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.
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.
