Allowable Foot Bridge Deflection
Allowable Foot Bridge Deflection
(OP)
Hello Everyone,
My company recently designed and built a stainless Footbridge/gangway for the NY Department of Environmental Protection. We have Signed & Stamped calculations showing that over a 65' span the max deflection is 4.25". This was originally approved by the project engineer at the DEP, except now they are saying that the deflection is too much. Is there a bridge standard that I can show them that this deflection is within acceptable parameters? Preferably with another NY agency or the state. I haven't found anything as of yet, & this bridge seems a bit small to apply ASSHTO standards.
Thanks!






RE: Allowable Foot Bridge Deflection
BA
RE: Allowable Foot Bridge Deflection
Back to my first question: if it's DL+LL you might be able to camber the beams for DL, assuming the beam isn't too shallow.
No offense but why would you assume a 65' bridge is too small for AASHTO? It exists for a reason.
Merry Christmas!
RE: Allowable Foot Bridge Deflection
The next question is the live load your team utilized in the design. I will (and I learned this from others, which, come to think of it, applies to just about everything I "know") very often design for a greater live load than the minimum required for structures where pedestrians might congregate unexpectedly such as a balcony or pedestrian bridge to watch fireworks or whatever might be interesting. If your design is for double the code minimum live load, and the L/180 is for live load, then you'll find your design at L/360 for the code minimum live load. But I'm guessing that since the "project engineer at the DEP" didn't raise the issue on review that no one questioned it until it was built and installed and evaluated as "too much" ... and I sincerely hope I'm wrong.
And then there are the additional observations noted above.
RE: Allowable Foot Bridge Deflection
RE: Allowable Foot Bridge Deflection
I do feel I need to clarify, we did originally figure the deflection to be MUCH smaller, about less than 1.5", and chose a proper channel section based upon that for a 54'-0" bridge length. Turns out they gave us the wrong length! But luckily we were able to add another channel to each side to stiffen it up. The Live Load is L/360, & the Total Load is L/180, so that's where the PE calculated the 2.125" LL & 4.25" TL deflections. Also, this is strictly an industrial bridge, so there is no public access. Because of that the DEP didn't include any sort of spec, as far as I have been told.
@bridgebuster, thank you very much for pointing out the ASSHTO Spec. I had a feeling it might be, but as we do not typically build bridges I'm not familiar with the ASSHTO specs. I wasn't sure if the spec even applied to this, because its described as a 65' long gangway with a 24" clear path. Not even described as a bridge.
@Triangled, We are subcontracted for the job, not straight to the DEP. The info we were given was to supply a stainless gangway to span the gap with Signed and Stamped Drawings & Calculations, which we did our due diligence and supplied. They were returned to us approved, and there was deemed no problem until the GC tried to install the gangway and found it to be 10' too short! Luckily this is for DEP workers, not the public, so it will never get the heavy traffic a public footbridge in the city would receive but still its causing everyone here to shake their heads.
@dhengr, Haha, I wish I had thought of that originally! Let me tell you, this project has certainly been a learning experience!
Merry Christmas Everyone
RE: Allowable Foot Bridge Deflection
RE: Allowable Foot Bridge Deflection
RE: Allowable Foot Bridge Deflection
RE: Allowable Foot Bridge Deflection
RE: Allowable Foot Bridge Deflection
Triangled, I know it must be surprising that the DEP didn't give any specifications on the gangway, but its true. We received far more spec's on the Handrail (OSHA standards of course) than we ever did for the gangway platform. Remember, they had screwed up the length of this gangway already which is why we're in this situation.
Before adding the additional 10' & the additional channel to each side to stiffen it up, the deflection was MUCH smaller. Luckily, our PE performed vibration & rotational deflection checks, & added a lot of cross bracing to prevent that, but this added to the weight of the gangway.
I'm going to explain to the Project Engineer at the DEP that we used the ASSHTO LRFD spec when doing the calcs, & see what he says. Graybeach is probably right, they're most likely cautious because of union grievances, which I can understand. Its just that they literally only specified the OSHA Handrail spec's, & did not provide anything else when we asked for clarification.
Thanks guys,
RE: Allowable Foot Bridge Deflection
RE: Allowable Foot Bridge Deflection
BA
RE: Allowable Foot Bridge Deflection
Mike McCann
MMC Engineering
RE: Allowable Foot Bridge Deflection
RE: Allowable Foot Bridge Deflection
BA
RE: Allowable Foot Bridge Deflection
It does, but I thought I'd mention it anyway. Have seen it done before for shorter spans.
Mike McCann
MMC Engineering
RE: Allowable Foot Bridge Deflection
RE: Allowable Foot Bridge Deflection
RE: Allowable Foot Bridge Deflection
RE: Allowable Foot Bridge Deflection
I have also seen the handrail used as a truss element to stiffen the walkway..
However, I caution this approach for any walkway that has people walking side by side. Not sure if you saw the footage of the footbridge that collapsed (I think in China) that was overloaded. When I watched the video... it appears that the top chord of the truss buckled outward, and it had a sudden failure. Possibly from the combination of vertical load plus outward hand rail load (albeit not 200#, but something significant).
So, my point being, make sure your handrail has very little deflection from out of plane loads if you're using it as a compression member in a truss!!
Buckling = BAD.
RE: Allowable Foot Bridge Deflection
You know, I've been resisting the impulse for four days now, and feel I've done pretty good, but I have to capitulate here... The answer to the OP's question lies in the name of the bridge. It's a foot bridge, so the maximum deflection allowed due for the load of a foot, is a foot. It's just that simple... It's also Friday.
Mike McCann
MMC Engineering
RE: Allowable Foot Bridge Deflection
RE: Allowable Foot Bridge Deflection