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# bridge posting

• To: "SEAINT SERVER (E-mail)" <seaint(--nospam--at)seaint.org>
• Subject: bridge posting
• From: Dave Handy <dhandy(--nospam--at)trg.ca>
• Date: Fri, 9 Oct 1998 12:11:13 -0400

```I don't know if US bridge codes are similar to the Ontario Highway Bridge Design Code, but I will pose the question anyway. (Maybe Thor will have some ideas?)

I have analyzed a bridge using the OHBDC truck and have a scale-down factor of 0.76. This represents the moment resistance - effects due to dead loads, divided by the effects due to live loads
Using the factor of 0.76 the table says rate the bridge for 18 Tonnes (~ 18 Tons). However the OHBDC truck is 38 Tonnes. One would think that the rating would be 0.76 X 38 = 29 Tonnes. They explain the reduction by reducing the rating to include for a load shift allowance and an overload allowance. This makes some sense.

However, If I design a bridge today and the beams are perfectly sized for the loads intended, and I check it tomorrow for posting requirements, I will have designed it for 38 Tonnes and have to post it for 25 Tonnes tommorrow. This clearly makes no sense. These allowances are not included for in the original design.

Any help would be greatly appreciated,

David Handy, P.Eng

```