Peer-to-Peer Standards Exchange

Keith MacBain posted 08-25-2023 01:25 PM

Hi Chris,

My interpretation is that terms normal or parallel to ridge pertain to the wind direction, not the surfaces.  If pressed for an answer, I believe I'd characterize the green surface as normal to the ridge, but I believe that's not how one enters Figure 27.3-1.  For your image the wind is parallel to the ridge, and if it were 90 degrees from what you show, it would be normal to the ridge.   

However, by this interpretation it appears that Fig 27.3-1 does not explicitly consider all faces of a hip roof. which might be the crux of your question.  In particular, when wind is parallel to the ridge, one enters the second table you show and does not find a value for a windward roof surface (i.e., your green surface).  Hmmm, not sure there.  I’d suppose the programmers for the software you use also wrestled with this and decided to go back to the first table you show, even though wind is not normal to the ridge as the heading indicates. 

This means that for the green face you’d use the value(s) for the windward face in the first table at whatever angle and h/L you have, even though the wind is actually parallel to the ridge (i.e., contrary to the heading of normal).  This also applies to the leeward face, which is hidden in your figure.  For the sides of the roof (pink, gray, and brownish) you’d return to the second table because the wind is indeed (still) parallel to the ridge.  I believe that’s what I’d do. 

Hope this helps,

Keith

Keith MacBain posted 08-25-2023 01:57 PM

Chris,

Another approach is that you could ignore the pitch of the green surface entirely and follow only the second table when wind is parallel to the ridge.  This means that the green surface would receive the same pressure as say the pink surface depending on the distance from the windward edge.  How realistic or conservative this is would depend on the angle.  In other words, if the green surface is very steep it would tend more toward a windward face (first interpretation) whereas a flatter pitch may approach this second interpretation.  

A similar difficulty arises with wind normal to the ridge, in which case the green surface is neither windward or leeward.  Naturally one can divide the surface along an extension of the ridge line but here again, this is not given explicitly.  Hopefully an ASCE person can give us a definitive answer for all angles and directions.  

Hope this helps,

Keith

Chris Rosencutter posted 08-25-2023 07:42 PM

Hi Keith MacBain,

Thanks for the feedback.  In this case I am the software developer for the Wind software and that's the reason I'm trying to get clarity.  I can see both sides of the argument and am hoping to get a definitive answer.  Mr. Ronald Hamburger from the ASCE 7 wind committee frequently responds to questions on this site, and so I'm hoping that he might offer some insight on this one.  

Thanks,

Chris Rosencutter

www.mecaenterprises.com  

Bill Allen posted 10-21-2023 10:36 AM

I would say parallel to ridge.

If nothing else, I believe the direction of the wind load is opposite than what you have shown. It should be towards the wind surface, not away from it.

Alex Thomas posted 10-23-2023 10:16 AM

Parallel to ridge but suction pressure is to be considered

Alex Thomas MIE REng CEng M ASCE

Chris Rosencutter posted 11-06-2023 09:56 AM

Thank you to all that responded.  I wanted to let you know that a response was received from the ASCE 7 Wind Subcommittee. They indicated that for the hipped end in the case where the wind is parallel to the ridge the bottom section of the table provided in Fig 27.3-1 (Parallel to Ridge for all θ) should be used.  

Best Regards,

Chris Rosencutter