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RE: structures & ethics

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With all due respect, your comment is preposterous. I will admit that if bending stress is within allowable limits then deflection is an issue related to appearance and even finish, but not life threatening. A good example of this is ceiling rafters for a 25-foot length. In most cases for a ceiling, if wood is used, the deflection while within code limits will crack the drywall tape as the deflection is excessive for finishes but not life threatening. However, to suggest that deflection is not an “indicator” of potential structural failure due to overstress caused by excessive bending due to overloading or improperly sized members is preposterous. Creep will exacerbate deflection due to bending, but this still needs to be considered by the designer as it indicates not only an unsightly and possibly difficult region to finish without future cracking or taping of joints, but it can lead to failure if creep occurs mid-span in a region that allows water to pond. With the possible exception of my area (and it rained enough on Monday to collapse a roof if water was not adequately drained from mid-span of the roof member.

 

What troubles me most is that many designers are pushing the limits to keep the cost of materials affordable and by doing this; they are reducing the factor of safety and pushing the limits toward failure. Furthermore, framers can install joists with the crown side down and this makes the problem worst over a long term. If the crown is placed down and there is a knot in the now tension side of the member, then all discussions of factor of safety is thrown out the window and failure becomes a more possible event.

 

I hope that I have misunderstood your comment. Creep in deflection may not be life threatening, but certainly before bending failure occurs in wood, creep is naturally induced as bending stress increases – it is in the nature of the material.

 

Please also correct me if I am wrong, but the Moment is the first integral of Shear and deflection is the second integral of shear or the first integral of the moment. There is a direct relationship between shear, moment (bending) and deflection. You simply can’t consider deflection an indicator for code compliance since left to its worst case, the result of failure is directly related to excessive deflection.

 

Dennis.

 

Thanks

Dennis

 

-----Original Message-----
From: ASQENGG2(--nospam--at)aol.com [mailto:ASQENGG2(--nospam--at)aol.com]
Sent: Wednesday, November 24, 2004 9:43 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: structures & ethics

 

In a message dated 11/23/2004 6:29:25 PM Pacific Standard Time, dennis.wish(--nospam--at)verizon.net writes:

I must, with all respect, disagree with you. Deflection is much more than an
indicator - it is a function of bending stress and while you should consider
service loading as a means to calculate if creep is the cause of deflection
(or some other issue such as installing crown side down), it does tell me
that there is a bending problem or that deflection due to bending exceeds or
comes close to exceeding the code allowable limits for deflection of wood
products. If I recall my calculus (and please - it's been over twenty years
- deflection is a derivative of moment or bending, which is a derivative of
shear. Therefore the relationship of bending to deflection is a real issue.
Furthermore, in a flat roof or low slope roof, deflection can lead to
Ponding problems that can exceed allowable bending due to increased short
term live load

Deflection is not necessarily an indication that bending stress stress has been exceeded and it is not a function of the bending stress.  Eventhough you have excessive deflection as long as the bending stress and shearing stress are within the allowable then the structure is still safe.  Deflection is only a serviceability issue.  It is not a life treathening issue.  Deflection are being controlled to minimize creeking sounds and bouncy floors on wood members,  unsightful appearance, ponding as you have mentioned, and etc.