Subject: Connection Pins in Old Steel Truss Bridges
From: Steven Mallett <smallett(--nospam--at)dillon.ca>
Date: Wed, 07 Feb 2001 10:30:55 -0500
I recently worked on a detailed structural evaluation of an existing steel
through truss bridge built in 1904 with a span of 160 feet. While all of the
members were determined to be sufficient to carry the specified design
truck load, the connection pins were found to be over stressed in
bending. These pins connect the bottom chord, vertical, and diagonal
members. A U-bolt hanger supports the deck floor beam from the center
of the pin. The steel pins are 3 inch diameter and approximately 14
inches long with yield strength of 30 ksi.
In the bending analysis, I assumed applied point loads by the members
(maximum member thickness is 1 inch). Using distributed loads did little to
reduce the bending moment. The pin bending capacity is as low as 40%
of the applied bending moment (i.e. Live Load Rating Factor of 0.4, as per
Canadian CSA S6 bridge code, Clause 12 evaluation procedures). Shear
capacity is no problem.
Now that the lengthy intro is complete...to the meat of the message.
Obviously, these pins have worked well for almost 100 years under the
design loads. This bridge does not have posted loads and is exposed to
normal truck traffic. Can someone suggest other analysis techniques or
past research on the subject of bridge pins in bending that would explain
this phenomena? i.e. acts like a short deep beam, etc.
Steve Mallett, P.Eng.
Dillon Consulting Ltd.
Halifax, Nova Scotia