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Lag bolt spacing in wood connections

Lag bolt spacing in wood connections

Lag bolt spacing in wood connections

I am designing strengthening for some existing wood trusses. The truss chords and web members are framed out of dimensional lumber (2X6 and 2X8 sizes).  The trusses were built many years ago.  As part of our work, my office visited the building and found that certain portions ( some of the bottom chords and web members) of the trusses were damaged by a fire a long time ago.  We are going to replace the damaged areas.

Analysis of the trusses under modern code loads indicates that new member sizes can be similar to the existing sizes, but connections are a problem.  The original connections have the 2x members lapped, with 12d nails in single shear. New connections are going to have to be much stronger. I need to lap the connecting members as before, but use of nails (too weak) and use of through-bolts (loss of section due to bolt holes too high) are not an option.  That leaves lag screws and conventional screws.  The analysis results require quite a few of them.  I may need to use a doubled bottom chord member just to fit all of them.

Now to my questions!  NDS standards indicate minimum spacing requirements for lag bolts placed in rows, based on the lag bolt diameter.  If I am installing lag bolts on both faces of the 2X member, does the spacing requirement apply to bolts on each face separately?  In other words, if lags are installed at 2" o/c on one face of a 2x6, and also at 2" o/c on the opposite face, and staggered so they don't hit each other, we will have a lag bolt shank at 1" o/c in the wood, but installed from alternating sides.  How should I consider the lag bolt spacing from a connection capacity viewpoint,  as 1" or 2"?

If you think I am pushing the envelope a little with this question, you're right.  The connections on this truss are a challenge.  No jokes about Elmers glue please,  ha ha.

RE: Lag bolt spacing in wood connections

If the lagscrews extend through all 2x plies as probably required for shear capacity, then every lagscrews exerts force onto each 2x member and the effective spacing would be 1".

Have you considered split ring connectors, timber rivets and probably the choice with the highest chance of being realistic: gusset plates with though bolts?

RE: Lag bolt spacing in wood connections

In addition to emceditor's suggestions, if you are concerned about losing cross-sectional area, you could use a stronger wood. Select structural Southern pine, or even a laminated product available in 1 1/2" thickness, could be used with the much higher axial capacity making up for lost area.

Because lag screws in reality tend to disrupt fibers, I would have a concern about using them instead of through bolts.

RE: Lag bolt spacing in wood connections

I would agree with Trussdoc on the wood damage comment.  Further, if the member is penetrated by the lag bolt, it should be counted in the spacing requirement.

I am assuming these are mostly end connections and not the diagonals.  Have you considered a steel saddle for the connection?

Why is your shear load so much greater than before, considering the member sizes seem appropriate otherwise?

RE: Lag bolt spacing in wood connections

Thanks for your comments.

The connection forces are high because I am using current gravity loads to design the repairs to these trusses.  The old connections are just large nails.  They won't work under modern loads.  I do not have the option of using any type of wood because in many locations I am connecting to existing (Looks like a good quality hem-fir).

Having said all that, I have found that cut screws can do the job.  They still need to be prebored but at least not in two steps.  Using a 12g wood screw instead of a 1/4" lag gives almost the same capacity because the lag single shear values get reduced by the embedment and group action factors somewhat.

I have never used cut wood screws for structural repairs before, though.  Anyone have any experience with them?  Will be using them in both wood to wood and metal to wood single shear situations.

RE: Lag bolt spacing in wood connections

In response to your closing question, I have use both cut and rolled thread screws in truss repairs with plywood gussets.  The NDS has a provision for the use of cut thread screws with ASTM A446 Grade A Steel Side Plates, but it is footnoted in regards to Fe of the plate and Fyb of the screws.  I would specify each in your report/plans.  I have seen unwitting laborers use sheet metal scraps and sheet rock screws for some repairs, only to have them removed and replaced when caught (I hate to think about the trusses that are not re-inspected by the engineer).

Check with you local building supplier and see if they can provide you with a spec sheet on the screws and plates they carry and then you can specify some of those in your report.

I prefer screws to nails in truss repairs.  They don't back out with time (movement, shrinkage, load changes, etc.)

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