# How much weight can my bridge support

I’m trying to determine if my bridge will support a 1700 pound stump grinder. The bridge span across the river is 24’ and has 3-2x12’s on 21” centers, as the joists. One on each side and one in the middle. The planks on top are 2x6. The 2 outer joists are supported on each end with 4x4’s. I’m not sure the bridge can support all that weight.

• To be clear, is it about 40” wide? Commented Oct 2, 2023 at 2:15
• 42” from centerline of each of the outer joists. Then add a a few more inches for the overhang of the 2x6 deck planks. Commented Oct 2, 2023 at 2:28
• Pictures or diagram please Commented Oct 2, 2023 at 5:35
• Lumber species and grade? If you don't know, then what US state are you located in? Commented Oct 2, 2023 at 5:37
• Any chance of erecting a temporary support mid-span in the river?
– MTA
Commented Oct 2, 2023 at 11:51

The deck isn't stiff enough to spread loads between the 3 members, so the strength of a single 2X12 will provide your "yes" or "no." A 2X12 is very deep and requires some special detailing to avoid strength reduction because of a phenomenon called "lateral torsional buckling." I'm suspicious that your bridge hasn't implemented this detailing, but I'm also suspicious that it will be inadequate regardless.

Assuming #2 grade Spruce-Pine-Fir (which is crappy wood that they get stuck with in Wisconsin, for instance) and assuming proper stability detailing, the adjusted bending strength of a 24'-0" span under the NDS's LRFD branch is 3420 ft-lb. I've used a load duration factor of 1.0 which corresponds to loads for 1 week to 1 month long. I suspect that I could have used 1.1 because this load would be on there for less than a week. Using 1.1 instead of 1.0 would have added another 340 ft-lb of capacity.

Putting 1700# at the midspan of that thing and applying the LRFD's 1.6 factor for live loads, I get a live load demand of 16320 ft-lb. Without even bothering with the weight of the bridge itself, this is 480% of the bridge's capacity.

For a span of this slenderness (24'/1' = 24), deflection instead of bending tends to be the active constraint (my intuition is that slenderness larger than 20 corresponds to deflection-constrained designs). For a one time loading like this, though, I could be persuaded to exceed the bridge's deflection limit.

Even under the relaxed deflection constraint and ignoring the bridge's own weight, however, the stump grinder fails that 24'-0" beam in bending at 480% the beam's capacity.

• Might it help to spread the load by laying down heavy mats on the deck? Say 4inx8inx10ft boards starting with one overlapping solid ground substantially? The idea similar to how they drive excavators over swampy ground. Spread over dozens of the 2x floor planks. Commented Oct 2, 2023 at 10:15
• What this doesn't take into consideration (and the OP hasn't asked about either) is the weight of the vehicle towing said stump grinder across the bridge. Unless it's going to be pulled across the bridge by a come-along or winch and the grinder is the only thing on the bridge, the weight is going to be even higher than this calculation. NOTE: This simply adds fuel to the unequivocal NO answer. Commented Oct 2, 2023 at 11:51
• @jay613, if I dumb-lucked into a good wheel spacing, then I would attach some sort of guides to hard constrain the machine's path. 28" center to center on two wheels is perfectly lucky in this case, I think. That sounds possible, so I wouldn't be so eager to explore your option without eliminating dumb luck first. Commented Oct 2, 2023 at 19:05
• @popham yes! If the axle is very different to ideal width he could build a cart to adapt it :) But my suggestion is in addition to that, to spread the weight along the bridge, across a greater number of floor planks and to never have the entire weight all at mid-span. Commented Oct 2, 2023 at 20:33