×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Allowable vertical deflection of footbridge

Allowable vertical deflection of footbridge

Allowable vertical deflection of footbridge

(OP)
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

RE: Allowable vertical deflection of footbridge

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

RE: Allowable vertical deflection of footbridge

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."

RE: Allowable vertical deflection of footbridge

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.

RE: Allowable vertical deflection of footbridge

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.

RE: Allowable vertical deflection of footbridge

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

RE: Allowable vertical deflection of footbridge

(OP)
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

RE: Allowable vertical deflection of footbridge

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

RE: Allowable vertical deflection of footbridge

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources