Seismic Weight & Semi Trucks

[cal91]

I am designing a multi-story warehouse with multiple levels of loading docks. Semi trucks will drive up ramps onto the loading docks and load/unload into the warehouse.

The dead load of the ware house is 75 psf, Live load (Storage) is 300 psf.

We are designing the warehouse using a seismic weight which includes 25% of the storage load per ASCE 7-10:12.7.2

The dead load of the loading docks is 160 psf, live load (Semi trucks) is 250 psf.

For the seismic weight of the loading docks, would you include weight from the semi-trucks? ASCE7-10:12.7.2 allows for public garages and open parking structures to not be included, but this is a little different of a scenario. Also, the code only prescribes minimum loads, and engineering judgement should be applied.

My inclination is to include 25% of the semi truck live loads in the seismic weight.

 

[MotorCity]

I would account for the seismic load due to a truck on a ramp. Seems pretty reasonable to me that a truck could be parked on the ramp during a seismic event, especially if the ramps can only hold one truck at a time. If the ramp is large enough to accommodate several trucks parked side by side you can probably rationalize that not all dock doors will have a truck at the same time. Maybe take a look at AASHTO to see how they handle it.

 

[TehMightyEngineer]

Second looking as AASHTO, they've already done the hard work figuring out the lateral load vehicles apply on a structure in a seismic event (limited by wheel friction as I recall).

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[cal91]

I've searched in AASHTO , this is the only thing I can find that pertains to Seismic Weight.

C3.10.1 "Earthquake loads are given by the product of the
elastic seismic response coefficient Csm and the
equivalent weight of the superstructure. The equivalent
weight is a function of the actual weight and bridge
configuration and is automatically included in both the
single-mode and multimode methods of analysis
specified in Article 4.7.4. "

So it appears that bridges are not required to have any vehicle included in the seismic weight.

 

[cal91]

Found this in C4.7.4.3.2c

The weight should take into account structural
elements and other relevant loads including, but not
limited to, pier caps, abutments, columns, and footings.
Other loads, such as live loads, may be included.
Generally, the inertia effects of live loads are not
included in the analysis; however, the probability of a
large live load being on the bridge during an earthquake
should be considered when designing bridged with high
live-to-dead load ratios that are located in metropolitan
areas where traffic congestion is likely to occur.

So it seems like AASHTO simply leaves it up to your judgement... Without doing a full scale statistical analysis... I feel 25% is conservative. The load is transferred through friction, not every loading dock will have a truck on it (there's 150 of them total), and not every truck is going to be filled to max capacity at the same time the big one hits.

Second looking as AASHTO, they've already done the hard work figuring out the lateral load vehicles apply on a structure in a seismic event (limited by wheel friction as I recall).

Could you direct me where this might be? I haven't found it in my copy. Thanks

 

[Deker]

To me there are two key phrases in the AASHTO section that you quoted:

1. "Bridges with high live-to-dead load ratios" - I'm not extremely familiar with AASHTO, but I believe they allow you the option to design the structure to remain elastic during an earthquake. In these cases, you would definitely want to consider any inertial forces that can develop due to live load where they are significant relative to the weight of the structure.

2. "Located in metropolitan areas where traffic congestion is likely to occur" - Lots of stationary cars with a high coefficient of static friction and low center of gravity have more potential to generate inertial forces.

For private garages that are expected to be at capacity day and night, I treat the live load as a storage load and include 25% as seismic mass. This isn't strictly required by ASCE 7 (Link), but I think it's rationale considering the reasoning above. If you expect that trucks will be parked near the loading docks day and night then I think it's reasonable to design for the additional seismic mass. But if the trucks will only be driving in and out during business hours, I wouldn't worry about it.

 

[TehMightyEngineer]

Could you direct me where this might be? I haven't found it in my copy. Thanks

Well, you got me there; I could have sworn I saw it in there but I can't find it either.

The more I think about it the more I wonder how much the live load can actually participate in the seismic mass of the structure. Vehicle suspension would likely dampen much of their contribution or at least restrict it's participation by isolating it from the base.

Best video I could find of vehicles in a seismic event:

https://www.youtube.com/watch?v=mJ1JHn3yEh4

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/