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Bottom Chord Scab Repair

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.         

RE: Bottom Chord Scab Repair

I don't think there is a "development length" like what you see in rebar.  I would say your conclusion to space out the fasteners correctly and see what sort of length you need on each side of the joint.  

There is no rupture, exactly, on wood as long as you provide the mininum required end distances.

RE: Bottom Chord Scab Repair

A couple of things
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

Additionally, remember that any additional holes in a tension member will reduce the capacity of that member in tension.

Mike McCann
McCann Engineering

RE: Bottom Chord Scab Repair

You say one plate pulled off, so i would assume the otherside plate is still in place...so designing for 1/4th the load is probably conservative anyway since the remaining plate is still doing a good bit of the work.

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

Do nailed connections require reduction of section?  I didn't think that was true of nailed connections.

RE: Bottom Chord Scab Repair

(OP)
ctcray,
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

looks like you can pick down to 3" spacing.  It's set up so Non-engineer designers can provide repairs for broken trusses.  I don't really know the ins-and-outs of how it designs for combined tension and bending.

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

I would specify pre-drilling for the nails.  I did a truss repair design one time, and the contractor used a nail gun which split the chord.

RE: Bottom Chord Scab Repair

A rule I use is a nail density no tighter than one nail per 5-1/2 sqin of board face. If nailing from both sides I use 11 sqin.

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

I like the Fastenmaster Truss-Lok Z screws.  No predrilling required.  Plus, they are color coded and have their length stamped on the end, so it's not difficult to confirm the right screw was used.  Can't say that for nails!

RE: Bottom Chord Scab Repair

You don't have to do net sections checks with nails without pre drilled holes because you are removing no lumber.  NDS has specifics on when predrilling is required.  You don't have to consider group action factor per nds till you get to 1/4" dia.  The 4" min spacing is more than nds requires, I believe most of the truss repairs use a min of 3-4" oc to ensure moment transfer in the scab.  

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