Allowable Foot Bridge Deflection 1

[Atops]

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!

 

[BAretired]

What is the design Live Load? Is 4.5" deflection due to Live Load or Total Load? How does the bridge feel when walking over it?

BA

 

[bridgebuster]

To reiterate what BA asked: Is the 4.5" the total deflection or just live load? If it's just live load you're way over. What code does your contract require? if DEP doesn't specify a standard, AASHTO as amended by NYSDOT Standard Specifications for Highway Bridges would govern. (Since DEP is a NY city agency, they'll refer to City DOT specs, which are the SDOT specs). Since this sounds like it's strictly a ped bridge, you need to follow the AASHTO Guide specs for ped bridges. If you're using the Standard Spec version your LL deflection is limited to L/500 ; if you're using LRFD your LL deflection is limited to L/360. Also, AASHTO requires a vibration analysis.

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!
[santa2]

 

[Triangled]

Looks like your team designed to L/180, but is this for live load or total load? I'm guessing aince you're asking the question that the NY DEP didn't provide a design specification up front, which would be aurprising to me. I suggest you review the contract specifications. I haven't seen a live load deflection specification for less than L/360 and sometimes something more stringent is recommended for pedestrian traffic for comfort.
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.

 

[dhengr]

Look at it this way.... If you hooked this deflecting structure to a bellows, and then to the bldg. duct work, you could call it part of the HVAC system. The DEP would be off your back, and as an extra benefit, they could use it as a very dynamic footbridge. They could get extra LEED points for this, what with the use of human walking (I hesitate to use the phrase human trafficking) producing a portion of the bldg. energy needs. And, you wouldn’t be practicing outside your field then either.

 

[Atops]

Thanks guys, all good answers!

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 [santa2] & a sincere thanks for all the help!

 

[graybeach]

It sounds like the bridge is some sort of catwalk within an industrial facility. The AASHTO ped bridge specifications would not apply here. Perhaps the NYC Building Code has some deflection limits. I can put myself in the DEP engineer's shoes because I work for a public utility. First of all, I try to catch things like this early on in the design process. Best, as Triangled suggested, is to firm up the design criteria at the beginning of the project. That said, whether or not something has been accepted by my utility is not relevant if there is a design error. The EOR is still responsible. Regardless of any code, your bridge just seems too flexible and will probably bounce around a lot. Even if you can somehow show a code is met, the bounciness could cause union grievances and such.

 

[mijowe]

65' long 2' wide, how is that vibration check panning out?

 

[JStephen]

If you get too much deflection vertically, and the bridge is narrow, you may have too much deflection rotationally also, although that may not be indicated in the deflection criteria.

 

[structSU10]

A vibration check should be done so the workers arent thrown around on this thing. It sounds like its natural frequency is gonna be around 2 Hz, which is well below what is recommended for walking (AISC design guide 11 states anything under 3 Hz should be avoided due to 'rouge jumping'). When you get to long spans vibration becomes a problem. For something like this the bridge likely wants to be 2-3 times stiffer.

 

[Atops]

I checked, & it looks like the calculations used the ASSHTO LRFD specifications to determine the Required Deflection Limits, which the gangway does adhere to. Unfortunately, graybeach is right, this is more of a catwalk style gangway than an actual bridge of any sort. As such the ASSHTO Specs do not necessarily apply. I found the section on pedestrian bridges from the NY Building code, which only mentioned that they cannot be made of combustible material. Its stainless 316 material.

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,

 

[bridgebuster]

If the beams are >14" deep, you could camber for dead load. If not, perhaps you could stiffen them;build up the section, add vertical bracing; just a thought.

 

[BAretired]

How are lateral forces accommodated in a 65' long deck approximately 2' wide?

BA

 

[msquared48]

By placing the deck between two vertical I-beams mid-height, and treating the whole assembly as a horizontal "I" beam to span the 65 feet laterally.



Mike McCann
MMC Engineering

 

[chicopee]

Cambering the steel with heat COULD BE your solution. There are good articles on the internet.

 

[BAretired]

Mike, that could be done but it would require web members to form a horizontal truss and, in any case seems a bit shallow for a 65' span.

BA

 

[msquared48]

BA:

It does, but I thought I'd mention it anyway. Have seen it done before for shorter spans.

Mike McCann
MMC Engineering

 

[a2mfk]

For this application I like the truss design that uses the handrails and superstructure all as part of a truss, it is very efficient and usually takes care of deflection and vibration. Throw in some horizontal diagonals under the walkway like Mike said for lateral. Just walked on an aluminum gangway (to connect to a dock) at a State park last weekend that used this design, but my wife just did not seem to find it as interesting as I did...

 

[bridgebuster]

women just don't seem to appreciate anything important

 

[chicopee]

women love to have fun and I love having fun with them.