Wooden Bridge Load Limit Determination

That look's like a dream house.

Bridge conditions tells that it is good enough for another decade. By the way how deep is Creek? Trying some FMEA.

Hi Rakesh,

Generally only 200-300mm, however, canbe upto 1m during winter flood conditions.

Regards, Neal

With the bridge being only 6m long, there is only likely to be a single axle set on the bridge at any particular time. Your "local" experts (neighbours) would be the first point of advice for such a venture as they would be able to tell you the history of what other loads have already passed over the bridge. If there's ever been a fully loaded cement mixer across it then you have one of the heaviest localised loads that you are likely to see.

Also have a look at the council bridges that are on the road. You should be able to verify their relative dimension compared to your bridge.

There is also more to the problem than just the bridge. If the raod comes down onto the bridge from one bank and then rises again to the other bank, the there will also be downward momentum when your trucks move onto the bridge.

If you're really worried about the load rating, then borrow some 6m long steel beams from the local yard and lay them along the bridge so that both ends are on the abbutment to take the significant load.

I once borrowed a semi trailer (just the trailer) and used that as a temporary bridge across a creek similar to what you've described.

I'd put in some blocking of the same size as the main bearers in between the main bearers every 2m to keep them from twisting over time.

I have load tables somewhere but i don't want to dig them out. IMO 50mm planking over a 1.3m span is inadequate for serious weight. Is it a full 50mm or somewhat less like a 2 x 4 in the US is actually 1.5 x 3.5?

I'd run 200mm x 50mm in between the cross bearers to reduce the span of the decking and then I would consider driving my car across.

Cement trucks crossing on 50 mm planks spanning 1.3m, I don't think so. I wouldn't be the first truck across that's for sure.

We are assuming that your timbers are free from knots or other defects. If not that would seriously jeopardise your venture.

A fully laden 6m3 concrete truck weighs about 20 000kg - 25 000kg.

Rear wheel weight about 6250kg (rear wheels take more than front wheels).

Given that you have 4 bearers that span and that the load will most likely not be shared by them (unless you can show otherwise), only one beam at a time bears the weight.

Maximum moment (non-factored) 6250 x 6 / 4 = 9375kgm (93.75kNm)

Capacity of a 300 dp x 150 wd beam is about 25kNm.

I think you might be miles off. Even if two beams share the load.

For normal loads such as light lorries, it might be ok.

If you like your bridge, it is probably best to put a temporary bridge next to it.

I would shore up the bridge temporarily from the underside with timbers or steel beams.

Good morning SureSpek. Without knowing the allowable bending stress for that particular material, and the personal inspection of the beams, and other details, a precise answer is elusive.

However, using a factored bending stress of 1500 psi derived from Douglas Fir #2, assuming only two of the beams carry the point load at the center of the bridge, and ignoring any benefit of the decking, initial arithmetic gives a figure of 7200# for a single axle point load at the center of the bridge. If another point load (axle) would also be present this would of course greatly affect the number.

Hi, Many thnaks for the insight. I have managed to get some detailson the timber as follows:-

The timber is dry, matured by 20 years plus. Routine delivery type trucks of 10 tonne+ have routinely traverssed the bridge since I have been here but ideally would like 20 tonne.

Regards

SureSpek

First off, I have no background as a structural engineer. That said, I look at it this way. Unless a great deal of study by a licensed engineer is done, you are risking lives and liability on something that you THINK will work.

I suggest that if your project requires any excavation, place a couple of culvert pipes in the stream and cover them with compacted fill from your excavation. Top it off with sufficient planking, first layer crosswise and second layer longitudinally, and you should be able to drive a 40 ton cement truck across with greatly reduced risk.

When you are finished, dig up the fill and use it for landscaping, sell the planking and culvert pipe if desired and you will be left with a quaint wooden bridge that hasn't been compromised. A small price compared to injury or worse.