Reinforcing a sagging Mono Truss 2

[BRNY]

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?

 

[msquared48]

Depends what extra structure your client wants to see.

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

 

[BAretired]

First, determine why the truss is sagging. After that, the solution may become self evident.

BA

 

[BRNY]

It is assumed that the attic space was previously overloaded with storage and snow. Yes, the top chord splice "kinks to the side approx 1/2".

 

[msquared48]

The beam is probably the simplest approach here.

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

 

[MiketheEngineer]

I agree that something "bad" happened to make a 17-6 mono truss deflect at all. And it would probably take a sizable over load to make that happen. Or really bad quality control.

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

 

[BRNY]

I need to clarify, the sagging condition applies to multiple trusses, spanning over the garage. 7 or 8 trusses are affected and I assume they need to be reinforced by a midspan beam below or adding sistered members to the chords. Which is the better approach?

 

[rb1957]

i'd reinforce the splice first off, maybe add a diagonal to react the down-thrust from the vertical member (from the ceiling end of the mid-bay vertical to the roof end of the longer vertical (clear as mud ?)

 

[msquared48]

If you have the headroom, the beam would be the simplest.

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

 

[BAretired]

A central support does not relieve bending in either the top or bottom chord as their span has not changed.

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

 

[BRNY]

Yes, the members are 2x4. Diagonals are tastened accross the vertical members, at the gable ends, from top chord to bottom chord across 3 bays ea. end.

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?

 

[TXStructural]

Metal plate-connected wood trusses? Are the edges/ends of the plates flaring away from the wood? Upon examination, the connections will probably exhibit a flared pulling out of the plates if they have been overloaded.

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.

 

[MiketheEngineer]

Good suggestions and adding the beam will/could allow you to "re-level" everything by using shims between beam and trusses.

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.

 

[BAretired]

Yes, the members are 2x4. Diagonals are tastened accross the vertical members, at the gable ends, from top chord to bottom chord across 3 bays ea. end.

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

 

[BRNY]

The trusses are only braced w/ diagonals perpendicular to the span of the trusses at each gable end. There is no ribbon. I will have a ribbon added to the diagonal chords. Your comment on loading for 2x4's is understood. It sounds like a beam, as well as larger members sistered to top & btm will be required for snow and/or storage.