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glue and bolt
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Get a bunch of 2x8s 18ft long and bolt a new joist to each existing joist with 1/2" lag bolts. Essentially gives you a 4x8 joist. I did this to some 2x6 joists in my attic and it significanlty strengthened the area.
glue and bolt
Field aplied glue has ZERO structural rating.
You need a shear calculation for the joists to determine the bolt patter,
ANything over 3/8 diamter usually does not work since the crsh load of the wood is exceeded.
Higher grades of lumber have lnger span limits.
If you switch to something other than #2 all sorts of nice lengths appear.
The wood costs more.
Simple sistering means you often have a lazy engineer.
The moment of inertia calculations for a 2x4 flat on the bottom of the joist are not all that hard, and for 1.5 inches of headroom loss you get
A large increase in strength and stiffness.
Adding steel strapping is even better.
For a joist to bend under load the bottom has to get longer (curves are longer than straight lines).
By converting the steel to an equivalent wood member (ratio of Young's modulus) you have added the equivalent of a HUGE wood section.
The shear loading determines the fastener spacing, and the holes must be almost zero clearance.