Replies

1. 
   Mike_Maines_ | Apr 10, 2001 02:08pm | #1

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   Jeffrey,
   Stiffness, or resistance to bending, is usually the most important factor to consider, and for this the deeper a beam is the stronger it is. At some point it will have a tendency to roll, which is one reason bridging or blocking is required for spans over 8'. I don't know of any rules of thumb, often there is a limiting factor on maximum depth, either availability of lumber or site conditions, or it makes sense to use a certain width--such as when putting in window headers. Bearing strength on the ends is also important, you need enough material so the end of the beam doesn't crush when loaded.

   Mike "knows just enough to be dangerous" Maines

1. 
   John_Sprung | Apr 10, 2001 08:34pm | #2

   *Jeffrey -- The reshaping you describe needs to stop before the beam becomes so thin that it buckles -- meaning that the top edge, which is under compression, bends or goes wavy. You might want to look at some engineering books and read about the section modulus, which is what makes adding height better than adding width. IIRC, it's proportional to the width, and to the square of the height. There's a good series by James Ambrose that doesn't require anything beyond high school algebra.-- J.S.

1. 
   Boss_Hog | Apr 10, 2001 10:44pm | #3

   *The section modulus John mentions is calculated by the followinf formula:Depth ^2 times the width divided by 6. (Depth and width in inches)So the taller the beam, the stronger it is - Up to a point. All beam design books and/or programs assume the beam is braced all along the top edge. You have to have enough width to keep the thing from buckling, but I don't really know how that's figured. Guess I also know "just enough to be dangerous"

1. 
   Jeffery_Bybee | Apr 11, 2001 06:19am | #4

   *Alas I cheated my way through algebra. I used a calculater and plain Mathamatics not the algebra. I have several popular engineering books like structure by je gordan and Salvidorie's three books " Art of construction , why building stand up, why building fall down" I just can't do the algebra. one thing I did figure out is that is the top and the bottom of the beam is held ridgedly it is 4 times stringer than one not hindered from buckling

2. 
   Jeffery_Wayne | Apr 11, 2001 06:19am | #5

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   Hi there
   I was wondering if there is a rule of thumb stating the minium width ratio to height of a wooden beam

   if a beam has 20 square inches of material in its cross section if it were 5 inches wide by 4 heigh it would be weaker than 4 inches wide by 5 inches tall. if the material was reshaped to 10 inches tall by 2 inches thick stronger still and so on what is the point that reshaping should stop ?
   thankyou