wind uplift 1

[scully44]

I am working on a job in massachusetts.

The Mass code requires that parts of roofs subject directly to wind pressure that are within a distance 1/10 of the least dimension of the building or ten feet wichever is less of the peak or eave be designed for an uplift of 1.7 times the base wind pressure.
My question is do I have to consider the higher pressure value over the entire length of the rafter or just that portion of the rafter near the peak & eave? It makes a big diffrence in the load on the collar ties & tie downs.

Thanks in advence

Pat

 

[csd72]

I havent done any work in mass, but this sounds similar to the Australian code that specifies a local pressure factor of 2 at the eaves and ridges.

The philosophy is that gusts can create small regions in excess of the average pressure that the codes are based on.

What you apply this to will depend on the tributory area, normally this will be cladding and immediate supporting members only (this is what the australian factor is limited to) but conservatively it could apply to the whole end truss where the wind is perpendicular to the truss.

Other situations will be up to your judgement depending on the proportion of supported area within these zones.

You may want to include these loads in your wall uplift also.

 

[RARWOOD]

I think it would be reasonable to apply the higher load only to the tributary area where the code requires the cladding to be designed for the higher load.

The way I would look at it is that the roof decking or sheathing in the area of the higher load would need adequate fasteners to resist the code required uplift. The decking outside the 10' or 1/10 zone would not. If the decking experiences the higher wind load it will transfer it to the supporting rafters.

Outside the high wind zone the decking only has to be fasten down to resist the basic wind load. Therefore the rafter shoukld only have the basic wind load applied to it.

 

[RARWOOD]

See uplift on steel beams posted on this same forum.

 

[UcfSE]

That sounds similar to the edge distance "a" used by the ASCE 7. I would apply it only over the distance specified and use the interior pressure otherwise. If you do the other, then you're applying the higher pressure over a greater distance than the code specifies.

 

[Tomfh]

I havent done any work in mass, but this sounds similar to the Australian code that specifies a local pressure factor of 2 at the eaves and ridges.

Local pressures only apply to cladding and cladding supports. What scully44 describes sounds more like AS1170.2's Cp,e coefficients behind windward edges.

 

[scully44]

Thanks for the advise.

After listening to your comments I think applying the increased wind pressure only over 1/10 d length is the intent of the code. Otherwise I think the code would just specify a 1.7 x base wind pressure for the entire roof.

I would also like to say that all of you seem very knowedgable on the subjects you post on & your comments have been very helpfull to me in dealing with building code issues as I have done a lot of stress analysis for power industry components, piping & supports etc. but am just know branching out into the residential/commercial construction industry