Replies

1. 
   sschefer | Jun 27, 2002 02:32am | #1

   Pike, I don't have the formulas but can relate to your needs. I helped build a bridge a few years back. It was on private rural county property but we still needed to have it designed by a certified bridge builder. We were all suprised to find one as easily as we did and the cost was very reasonable. This particular bridge was the primary access to the main home on the property. It had to be certified for the heaviest vehicle that might need to pass over it. These vehicles included fire trucks, line trucks, ambulances, etc. The speed at which they would travel over the bridge was also part of the calculations.

   Just some food for thought if you truly want this to be a safe bridge.

   Steve - in Northern California

2. 
   BenM | Jun 27, 2002 07:53pm | #2

   Providing a few formulas isn’t going to help you very much. Unfortunately, the answer to your question is not simple at all. Additional information is needed, such as:

   1. Materials to be used. (Wood?, Species?, Grade?).

   2. Span of deck (stringer spacing).

   3. Number of lanes.

   4. Number of stringers. (determined from items 2 and 3).

   5. Design vehicle (number of axles, load per axle, axle spacing). How did you come up with 500 ft-k? What is the heaviest vehicle that could use the bridge (either on purpose or accidently)? For a 40 foot span vehicle-loading will likely govern over lane-loading. If you do use lane loading you will want to account for concentrated loading as well.

   6. Other loads to consider: seismic, longitudinal, wind, impact.

   7. Geometric constraints (for example, do you have a minimum vertical clearance requirement under this bridge?) If the bridge is to cross a stream, what is the maximum height of water during periods of flooding? Would that affect the substructure?

   8. Does the bridge have to be designed/built to a specific code?

   9. Parapets, curbs, bearings, substructure?

   10. Drainage.

   11. Any utilities to be carried by the bridge?

   12. Subsurface conditions?

   That will get you the basic structural sizes. Then consider deflections, camber, cross-bracing, fasteners, maintenance, etc. and you’re done.

   The best advice I can give you is to discuss your project with a structural engineer.