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Wind exposure information for porches on truss design sheets.

Wind exposure information for porches on truss design sheets.

Wind exposure information for porches on truss design sheets.

(OP)
I currently am a building plans examiner, and am talking about residential trusses. If I'm looking at a design with a porch, how can I tell if that section has been designed as exposed.  I see Alpine, MiTek, and Robbins design sheets.  MiTek provides the most information, and will say if it is an exposed cantilever, or exposed porch.  Robbins provides some information, but it's generally a line that says that portion is specially designed.  Alpine shown litle information other than interior or exterior location.  

If you are the Truss Designer for these truss companies, how do you know what's been done?  And shouldn't the EOR or reviewer get this information?  Shouldn't this information be required to be provided?

And what do you think is the proper design for a porch, as an overhang, or as partially exposed?

RE: Wind exposure information for porches on truss design sheets.

I do not know what other software does but MiTek does indeed have this information shown clearly under it's design parameters.  It should be addressed.  This generally only affects the uplift reaction.  I have rarely ever seen even plates change as a result for cantilevered porches or mono-pitch prch trusses.

woodengineer

RE: Wind exposure information for porches on truss design sheets.

Correct me, if I am wrong, but as the plans examiner don't you have the authority to request the information.  If you believe additional information is required to complete your review then ask for it.

In the past when I was the EOR on a project, I often found it necessary to request additional information from the truss supplier.  

You might sit down and talk with one of the local truss suppliers who has a staff engineer.  Normally they are more than willing to help educate people about their product.  They should be able to give you a better understanding of how they treat porches in their designs.

How you approach the problem also depends on whether or not there is a EOR for the project.  If you are dealing with single family homes with no EOR then it may not be as critical as whether the porch is treated as an overhang or not.  In a single home the important thing is that the design provides some resistance to the wind load imposed on the trusses by the porch.

RE: Wind exposure information for porches on truss design sheets.

Robbins software indicates if it is designed as an exposed region.  It will say "USER DEFINED WIND - EXPOSED..."  Then it will give you the region of exposure (i.e. from joint A to joint B along with the coordinates)

RE: Wind exposure information for porches on truss design sheets.

(OP)
Yes, you are all right.  I have seen the information on Robbins and on MiTek.  I have also seen Mitek apply exposed loadings on trusses with three supports but not a porch.  I have, often asked the truss company, and the engineer about design parameters.  They have all been helpful, but  some of the designers like an understanding of the parameters, and the engineers must look at hundreds a day.  But the information should be more available, because mistakes are easy to make.  It shouldn't require a request for information to check the truss design.  I guess on that point perhaps I am just complaining.

Thanks for your responses, and thanks woodengineer for pointing out that the design of the truss itself is not very sensitive to the exposure.  That helps.

RE: Wind exposure information for porches on truss design sheets.

DRsquare one of the truss mistakes I see all the time is the treatment of residential sturures as enclosed, when they are partially enclosed.

RE: Wind exposure information for porches on truss design sheets.

To all,
I am interested in knowing what is the problem with this "exposed or not exposed" consideration. In other words, what behavior problems have you seen when this consideration is overlooked? Does the problem occur often? That last question could apply to both the problem of this condition being overlooked in the design and any behavior problems with the "in use" condition.
Don't get me wrong, I want things to be done right too so I am not questioning the need to consider this item, but I occasionally find mysef in the middle of a situation where the parties involved are wondering why they need to re-do something or why something might be rejected when some of these considerations would often go without notice or incident.
I just want to know what field problems would you expect to develop if such a consideration is not taken into account.
Thanks

RE: Wind exposure information for porches on truss design sheets.

houseguy, the problem would be with an underdesigned truss or connection.  Being wrong is good enough for sending it back and having it changed.  Just because someone got away with it before is no reason to continue nor is it justification for performing an incorrect design.  Not having problems in the field or the fact the something is standing after "xx" years only means that a design event hurricane didn't come through yet, it doesn't mean it's correct.

RE: Wind exposure information for porches on truss design sheets.

UcfSE,
I was hoping you wouldn't focus on the answer you gave. Perhaps it's unavoidable. I am an engineer too so I spend much of my time in the position of having to encourage that way of thinking.
I am looking for insight beyond that. I appreciate your reply though.

RE: Wind exposure information for porches on truss design sheets.

Where were you going with that question, what type of response are you looking for?

RE: Wind exposure information for porches on truss design sheets.

Actually you bring up a good point about the idea of a design event hurricane. Maybe we should start a different thread because I often wonder how many structures that are insured, get their coverage without regard to whether or not they were designed correctly. I've seen numerous tornadoes and everybody marvels at how "random" the tornadoes seem to be when one structure will be blown away and another right next to it will not. Nobody seems to considerthat one might have been designed or built better. (I know many subdivisions are built with all houses the same design).
What I was looking for was some insight into what specific structural problems sould be expected to be the result of the design oversight that was indicated.

RE: Wind exposure information for porches on truss design sheets.

I doubt there would be a physical problem.  They'll still have the cut sheets and the ability to build, erect and anchor the truss, per the truss engineering or structural plans.  Just what they designed and therefore installed won't necessarily be what it should.  I can't think of anything that would actually be a field problem unless it came back so underdesigned that it set off red flags in everyone's minds.

That might be an interesting idea for a thread, considering the general public is oblivious to what we do for them.

RE: Wind exposure information for porches on truss design sheets.

(OP)
From what I've seen, the answer is "it depends".  ASCE-7 is unclear how porches should be designed.  In the Wind Load Design Guide be Metha and others, he designs a porch as an overhang when the wind is directly towards the inside wall of the porch.  Overhangs use the pressure on the roof plus the  wall pressure pushing up from the bottom.  The "partially enclosed" pressure is figured like enclosed, but with coeficients 0g =/- 0.55 instead of =/- 0.18.  The overhang value is always greater but the difference is a function of roof slope.  The effect on the truss reaction varies depending on the length of the truss and the size of the porch.  As woodengineer said, I have seen that it may not affect the truss design, only the design uplift forces.  I have not experimented with it enough to know when it is a big problem.  But it would be nice to know how it was designed so that I can check to see if it is a big porblem, with out having to send it back for redesign.

To the general public and the residential contractors, I'm just someone that's delaying their house.

RE: Wind exposure information for porches on truss design sheets.

Truswal has this option, and it is clear on the design engineering when chosen. As design manager, our biggest issue is to WHEN to design as partially enclosed, fully open, or enclosed. We are in Florida, and have a real hard time knowing if the building was designed as enclosed or partially enclosed. We are lucky to even get the wind speed on the plans. I cannot figure out how a plan examiner can even do his job with the information provided on most plans drawn by some "CAD tech" in their living room.

If you would like to have some example profiles, along with the load case reports showing exactly how the trusses are engineered and the coefficients, please drop me an email.

RE: Wind exposure information for porches on truss design sheets.

(OP)
Trussme68,
I'd like to learn all I can.  I think Truswal is a great idea.

I know emails are frowned upon.  I wish there were an internal message exchange based on Forum ID's.  Could you post any useful information so ALL could benefit, or show a link that others could review your examples?

RE: Wind exposure information for porches on truss design sheets.

(OP)
Trussme68
As you may have noticed, My response got edited.  I did find trusswal.com and truss-frame.com which seem to have some info.  Thanks

RE: Wind exposure information for porches on truss design sheets.

My bad. I assumed by picking on a name, you could email a person. I never tried it, but now know this will not work. www.Truswal.com should be good. Feel free to contact one of their engineers. They like to answer questions.

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