Bottom Chord Scab Repair
Bottom Chord Scab Repair
(OP)
I have to provide a scab repair for the bottom chord of a 4-ply girder truss. A splice plate has pulled out of one face of the 4th ply of this truss. The bottom chord is 2x8 SYP #1. The following are the forces at the location of the splice joint:
Tension: 12522 lb
Shear: 1837 lb
Moment: 23580 lb-in
These are the forces on the composite truss, so forces in individual ply would be 1/4 of these values.
I am using a 2x8 scab centered over the splice. I determined that (30) 16d Sinkers are required to transfer the tension force (due to bending plus axial).
How do I determine the length of the splice board? Should I just determine the minimum spacing required for the fasteners and use that length, or is there some "development length" that I should be concerned with?
Also, I was planning on placing the fasteners symmetrically about neutral axis of the cross section of the scab. Is this ok considering the axial force distribution is not uniform due to the bending or should more of the fasteners be located in the higher tension zone of the member? I appreciate your thoughts on this.
Tension: 12522 lb
Shear: 1837 lb
Moment: 23580 lb-in
These are the forces on the composite truss, so forces in individual ply would be 1/4 of these values.
I am using a 2x8 scab centered over the splice. I determined that (30) 16d Sinkers are required to transfer the tension force (due to bending plus axial).
How do I determine the length of the splice board? Should I just determine the minimum spacing required for the fasteners and use that length, or is there some "development length" that I should be concerned with?
Also, I was planning on placing the fasteners symmetrically about neutral axis of the cross section of the scab. Is this ok considering the axial force distribution is not uniform due to the bending or should more of the fasteners be located in the higher tension zone of the member? I appreciate your thoughts on this.





RE: Bottom Chord Scab Repair
There is no rupture, exactly, on wood as long as you provide the mininum required end distances.
RE: Bottom Chord Scab Repair
1. Make sure all 4 plies share the load - are the securely fastened together.
2. Why did this plate pull out??
3. That many nails might "shatter" the board. You might use gunned nails or even screws - pre-drilling might be helpful but time consuming.
Good Luck
RE: Bottom Chord Scab Repair
Mike McCann
McCann Engineering
RE: Bottom Chord Scab Repair
Alpine's View software will do a scab repair design for a "damaged" bottom chord. You specify how much of the chord is damaged, say 1", and it will tell you the required "development length" that your talking about. their repair designs typically don't have nail spacings closer than 4" on center, and I've rarely seen the splices much less than 48" each side of the joint. I don't think their software will look at a single ply of a girder truss though.
RE: Bottom Chord Scab Repair
RE: Bottom Chord Scab Repair
I believe if the fastener diameter is less than 1/4" you don't need to do net section checks (per NDS 2001 edition).
4" spacing seems a bit large for 16d nails. The min. spacing per NDS is 4D, which would be about 0.6". I assume that 4" is an Alpine standard?
As JAE mentioned, I don't think there is a "development length" requirement. Is the 48" each side of the joint an Alpine standard as well?
RE: Bottom Chord Scab Repair
It doesn't call it a development length, but from my experience using it, most often the repair scab is no less than about 4' in each direction from the repair point.
RE: Bottom Chord Scab Repair
RE: Bottom Chord Scab Repair
In each row, no closer than 2-1/2"oc spacing.
Switch to Simpson SDS 1/4" when length and nail quantity dicate.
RE: Bottom Chord Scab Repair
RE: Bottom Chord Scab Repair