Recycled Pedestrian Bridge 1

[phamENG]

I'm analyzing a maintenance catwalk (designed and fabricated by never installed) for use as a small pedestrian bridge. Span will be about 30' to cross a drainage ditch between the parking lot and industrial facility. I've run the analysis and it doesn't quite work considering the 90psf load from the AASHTO pedestrian bridge specs. This thing is only 3 feet wide - no passing. It's also not in an area that would be subject to crowd loading. 90psf seems excessive to me in this instance. If you assume 250# workers walking in a single file line on the bridge with 3ft per person, you have 250#/9ft2 = 27.778psf. I wouldn't drop it this low, but given the relative predictability of the usage here what are the general thoughts about reducing to 40psf?

Vibrations is another issue. In this case, I'm inclined to give a warning that it is likely to vibrate - either spend the money to fabricate a new structure or put up with vibrations. The girders are W12x14's. They are thoroughly braced and strength is no issue, but it's flexible. A quick rundown of the DG11 checks for approximate natural frequency and acceleration puts my minimum Ix at about 1110in4 to be within the suggested limits for an outdoor pedestrian bridge.

Thanks.

 

[phamENG]

Climate is mild, minimal snow throughout the year, and the only ice to consider is a slip hazard. Deck surface is serrated bar grating.

BAretired, that's precisely what I did. I've informed the facility managers what load it can carry within acceptable deflection limits, and warned about possible vibration issues. I also informed that what my targets would be for a new structure. They can decide to go with it, or I'll design a new one.

I appreciate everyone's input.

 

[glass99]

AASHTO would only apply if it was over a highway. It sounds like an ASCE07 "Walkway and elevated platform" which is 60psf.

 

[phamENG]

glass99: Thanks for the input, but I don't think I agree with you about the applicability of AASHTO (and more specifically in this case, the AASHTO Guide Specifications for the Design of Pedestrian Bridges). While it is published by transportation engineers and sponsored by highway officials, it has broad applicability. I've designed pedestrian bridges to that standard in places that were nowhere near a road. Application of environmental loading and the particulars of live loading on a bridge structure are more thoroughly and accurately covered in AASHTO than in the ASCE 7, which focuses primarily on buildings.

My question wasn't so much about the applicability of the standard, but about the rationale of the live loading requirements and others' opinions on the engineering judgement I was proposing to use.

 

[MotorCity]

Deflection and vibration are serviceability criteria. If the owner wishes to reuse a bridge and it is structurally sound, they may have to forfeit user comfort and live with noticeable deflection and/or vibration to save a penny. As for the minimum design load, I'm in the 60 psf camp.

 

[phamENG]

Thanks, MotorCity. The general consensus seems to be converging on that.

 

[BridgeSmith]

I like Ron247's idea of suggesting to the owner to post a 'maximum occupancy' for the bridge. If 60psf works, that's 18 300lb guys.

I haven't looked at the guide spec for pedestrian bridges, but in the AASHTO bridge design spec, the loading for deflection is different than for design. Are you sure the loading for the deflection check is 90psf?

Rod Smith, P.E., The artist formerly known as HotRod10

 

[glass99]

phamENG: Of course you might choose to go beyond code requirements and worry about vibrations and tighter deflections in AASHTO, but generally you only have to meet IBC + ASCE7 + AISC. Not only that but responsible engineers do so all the time. Every bridge ever built inside a building for example.

 

[phamENG]

BridgeSmith: 90psf is the service level loading (factor = 1.0). Its increased for strength (1.3, 1.75, etc.) in the Strength LCs.

glass99: I'd say it comes down to the AHJ, as many have mentioned here. The pedestrian bridges I've designed (prior to this one) have been under the authority of a municipality and, regardless of location, have insisted on AASHTO specs. It's a requirement that I agree with as it was written for independent bridge structures. I agree with you that a bridge like structure within a building would not be designed to AASHTO. It is part of a building structure and not subjected to wind or other environmental loading. It would fall squarely under the IBC as you say. I would still check vibrations per AISC DG11, though, which has more stringent recommendations for interior pedestrian bridges than it does for exterior. I would also be hesitant to use the 60psf in that situation, though, as I can think of few times where it couldn't be called a corridor if it's accessible to the public.

 

[BridgeSmith]

To add to glass99's comments, guide specifications are not legal code requirements, unless adopted by the AHJ.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[glass99]

phamENG: Where are you building this? Which jurisdictions in the past have required AASHTO? Its not a reference standard in either NYC or California. (I am partly asking bc we have a footbridge project coming up in NYC)

 

[phamENG]

This particular one is in NC, but the ones I've done in the past were in Virginia. The municipality stipulated the AASHTO Guide Spec as the minimum standard in the RFP and design contract.