Photos of trusses we have made for barn restorations

[Pete Bannister]

The roof trusses/ roof was formally designed by a Struct Eng. I reviewed the calcs. Nothing wrong with the design. It would have worked but for the slope of grain issue!

Agg, we visually grade softwood and hardwood. Softwood’s easier to do by far. The grading rules for h/w are more complex.

[agg221]

Ouch!

 

Interesting failure. I know that's not the longest of grain, but it's not that short either on the side - it looks to be the combination of the two axes. I can see why they thought that would be acceptable, but I don't like notches myself - stress raisers. I've kept them to the minimum, with pegs in the neutral axis.

 

Wasn't aware you could visually grade s/w for structural use - I've only dealt with big mills where they use 100% testing and stamping.

 

Alec

[farmerjohn]

Here's a newish green oak truss roof that didn’t go so well!

I’m posting it because the cause of failure was down to the grading of the timber; specifically slope of grain. Oh and yes, the inevitable fissuring that occurs as green oak dries.

The raised tie beam result in really rather high bending moment at the connection to the rafter. The very artfully crafted dovetail jointed trusses at the start of this thread have a similarly high tie beam and relatively high moment. However the span of the trusses is very different.

Note the pegs haven’t failed in my example. The timber failing along its weakest axis. The eaves distortion resulted at both sides of the building pretty much equally. Oh and the ridge had a nice dip of about 300mm too. The first photo is not a 'before'. its a neighbouring truss at the other end of the roof.

 

 

was that truss designed up by a engineer?

i would like to point out i am not one, but i work with one on a regular basis, i would not have used a raied tie beam truss on a pitch as shallow. In my opinion, and my opinion only i would have thought the truss has failed as the principle rafters are on a very flat pitch and where the tie beam as been cut in it weakens the timber. as there is no triangulation compression as the angle is so flat, that would be my assessment of it from my knowledge, but it is no way fact.

[Pete Bannister]

yeh its odd that, does anyone know the reason for this? i asked for good oak but not HT graded (i think thatswhat they call it) so i dont get any paperwork with it, but it should be pretty good and only get charged green oak cost.

i had a structural engineer work out sections for truesses, purlings and ridges. only so i know that if we made it to a calculated spec it will not have any problems in the future.

 

farmerjohn, green timber ( timber above 20% m.c.) cant be graded within the B/S EU standards. So you cant buy graded timber if its above 20% m.c. Allegedly

[Pete Bannister]

Farmerjohn, yes it was formally designed and had already gone through Building Control. You're exactly right, the design does stress that joint very highly for the reasons you surmise. However, it would have worked ( in fact it did work in about 8 other trusses in the same building) but for the slope of grain.

 

BTW there's a handy little guide that TRADA publish on using green oak. It provides a grading criteria that is quite well considered and I used it as reference whilst reporting on that failure.

[farmerjohn]

Hi Pete, i will look for the TRADA guide as i will find it very useful.

 

Thanks for sharing the information, i have not come accorss the problem regarding the slope of the grain before and would be intrested if there is any 'bedtime reading' on it so i can avoid that pitfall.

 

Hope all got sorted with the truss in the end.

 

regards, John

[Pete Bannister]

Ouch!

 

Interesting failure. I know that's not the longest of grain, but it's not that short either on the side - it looks to be the combination of the two axes. I can see why they thought that would be acceptable, but I don't like notches myself - stress raisers. I've kept them to the minimum, with pegs in the neutral axis.

 

Wasn't aware you could visually grade s/w for structural use - I've only dealt with big mills where they use 100% testing and stamping.

 

Alec

 

Alec, yes hardwoods and softwood may both be visually graded for structural use. Visual graders are licenced and stamp their licence number on the individual timber. Its then a licenced commodity. The EuroCode (B/S) lists common species used in northern Europe for structural use. If you have timber that’s not listed in the 'Code' then it can’t be graded. However, that doesn’t prevent anyone from using it effectively ( or not as the case may be)

The slope of grain in the failed rafter was 1:5 it should have been 1:13 or better and had it been so the failure would not have resulted

Edited November 18, 2012 by Pete Bannister

[Pete Bannister]

Hi Pete, i will look for the TRADA guide as i will find it very useful.

 

Thanks for sharing the information, i have not come accorss the problem regarding the slope of the grain before and would be intrested if there is any 'bedtime reading' on it so i can avoid that pitfall.

 

Hope all got sorted with the truss in the end.

 

regards, John

 

I was called in to advise on the failure and on repair. It was sorted by altering the design (inserting raking 'posts') to remove the moment and some serious glue. I'll put a ref list together on grading timber. Im not in my office right now so give me a couple of days and Ill post something up for you. :thumbup1:

[delabodge]

great work,,,,,nice to see,,,

[hplp]

How have they repaired that? Is it a complete roof off( and very costly) or can/has it been repaired cheaper?

Thanks,

H