Estimating lateral bracing requirements

[robartsd]

I've been asked to design the set for a production at my church. One of the set features is a platform 5-6 feet off the deck. This platform runs the width of the stage and I would like to keep the space under it as open as possible. For the vertical loads, I plan to use stressed skin panels with wals running upstage-downstage. If I can get away with it, I'd like to have the cross stage bracing entirely in the wings (intregal with the off-stage stairs), but some cross bracing on stage could be worked into the set design. I have a BS in structural engineering; but I have no useful reference as to how much bracing is required for such an application. I plan to use 40 psf for vertical design load - just supports characters who are standing, siting, or walking - no dancing or other highly dynamic loading. My structural engineering education taught me to design lateral systems to resist wind or seismic forces; however, as this structure is indoors and will not be fastened to the deck seismic and wind loading does not apply.

Here are some ideas I've come up with so far in brainstorming possible approaches:

• Estimate the availible lateral load transfer to the deck due to friction and design bracing to resist at least that much force.
• Estimate the forces based on vertical support geometry being some amount out of plumb (several times a design allowance for deflection and construction tolerances).

Any ideas on how to determine bracing requirements would be apreciated. I am capable of designing the bracing systems, but I don't know the magnitude of the forces to resist.

 

[BillConnerFASTC]

I've used 20% of live load for a lateral or "sway" load. From NFPA 5000:

35.6.2.3.2 Footboards in reviewing stands, grandstands, and
bleachers shall be designed to resist 120 lb/linear ft (180 kg/
linear m).
35.6.2.3.3 Reviewing stands, grandstands, bleachers, and supporting
structures shall meet the requirements of 35.6.2.3.3.1
and 35.6.2.3.3.2.
35.6.2.3.3.1 Reviewing stands, grandstands, bleachers, and
supporting structures shall be designed to resist a horizontal
swaying force applied to the seats as follows:
(1) In a direction parallel to the length of the seats, 24 lb/
linear ft (36 kg/linear m) of seats
(2) In a direction perpendicular to the length of the seats,
10 lb/linear ft (15 kg/linear m) of seats
35.6.2.3.3.2 Sway loads shall not be required to be applied in
both directions simultaneously.

I've seen this referenced elsewhere but not finding it now. Our portable platform spec requires:

2. The staging system shall be subject to side loads in both direction axes plus or minus 20%
of the average vertical load described in item 1. This shall also be cycled in phase through
10,000 cycles.

Though that assumes 125 psf design live load, so maybe 20% is not enough for a much lower live load.

 

[kicknargel]

40 psf seems low. Remember to design for possible loading scenarios rather than intended ones. Might the director decide there should be dancing, or might people be messing around during rehearsal, or might other people have access to the space?

 

[Footer]

40 PSF??? Really? Also, if your not securing it to the deck, you are going to need to do a lot more bracing. Truthfully, I would brace every 8' at the minimum. You could always do it with steel for a cleaner look. 3/4" Ply stress skinned decks and 2x4 studwalls will end up with a much higher rating the 40PSF anyway. For the horizontal wall, figure 15 people running and stopping. NEVER design for the best case scenario. Always design for ladders and all of your cast to be up there. I guess you could design anchors on each side of stage that could take the lateral load, but without screwing into the deck that is not really good in my book. If the whole thing slides an inch you could lose it.

 

[BillConnerFASTC]

While you can't anchor it to the floor, any chance you can connect the vertical supports and stairs at floor level? A continuous run of ply or maybe a couple of flat steel bars? The "triangles" at each end should keep it upright and rigid.

 

[robartsd]

Yes, I was planning on using a horizontal support at floor level (most likely plywood or OSB with cleats for the studwalls). The whole thing slides and inch would mean the whole thing slides an inch. Both the platform level and floor level should be designed to transfer the entire latereal force to the bracing in any direction.

Bill, thanks for some code refferences.

Footer makes a good point about using the force of accellerating/decellerating people to estimate the latteral load. With average accelleation of sprinters (at the start of a race) around 3.5 m/s^2 and the accelleration of gravity at 9.8 m/s^2, it is unlikely that a live load consisting only of humans could generate a latteral force greater than 40% of their weight.

 

[BillConnerFASTC]

The 20% is in fact based on crowd movement. Unlikely that platform is fully loaded - a person every 4-5 sq feet - and all start off fast in same direction at same instant. The very essence of load diversity.

 

[robartsd]

In structural engineering we are sometimes allowed to reduce live loads based on tributary area and occupancy type because the full design load is not expected to occur over the entire area at once. For gravity loads, no element in my system would have enough tributary area to quailify, but the tributary area for lateral bracing in the cross stage direction might. Based on our discussion here, I am confident that the design lateral load should be between 20% and 40% of the design gravity live load. Thanks for your help.