Follow up to CSA S6 Bridge Deflection Criteria Thread

[pbc825]

Hello all,

I previously made this post

http://www.eng-tips.com/viewthread.cfm?qid=426751

It was related to what the deflection limits on a single span bridge girder were in accordance with CSA S6. I've since done some calculations and have a follow-up question regarding CSA S6 deflection limits. If I follow along with all the clauses, I believe I understand that the load on the bridge to be considered for deflection limits in Figure 3.4.4 is to be 90% of the CL-W wheel load multiplied by the dynamic load allowance (DLA) which is 1.25 for considering three or more rear axles in the CL-W distribution. Dead load is set to 0 (of course).

Can a Canadian bridge engineering weigh in on this and confirm I'm doing this correctly? I'm specifically interested in whether the DLA is applicable to the load scenario. It feels like the deflection limits are overly onerous. For example I have a 60' bridge and to meet ULS criteria, the girder size is W24x68. Considering DLA and the deflection limits as I understand them, the appropriate girder is W36x150 (121% heavier and ~400% stiffer). Looking at it from a structural steel perspective, the deflection limit is L/2300. Yikes.

Thank you in advance for your response.

 

[cooperDBM]

Yes, DLA is applicable. The bouncing of the truck will add to what a pedestrian feels in vibration. To clarify your procedure, are you distributing the load effect (deflection) using an exterior girder live load distribution (5.6.6)? You can account for the extra stiffness of the sidewalk and concrete barrier if they are integral for a vibration check. What sort of first flexural frequency are you getting?

 

[pbc825]

Thank you cooperDBM

This is a special sort of temporary steel bridge. No sidewalks. Steel plate deck connected via members perpendicular to the direction of travel. The deck is constructed such that there is no appreciable difference in inside girder and outside girder deflection.

Given there's no pedestrians, would the deflection criteria be based on comfort of the drivers?

As for the frequency, I've calculated 4.7Hz via a FEA method and 4.4Hz via a hand-calculation method. Both have a reasonably accurate mass and stiffness distribution. The mass of the vehicle is not considered in the modal analysis. In the W36x150 model, FEA method and hand calcs return 8.1Hz.

I'm using an older version of S6 (2000) which does not have a 5.6.6. My printed copy of S6-14 should be in the mail soon.

 

[cooperDBM]

A limit on vibration wouldn't just be for pedestrians or drivers. A stiffer bridge will provide more dampening, reducing the DLA effects on the structure, and vehicles I suppose. The code gives a different limit for no sidewalks/pedestrians. In any case these limits are intended for permanent roadway structures. Because your structure is temporary you may be able to justify a relaxed limit, particularly if speeds are controlled, though I don't have any guidelines. I believe the code assumes highway speeds of 120 km/hr. You may also be able to reduce the DLA for lower speeds.

Your frequencies are on the lively end, particularly 8 Hz. The code is aimed at heavier concrete decked bridges with much lower frequencies. Low enough not to resonate with truck suspensions (I'm not an expert here).

S6-00 has an equivalent simplified distribution clauses though again they're for concrete decked bridges which isn't applicable in your case. I assume that you've figured out a way of sharing the truck weight among the girders.