration considering:

1) If the mass does not separate from the structure, then all of it is being accelerated to match the acceleration of the structure at all times.

2) If the mass does separate from the structure, then it probably pounds it on the way back down.

 

[JAE]

3) The effective seismic weight requirements in the code are minimums. The engineer is free to use discretion and increase the weight if the engineer deems it rational to do so based on the anticipated actual average storage load expected in the facility.

 

[human909]

I deal extensively with silos. The bulk storage material is a transient live load and absolutely needs to be treated as such for things as windload and uplift. However if you don't take it into account the bulk material for seismic load you are setting yourself up for trouble.


Also AS codes require the following combination factors for live load (Q):

Wi = ∑Gi + ∑ψcQi
ψc = earthquake-imposed action combination factor
= 0.6 for storage applications
= 0.3 for all other applications