Existing Wood Trusses are Failing... 3

[StructuralJoe]

I have any interesting project...

It is a 1-story plaza in Hernando County, FL

It has wood roof trusses spanning ~50' between load-bearing CMU walls

It was built in 1985 and from discussions with the owner the roof originally was constructed of plywood sheathing with Clay Roof Tiles...

Upon inspection the roof has 1 member on each truss that has buckled, typical on every truss

The owner had originally contracted a "handyman" to provide "repairs" although no engineer specified the repairs.

Members where added randomly as shown in attached photos.

I have since been contracted and modeled the roof truss with all applied loads to current code...

It appears that the trusses have no horizontal bracing

My questions:

The only member that fails (on-site) does not fail upon analysis... but other members do, why?

Bad wood grade on every truss?
Load transfer?
Lack of proper permanent bracing?
Current wind loads to excessive?

I have modeled the wood grades to be the minimum that will work for the minimum loads applied and not fail any members... but I don't feel comfortable assuming that the trusses where originally designed to not have any lateral bracing (rat-runs)

Any thoughts on this process...

Any suggestions or recommendations?

 

[BAretired]

kslee,

The way he show it, it is not unstable. It is redundant because he has two roller supports each end which is equivalent to pinned each end...redundant by one.


BA

 

[kslee1000]

I see. Thanks.

 

[StructuralJoe]

Bruce, I thought the same thing at first but because the truss is not symetric the forces do change...

 

[BAretired]

Sorry Joe. If the forces change then you are doing something wrong. The only think that should change is the horizontal deflection of each node.


BA

 

[StructuralJoe]

Bruce, it's possible... I will double check my results...

 

[kslee1000]

For gravity load alone, it shouldn't make any difference. If lateral load is involved, the reactions could change.

 

[BAretired]

Joe,

It just occurred to me that if you are applying horizontal loads then you will get different loads in some members. What I said earlier was correct for gravity loads.

For example, if you apply a horizontal load to the left end of the bottom chord and that node is pinned, no force will go into the chord. If the R.E. is pinned, the same force will produce a compression throughout the Bottom Chord.


BA

 

[snaggle]

Joe,

As mentioned by BAretired, you should get the same results from pin-roller and roller-pin.

The horizontal deflections/reactions come from the wind loads. If you are only running one load case (wind from the right for example) then you will see different results in the pin-roller roller-pin models. You need to run multiple load cases (wind from the right, wind from the left, and wind parrallel to the ridge.

Also, as mentioned earlier, you will get the most important stress reversals for the .6D+W load case.

 

[StructuralJoe]

This change of failed members occurs in combinations involving wind loads which are perpindicular to the roof surface... therefore yes there are horizontal forces...

 

[BAretired]

If there are horizontal forces involved, find the horizontal reaction 'H' at the pinned end. Assume H/2 at each end and modify the forces in the bottom chord by H/2. I believe that the bottom chord is the only member which should change as a result of switching the pin and roller. Others may wish to comment.


BA

 

[kslee1000]

Internal force changes in both top & bottom chords, since the chord segments at ends have different tilt. Also, for pin-pin, I afraid the horizontal reaction is defferent at each end as well, because the truss is not symmetrical.

If you have confidence in your program, trust the results it provides. It's not worth the time & efforts to solve by hand for a statically indeterminate truss, unless you want to refresh the memory of those good, old colledge days.

 

[msquared48]

I believe you could check this graphically with a Maxwell Diagram, and rather quickly.

Mike McCann
MMC Engineering

 

[StructuralJoe]

The change was minor enough that it isn't a big deal... I have modeled it pin-roller, roller-pin and pin-pin and my fixes still work... so no biggie

No need to relive the good ol college days... too much beer involved

Thanks everyone for the great posts and take care!

 

[BAretired]

If all joints of the truss are considered to be pinned, then a unit load applied at each end of the bottom chord affects only the bottom chord. No other member should be affected. If joints are considered moment connections, then all members will be affected.

If the horizontal resistance is deemed to come from the roof diaphragm, then the truss could be modeled with a series of unit loads, one at each node, each parallel to the top chord. You would then apply just enough of that load to eliminate the horizontal reaction at the pinned end and combine forces for the final result.

I don't know of anyone who would go to such lengths, but perhaps there are some.


BA

 

[kslee1000]

Cheers

 

[Ron]

Joe...agree with SRA...use liquid nails. I've used it before (I'm in Florida...NE)

BA and Mike really hung in there on this one. Virtual beer for both!

 

[StructuralJoe]

Ron, I plan to, it is a little confusing, where are the calculations for "glue"?

Should we just trust the maufacture or can the bond be calculated similar to shear flow? If shear flow where can the shear strength of the adhesive be found?

I will most likely develop the force in shear thru the screws and use the glue as addition F.S.

Virtual Beer for everyone!

 

[hokie66]

Joe,

I think taking all the load in screws is the right approach. The temperature in the attic space can exceed 140F in Florida. Liquid Nails does degrade with time and temperature. The manufacturer says the expected life is 20 years. I think you need the solvent based glue, not the water based one.

 

[msquared48]

H*&^... Just rip off the roof and and install new, properly designed trusses... I'm gonna have that beer, or two.

Mike McCann
MMC Engineering

 

[BAretired]

Hey Mike, wait for me. I'm getting thirsty too!


BA