Load sharing across nailed joint?

[Hatman]

I have a horizontal timber ledger that is nailed at 200mm [8"] centres along its centre line to a supporting timber structure. The ledger will need to support a substantial point load.

Am I right in thinking that if the ledger is stiff enough I can assume that the shear load will be shared across a number of the nails? If so how can I determine the load sharing [and the required ledger stiffness] to check that the joint is not overloaded?

Thanks [if it helps the design will be checked against the UK timber standards]

 

[msquared48]

I would not try to analyze the support of the point load this way. I would be more direct in the connection.

Depending on the size of the load and the size of the ledger, I would use lag screws rather than nails, into the studs that are within a 2 to 3' length of the ledger. If necessary, install more or larger studs in the area of the load to attach the lags to. If the lags in the wood ledger are insufficient, consider adding thru-bolts with a steel bearing angle under the wood ledger.

Mike McCann
MMC Engineering

 

[Lion06]

You could try treating the nails as spring supports, but it would be difficult to come up with the stiffness properties. I would just assume a simple support condition between the two attachments on either side of the point load.

 

[CJJS]

Typically, in wood, it is the horizontal shear that is the controlling factor. If your point load falls between two of the fasteners, the fasteners will not do much in helping resist horizontal shear stresses. So, you should design the support of the large point load connection more directly.

 

[RVSWA]

...to further complicate this calculation, there is always going to be some "slip" with regards to the mechanical fastener. You would need to anticipate this before you can define the spring in your reaction. For this reason and many others, a simpler and more "direct" connection or load transfer is often preferred. I occasionally have some rich clients but none are likely interested in jumping over this dollar to pick up a dime... If your condition is existing and requiring retrofit, then I might consider entertaining the argument to a degree, if it is new construction, add more hardware and save everyone some money. In my opinion, given a choice between spending the same money on engineering vs. hardware, hardware should usually win, at least in the end you have a stronger structure. You might find that after a little damage/deflection, the load will redistribute and the connection will work as you assumed, how much deflection can you tolerate, that may be the question to your answer.

 

[FSS]

I think I would need to know the magnitude of the load before trying to determine the feasibility.

 

[Hatman]

It was really some advice on the general design approach that I was after.... but if it helps - the ledger needs to withstand an accidental load of 4.3kN, the position of the load along the ledger is not fixed [will depend on where fall occurs!]. The ledger is currently a 38x89mm C16 timber, 1.9m long, nailed at 200mm centres.

Thanks for all the thoughts so far