Truss for softwood 1

[Fulat]

Hello. I'm new here and I found this site greatly helpful to fresh engineers like me. Currently I'm testing few trusses made from relatively light and soft timber. Although they passed minimum requirement for strength, deflection etc given in Standards (BS and AS), they still generally failed at the member. Is it possible or practical in real situation to reduce the length of members by introducing more joints/splices to force the failure to the joints instead?

Expert opinions and any suggestions are deeply appreciated. Sorry for the language...

 

[boo1]

What is the failure mode and size on the failed members?

 

[Fulat]

The main members (rafter and ceiling) are 35x100 (mm). 5m span truss with 17.5 and 22.5 pitch. Mostly shear failure at rafter but some rafter seems to "buckle" when we look from the side... thanks

 

[Trussdoc]

Continuous bracing such as a roof diaphragm, or closely spaced purlins would dramatically increase the strength of the top chord by reducing the buckling. Also, look at the way that the chords are connected at the truss ends, you may be creating a high shear concentration there.

 

[Fulat]

Thanks for the advice Doc. Is there any other way to increase the strength, that is normally applied in actual situation nowadays?

 

[AJGIII]

It seems to me your testing is showing you that your design is not adequate, either in spans or sizes of members. Adding continuous lateral bracing will reduce lateral buckling, as will adding members onto the buckling ones in a "T" fashion.

You can add more members (and joints) to reduce stresses. I don't mean to be disrespectful, but it sounds as if your need to go back to statics basics. Or, accept your test results as providing you good info, and react accordingly.