Wooden bridge beams, revisited

[LD1]

LD1, I have no idea why the tables show different values. I always went from the wood beam loading tables in Marks Handbook for Mechanical Engineers. It says that the loading in the tables are for uniform loading and to use half the value for a single load at mid-span. Other loading you have to do a detailed calculation. I think the 1/2 value comes directly from the stress formulas. The same relationship is shown for steel beams.

In any event, using a healthy factor of safety when using wood beams is always a good idea, especially if they are not gluelam.

If you look at the chart though....for most common spans like 8' to 16' the numbers are pretty darn close to 1/2.

But the real short or real long spans is where deviation occurs.

Which makes sense. Because long spans...even with steel, stress is rarely the limiting factor....rather deflection is. And on really short spans....deflection is hardly a factor. Rather local stress or deformation right at the supports.

But it's real tough to find any good charts for wet-service and treated lumber. And with the cost of lumber and posts vs some scrap yard steel beams.....it's usually cheaper to build with steel anyway.

But I'm still curious as to the conditions of this proposed bridge. Span and potential loading, etc.

 

[paulsharvey]

I was also going to point to Southern Pine Council for info, but someone beat me to it.

 

[paulsharvey]

No idea what size creek this is; but it might be worth looking at running double barrel 48" culverts? That's gonna cleanly span a 9 ft wide stream, without limiting flow; ans depending on what your allowed to do in the water (DEP/ACoE), you could probably choke down a 12 ft wide, 1 ft deep creek to easily flow in double 48".

Edit: might consider 38"×60" ERCP if you don't want the height of 48" pipe; and this also makes for a wider pipe crossing; double 38x60 ercp would span 11-12 ft wide (12" gap between, on upto 24" gap); and 38" high (actually OD is probably close to 44"; but 2.5" is going to be below the invert). Them figure 12" coverage; so top of embankment would be roughly 53.5" above your pipe invert/flowline. You would want either a headwall, soil cement bags, rip rap, or poured mitered end section, to avoid erosion in flood stage.

 

[paulsharvey]

Getting a bit crazier; people have used 8 ft diameter steel tanks as a culvert; if it's a ravine; all this depends on legal issues; ie navigatable water way or sovereign submerged lands, ect.