Wind Loading - AASHTO Pedestrian Bridge, AASHTO Sign and Luminaries, with AASHTO LRFD 8th, Oh my.

[MIKE_311]

where to start..

So the AASHTO Pedestrian Bridge Guide Specification (2009 w. 2015 int, LRFD) say to use the AASHTO Signs and Lunimaires (2002, ASD) guide spec to develop wind load pressure for pedestrian bridges. That worked until AASHTO 8th where they changed how wind load pressure were calculated and also changed load factors for Str III from 1.4 to 1.

How to handle this? Should the wind pressures be calculated using AASHTO Signs and then increased 40% when evaluating strength III or should we just just AASHTO 8 for the wind loading with current load factors. The former method produces high wind pressures, I wonder if AASHTO is realizing that the wind pressures were just excessive.

Compounding the issue is that this bridge (if you read my other post) is large, very, 400' feet long, 16' wide, with piers that will be 25' feet tall (15'-16') above water). An excessive wind pressure may be adding unnecessary size, reinforcement and also more piles. We thought about just checking the girders for the higher wind pressure in StrIII and just using AASHTO 8 for the substructure design, but what to do about the bearings? I really don't think we should be mixing codes and approaches.

Edit: for discussion:

AASHTO Signs with AASHTO 8 load factors, Pz = 52.9 psf
AASHTO Signs with AASHTO 7 load factors, Pz (x1.4) = 74.1 psf
AASHTO 8th, Pz = 51 psf
AASHTO 7th, Pz = 54 psf (includes 1.4 LF)

I'm going to go to the client with this and have them decide, but I need to formulate my position first.

Anyone have experience with this or can provide some guidance?

 

[BridgeSmith]

It's always a pain when they do that to us. Hopefully, I can make you feel somewhat better about your options by letting you know that in the 8th edition, they didn't change the wind loading as much as it seems. The load factors changed, but the wind pressure calculations also changed, so that the factored wind pressures for the various load combinations remain much as they were. In the 8th Ed. the Strength V and Service I wind speeds (80mph and 70mph, respectively) are now independent of the design wind speed for Strength III from the map (instead of essentially being a percentage of the design wind speed, which varied depending on your location). If your design wind speed under the older spec was the 100mph that covered most of the US, the wind pressures for the Strength V and Service I combinations are about the same.

That said, I recommend using the edition of the bridge design specification active at the time of publication of the sign spec. edition you're using.

I think you'd be justified using whichever version of the specs you choose, so long as the load factors and base wind pressure calculations are from the same version.

Edit: I noticed you were using the standard sign spec., so for that you would use the ASD provisions of AASHTO Standard bridge design spec. (17th Ed.)

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

 

[bridgebuster]

The Guide Spec commentary gives a good reason for using the Sign & Luminaire Manual (In C3.4 I saw AASHTO Signs but no reference to ASD 2002 version.)I interpret 3.4 as being for the superstructure only unless you have some sort of funky, piers. If you have piers that are similar to what would be on a highway bridge, I would use the LRFD spec for the substructure. If you could demonstrate that the superstructure isn't what AASHTO envisions perhaps the owner would go with using the LRFD spec. You're not precluded from using LRFD to design a ped bridge, although there are some minor inconsistencies in LRFD regarding ped bridges.

The last ped bridge I did was 20+ years ago, a 160' cable stay. The Guide Spec was only in ASD at the time. Wind requirements were different.

 

[TrussBridgeboy]

Sometimes with pedestrian bridges you have to use some engineering judgement. The ped. bridge guidespec was generally written to cover truss bridges which were light and narrow, like an overhead sign structure, compared to regular highway bridges. Lateral loads and fatigue from wind could easily control those designs in some instances. What you have seems much more like a regular girder bridge so the wind loads should be close to what you would expect for that type of design straight from the 8th ed. Bridgesmith's comments seem to me to be on the right track.