I-Beam Strength

I do not know who you are but, whomever, you wrote a blinding post there sir!

A GA has been added!.

Not sure if it makes any different as you are a guest but it shows you have thought this out. If you are not a member please join! It is a good place with clever people and the odd nutter! But it is a good laugh sometimes!

Take care and have a wonderful holiday....................

I am not being funny but go to 84 lumber and they will size it. In addition, go to The Southern Pine Association.

Hello Guest,

Not quite sure why you started your post as you did, but, what you write is not obvious or correct in any way shape or form, which it seems you imply!

I live mostly in the UK, and the beam supplier has no obligation to tell you what sized beam you should need!

The point you missed is, you should ask your local council/Authority Planning Dept. They are the ones who will come and check it anyway.

The best bet for easy calculation of various beams & etc. is a program named Beam 2d by Orand Systems. It is easy to use and inexpensive to purchase. Or, you can get many uses at no charge. Go to www.orandsystems.com.

Not my area, but wouldn't ANSI or ASTME have some specs. ?

Hello DRFREON:

wouldn't ANSI or ASTME have some specs?

As you say these standards should apply. Or perhaps the software someone else advised to work out the size needed.

The guest.......I can't stand arrogant people and the guest seems like like. If he wants to tank the big talk let him join and we can see his colours. Sorry for the rand.

Take care and have a wonderful holiday...................

ANSI and ASTME will have specifications on the dimensions and materials used, but not on applications. For this purpose, local building codes would control.

You stated that you wished to use an "I" beam, which is usually called a wide flange beam in the industry. These beams may be made using many different grades (think strengths) of steel. The specification that is referenced by every building code I have ever worked with in the United States is the "Manual of Steel Construction" published by the American Institute of Steel Construction (AISC).

The first reply to your query was correct in a technical sense, however if you use the AISC manual you will have to be careful as S denotes the section modulus, and in structural engineering circles stress would usually be denoted by σ.

First, calculate the bending stress induced in the beam:σ = (PxL)/(4xS). (Note that I have not included the WL/8S portion of the formula referenced by another respondant, as that does not apply where there is only a "point" load at the center of the beam.) In this case you have (2000lb x 25 ft x 12in./ft.)/4xS, which boils down to bending stress = 150,000/S psi.

Allowable bending stress usually is 0.66 x minimum yield point of the steel. Most structural sections have a minimum yield point of 50,000 psi, but older steel (if you are using used sections) may be lower, around 36,000 psi. Being conservative this leaves you with an allowable bending stress of 0.66 x 36,000 psi, or 24,000 psi.

Now solve for S and you know the minimum section modulus (a number that mathematically describes the resistance of a certain shape to deformation.) Now go to AISC and choose a section with at least that section modulus.

You are not done yet! As the other respondant noted, this simply means the beam is strong enough to handle the load. It might sag 2 or three feet when the load is applied, which according to any building code I have seen would be unacceptable. Usually you want a maximum sag or deflection of L/240, less in many cases. In this case then, the maximum deflection would be 25ft * 12in/ft / 240 = 1.25 inches. (Again, you may require less sag depending on what this beam supports or what is above it.) This 1.25 inches is the "y" referenced by the other respondant, and you would substitute this into the equation y = PxL³/48xExI (again, the rest of the equation referenced would not apply in this case), and solve for I, the moment of inertia, which is also listed in the AISC manual for steel sections. Choose a beam with at least that moment of inertia and that section modulus, and you will be OK if our assumptions were correct (i.e. there really is only a load of only 2000 lb at the center of the beam and the ends are simply supported etc.)

Another respondant noted that a hardware store or perhaps a supplier would size the beam. I know that this is sometimes done, but use caution as I have been retained to perform forensic engineering in failures where the cause of the failure was just this. Hardware stores use tables intended to give an idea for estimating, but the clerk at the hardware store can not tell you if the assumptions are correct, or what would be a good support at the end, etc. Garbage in ....

I would suggest that if you are at all uncomfortable, find a licensed structural engineer. Sizing the beam is relatively simple and will not cost you a lot of money, and in the end you are assured of a positive result.

Hope this helps. YE