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RE: Timber Beam Deflection[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: <seaint(--nospam--at)seaint.org>
- Subject: RE: Timber Beam Deflection
- From: "Dennis S. Wish PE" <wish(--nospam--at)cwia.com>
- Date: Sun, 23 Aug 1998 14:58:18 -0700
Roger, you may have a point, but if I were guided by logic I would think that creep is the effect caused by an equalization due to tension stress in the wood fibers. It would seem reasonable to assume that if the maximum tensile stress is not exceeded there can only be a maximum amount of creep that can occure regardless of the number of times you reload the beam. I would also think that since fatigue is not an issue with wood, the maximum amoung of creep would not exceed the original deflection amount unless the length of time that creep occurs has not reached maturity. Therefore, I still can't understand how the deflection will increase by unloading and reloading the beam. Here is another analogy, although it is based upon other organic and inorganic materials. Guitar or piano strings (both nylon, gut and steel) will stretch when first installed. In fact, Piano strings tend to last in excess of 50 with retuning due to slipage of the pegs or from evviornmental changes. There is an adjustment period before the elongaton due to tension in the wires reaches equalibrium. Once that point is reached, no further elongation is expected unless there in a change in the enviornment (temperature, humidity etc.) I would find it difficult to expect a beam to act any different but the problem is an interesting one at any rate. Is there any test data on this that I can learn about to understand your definition better. Dennis Wish PE -----Original Message----- From: Roger Turk [mailto:73527.1356(--nospam--at)compuserve.com] Sent: Sunday, August 23, 1998 12:54 PM To: seaint(--nospam--at)seaint.org Subject: Timber Beam Deflection As one of those that suggested that the final deflection was 0.75 inches, I would like to explain my thinking. When creep occurs, strain continues without increase in stress, contrary to Hooke's Law. When the load is released, the stresses in the beam return to zero, the stress condition that the beam was in when it was first loaded. When the beam is reloaded, I could see no reason that it wouldn't behave in a manner similar to when it was first loaded. It is acknowledged that creep diminishes with time, becoming negligible after about two years (Ref. USDA "Wood Handbook"). But that is with the load constantly on the beam. I don't know of any research that has been done with wood to measure creep due to cyclic loadings, similar to what has been done with concrete. I think that your hypothetical question is a good one for a research project. A. Roger Turk, P.E.(Structural) Tucson, Arizona Roy Levy wrote: . > I wish to thank all the responders for their prompt replies. They all . > made some sense, although the conclusions may have differed. . > . > This was a theoretical problem and the assumption was that the beam was . > straight without initial camber. . > . > For those who suggested that the final deflection will be of the order of . > 0.75 inches, does that imply that sufficient repetitions of the . > described sequence could cause the deflections to increase without bound?
- Timber Beam Deflection
- From: Roger Turk
- Timber Beam Deflection
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