Lateral Loads for Large Live/Dead Load Ratio

[theonlynamenottaken]

I've designed several "roof-top deck" structures for 100 year old, brick town-square buildings recently. The structures are stand alone (from the brick building structure), with columns extending down to new foundation. BUT, they usually have very large live/dead load ratios and they have absolutely no wind area. What do you use as your lateral load?

Seismic is not the lateral load I most worry about. It is the group of people "swaying" forces I'm pondering. You know, a bunch of people dancing together - which is a very real possibility for roof-top assembly areas at restaurants (wedding receptions).

What I have been using for lateral load is a percentage of the live load. Of course, this isn't identified in codes or literature anywhere. Using ASCE 7 I can easily determine the seismic response coefficients and base shears based off of structure dead loads, but this doesn't seem to be enough load. Also, I worry about service drifts - i.e., excessive drift just from people shifting their weight from side to side. ASCE provides "people sway" lateral forces for stadium seating/bleachers, but those aren't directly applicable...

Sure, I can set a service drift limit - say H/400, 500, etc. But what load do you use to evaluate that??

Any thoughts???

 

[Lion06]

Include some portion of your LL in your mass for determining your seismic load.

 

[theonlynamenottaken]

That seems like an "allowable stress" approach to this dilemma. Arbitrarily selecting a portion of LL to include in seismic calcs to determine a non-seismic loading. And, how much is "some portion"?

 

[theonlynamenottaken]

StructuralEIT - Let me clarify. I do include a portion (usually 50%) of the LL in the seismic DL. But, in this part of the country where Cs is usually less than 10% I fear that this increased seismic load is still not as large as the "people induced" lateral load would be....

 

[prsconsultant]

Are you concerned about the vibration of the roof system? If the roof is accessible, I guess your roof deck has a concrete fill.

I don't think a bunch people partying on a roof top would excite the struture to be concerned about the lateral effects.

 

[JAE]

What about applying a code wind load to the "area" of the group of people on the deck? i.e. human sails.

Otherwise, I would suggest doing what you are doing and that is to take a percentage of the dead load and live load as a lateral force.

In low seismic areas (SDC A) you get 0.01 x W for the required lateral seismic demand on the structure.

In my city, the code has been amended to use 0.02W.

Perhaps you could estimate a lateral surge force from a group of people all running to one side of the floor and stopping at once. This might be somewhat analogous to the 20% lateral force required by AISC for crane side-loads.

Just some thoughts.

 

[Galambos]

even better, how 'bout applying the stadium sway loading from the IBC live load section?

 

[slickdeals]

I second Galambos opinion of using stadium bleacher sway loads. We have done this in the past. Atleast you are following a recommendation from the code, in case anyone questions it.