×
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

Wood vs Steel Deflection Criteria

Wood vs Steel Deflection Criteria

Wood vs Steel Deflection Criteria

(OP)
In a bridge design manual developed to guide designers on the design of trail bridges (hiking, biking and snowmobiles) they have two deflection criteria.  For wooden bridges 1/200 for steel bridges 1/500, yet the loading criteria are the same for both types.  These bridges do not carry highway traffic or emergency vehicles.  The maximum load is self weight, 30 psf over the whole deck + 10,000 lbs so, they really don't follow AASTHO guidlines either.

Can anyone provide the logic behind this?  Why not the same criteria for both?  Why should the steel bridge criteria be more restrictive than the wood bridge?  

RE: Wood vs Steel Deflection Criteria

The American Institute of Timber Construction (AITC) recommends live load deflection criteria of L/300 for timber bridges. AASHTO recommends L/500; other criteria are stated to be acceptable. Considering traffic volume, speed limits, relative deflections of components in the bridge structure, and field experience, a deflection limit of L/300 is generally sufficient for timber bridges.
The logic behind this approach is primary very low Young Modulus for timber - so deflection for a timber bridge is usually large. Limiting the deflection to L/500 will lead to oversized members, as the displacements will govern the design.
Also, due to the excellent vibration-damping characteristic of the timber structures, larger deflection will be not so uncomfortable to the users.
It should be added that the deflection criteria is mostly a "comfort" requirement in the design process, and large deflection does not necessarily mean that the structure is unsafe.

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