Reinforcing a sagging Mono Truss
Reinforcing a sagging Mono Truss
(OP)
I have a sagging mono truss that my client would like to reinforce to not only remove the sag, but accommodate light storage. My thought is to simply add a beam at midspan, under the bottom chords, directly below the midspan vertical chord. This would not only provide direct bearing for the top chord, but reduce the span of the bottom chords to allow for additional dead load. Is this a structurally sound enhancement to the system? OR is there a better approach?






RE: Reinforcing a sagging Mono Truss
I would ask why the truss is deflecting, and by how much? Is it damaged?
Is there a "kink" in the roof at the top chord splice location?
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Reinforcing a sagging Mono Truss
BA
RE: Reinforcing a sagging Mono Truss
RE: Reinforcing a sagging Mono Truss
I would also install new 2X blocking transverse between the splice locations to control the rollover you are seeing, and sister the splice if the connection is deteriorated.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Reinforcing a sagging Mono Truss
Yes - you could put a beam under the central vertical - but then all kinds of forces will change. Example, the central vertical will now go into compression while the diagonal will go from compression to tension.
Most likely the lumber will be OK - but the truss plates on the diagonal may be inadequate.
You might want get a truss company/engineer involved.
Good Luck
RE: Reinforcing a sagging Mono Truss
RE: Reinforcing a sagging Mono Truss
RE: Reinforcing a sagging Mono Truss
To sister on to the bottom chord might help, but this is basically a tied three-hinge arch considering the kinked top chord splice. Consequently, to spread and deflect, one of the two end connections must have failed in all, or most of of the trusses. I would look at those connections real close.
Rather than trying to make those end connections good, which could be a real headache with the restricted space, if you have the headroom, add the beam below.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Reinforcing a sagging Mono Truss
The sloping member to the right of the "spliced top chord" carries bending but no axial load. The diagonal below carries compression and should be laterally braced to prevent buckling. Its length is 117" and its width is presumably 1.5" so l/d = 78.3 without bracing. Maximum permissible l/d = 50 by code.
What are the members...2x4? Where are the braces, if any?
BA
RE: Reinforcing a sagging Mono Truss
It appears that the sag occurs at midspan where the vertical members are. If the verts are put into compression by adding a beam, removing the sag. Wouldn't all of the members become simple spans for roof and floor loading?
Mikes comment to do this and add blocking at the kinked top chord splice makes sense to me. Agreed?
RE: Reinforcing a sagging Mono Truss
To repair this, you could support the trusses and replate the joints with larger truss plates - do not try to repress the existing plates.
Normally, putting a support at midspan reverses forces in some members and creates increased loads for some connections. In you drawing, if you support the middle node, you compress the vertical; check its capacity, unbraced, in compression.
A 2x4 spanning about 9 feet is at or over the limit for ceiling joists and snow-loaded rafters, depending on species of wood and exact loads (SYP span tables indicate that No. 2 SYP reaches its limit at about 9 feet.) This is consistent with truss manufacturing industry practices (using every available inch) to reduce material cost and waste.
RE: Reinforcing a sagging Mono Truss
Be carefull doing this - as noted there were probably some hefty loads at one time and moving it back may take quite a bit. Also -by doing this - will you affect the roof deck plywood and shingles in some adverse way.
RE: Reinforcing a sagging Mono Truss
I don't quite understand your description but it doesn't sound like the diagonals in the central trusses are braced at all. They should all be tied together with a continuous 2x4 ribbon nailed to every diagonal.
What is the snow load on the roof? A 2x4 top chord seems pretty light for snow and a 2x4 bottom chord will not accommodate much storage on a span of 8'-9".
BA
RE: Reinforcing a sagging Mono Truss