Temperature effects on lumber 3

[PostFrameSE]

I have a project where a customer reported that his attic temperature was 170 degrees on a 90 degree day. That seems a little warm to me. Obviously with temperatures like that, the building is only subjected to dead loads or wind loads. The trusses and purlins are designed for snow loads more than anything else. Will there be adverse effects on the lumber due to short term high-temperatures? I know that the NDS has reduction factors due to high temperatures, but my building will never be subjected to high loads at high temperature.

Also, what is an acceptable temperature differential between an attic space and outside air?

Thanks.

 

[tngolfer]

170 in an attic sounds a little hot to me also. Honestly I don't have anything to challenge it though.

Obviously with temperatures like that, the building is only subjected to dead loads or wind loads.

Why do you think you don't have to design for live loads?

my building will never be subjected to high loads at high temperature

What a naive comment. How do you know? What makes you think you wouldn't have dead and live load or dead and wind on a hot day.

 

[PostFrameSE]

I evidently wasn't clear enough. Truss member sizing is determined by snow loads far more than wind loads, live loads, or any other loads. Therefore I can state that the controlling loads that determined the actual member size of my truss will never occur at the high temperatures. We have very light single story structures with slick metal roofs with pitches that aren't easy to walk on. Wind, snow, and dead loads is all that can be practically expected on these structures.

Thanks.

 

[frv]

PostFrameSE-

I'm actually going through the process of teaching myself wood design. I've purchased the Breyer book which is, by all accounts, the "Salmon & Johnson" of wood.

They address this very issue and bring up the fact that in many places in the US the temperature routinely exceeds the threshold temperature at which the temperature factor is applied. Is it then necessary to use it? Usually no. Precisely for the reason you mention (likelihood of max temperature and maximum load occurring simultaneously) and because the factor is intended to account for PROLONGED exposure. Prolonged isn't defined, but it is more for things like industrial facilities that will continually be operating at elevated temperatures.

tngolfer- I think a little common sense would lead you to conclude that the attic is not going to be full of people if the temperature is 170 degrees. I don't think PostFrameSE's assumptions are far-fetched or naive.

 

[MiketheEngineer]

Prolonged exposure to 150 egrees F will adversely affect the capactiy of the wood. Can't remember where I read that... Maybe APA or SYP Council

Put in some vents, power vents, ridge vents - whatever to get the temp down.

 

[aayjaber]

Wood is mainly cellulose and 170 F will not alter its mechanical properties if the wood is not damp. Water and fire are the main enemies of wood.

It is a bad idea to allow so much heat to accumulate in the attic. The ventilation design must be bad. Such high temperatures will damage the roofing composite shingles and in a year or two the roof will be leaking and the damage will happen to the wood when it starts rotting.

 

[apsix]

"the attic is not going to be full of people if the temperature is 170 degrees"
True, but live doesn't only mean people. Who knows what might be stored in the attic; books, bags of cement, nanna's china collection ...

 

[tngolfer]

frv,

PostFrameSE didn't say anywhere in his OP it was the bottom chord of the trusses. Actually he said the trusses and purlins which won't be affected by attic loading so thanks for the common sense comment which made no sense.

 

[PostFrameSE]

tngolfer,

I'm not sure what's wrong with you as you are certainly defensive, or smarter than the rest of us, or something.....as your choice of words would suggest. Most of us aren't idiots, and we understand loads, and combinations thereof, and we do have common sense. I obviously didn't share the whole story, what type of building I'm dealing with, etc for the sake of space. Suffice it to say that I do know the codes, how they are applied, and I do know the application. If you would just stick to answering the questions instead of criticizing other people's judgement and posts we wouldn't have people having to defend themselves or others. Thanks but no-thanks if you keep responding to posts like you have.

Thanks to the rest of you who have objectively answered my question.

 

[frv]

apsix-

This is true, but refer to my paraphrased comment by Breyer. Short-term temperature increases will not adversely affect the members and the likelihood of maximum loading and maximum temperature occurring simultaneously is very low.

tngolfer-

It appears you became offended at my comment. I wasn't insinuating that you, personally, lack common sense. My comment was more in line with a thread that graced these discussion boards a few weeks ago in reference to whether or not a 5% "overstress" was acceptable.

The idea is that engineers should exercise some judgment in these matters. My point was that in the case of an attic at 170 degrees, common sense (read professional judgment) should make obvious the points I made to apsix.

 

[mudflaps]

Just happened to look into temperature and wetness effect on wood when updating my spreadsheet stress modules. The National Design Standard (NDS) gives a good discussion and indicates the main concern is SUSTAINED high temperature with much less concern for day/night fluctuations. In the end it seems to indicate,"Use your engineering judgement".

old CA SE

 

[frv]

aayjaber-

The cellulose can be considered the "solid" part of the wood. But I don't think you can categorically dismiss the effects of 170 degrees on wood. What about the lignin? The lignin binds the cellulose. If it is affected by the temperature, then the entire cross section is affected. I really don't know to what extent it affects the lignin, but PROLONGED exposure to 170 degrees would indeed affect the mechanical properties of the wood, regardless of the moisture content.

In my neck of the woods, disease and mold are also important factors to watch out for in wood.

 

[RARWOOD]

From the "Wood Hand Book', " In genreral the mechanical properties of wood decrease when heated and increase when cooled". The decrease in mechanical properties is generally reversabile, however prolonged exposure to temperatures over 150 degress can result in a permanent loss of strength when the wood is cooled.

The commentary to the N.D.S. indicates that adjustment to design values for ordinary fluctuations in temperature is not required.