Allowable vertical deflection of footbridge 1

[valleyboy]

Hello all.

I'm looking at the design of a footbridge. The form of the bridge is a steel through girder vierendeel truss.

I have been looking in BS 5400 for an indication of deflection limits under dead + imposed loads. There do not appear to be any such limits in BS 5400, but there is a statement that the deflection should not affect the servicability of the bridge, or affect the applied corrosion protection.

I wondered what type of limits other engineers employ on such structures? The deck is to be stiffened steel plate, and as such there are no brittle finishes.

VB

 

[dicksewerrat]

Many people are affected by very small deflections in floors, etc. Go with something more than minimum, L/480 or L/600.

 

[PEinc]

In AASHTO's Guide Specifications for Design of Pedestrian Bridges, 1997, Section 1.3.1 Deflections, it says, "Members should be designed so that the deflection due to the service pedestrian live load does not exceed 1/500 of the length of the span. The deflection of cantilever arms due to service pedestrian live load should be limited to 1/300 of the cantilever arm. The horizontal deflection due to lateral wind load shall not exceed 1/500 of the length of the span."

 

[miecz]

AASHTO's service live load for the deflection limits cited above is: w=85 psf*(0.25+(15/sqrt(A)), where A is the deck influence area in ft2. The lower limit is 65 psf.

Here in New England, the major supplier of pedestrian bridges limits vertical deflection of the main trusses due to pedestrian live load to 1/400 of the span, unless specified otherwise. Other members such as floorbeams and stringers are limited to 1/360 of the span.

 

[blake989]

I do not have any further input for deflection other than those listed above, but I will share some other factors I have learned about pedestrian bridges.

Dependent upon who will be using this bridge, general public or manufacter/construction workers, "structural aesthetics" will play an important role. For instance, if a 3/8" sag rod could support the whole bridge, place a 2" diameter pipe around the 3/8" sag rod.

Also keep an eye on vibration, a little extra dead load may be beneficial to dampen out foot traffice on a light structure.

 

[bridgebuster]

The AASHTO Guide Spec also requires checking the natural frequency of the bridge. There's an example in the appendix.

 

[valleyboy]

Thank you all for your comprehensive replies.

I'll limit deflection to L/500 under live loads.

I also take on board the comments regarding vibration. Here in the UK the Design Manual for Roads and Bridges specifies a minimum natural frequency of 5 Hz for vibration in the vertical plane, and 1.5 Hz in the horizontal plane.

Thanks again !!

VB

 

[miecz]

Anyone know the difference between a "Guide Specification" and a "Specification"?

 

[bridgebuster]

A "Guide Specification" is not legally binding. It is more of a "suggestion"; of course it's based on sound practice. To become a legally binding specifcation the issuing organization's members must officially adopt it; in this case the various states that make up AASHTO.