[Newbie] I Beam in the shop.

[Moderatemixed]

Hey guys and gals.

I purchased a 7'2" long section of I beam to put into the rafters of my basement shop. It will be supported on both ends by concrete. I got out my Machineries Handbook and am trying to determine what the maximum load in pounds can be at the middle. I am not looking for a freebee answer here. What I am asking is would someone explain to me how to get the answer using the formulas and guidance in the book. I am using an old 16th edition. Page 378 for I beams and my my beam is a 4 x 2 5/8 7.7. Then over to page 388 Case 2. The symbols are different than what some of the calculators ask for as are some of the terms. I am hoping that it will support 400 lbs as that is the weight of my surface plate and I want to pick it up and move it to another bench, transiting the entire length of the beam. Thanks for the guidance and the learning opportunity.

Derek


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[Moderatemixed]

So I looked in my Marks Handbook, for those not familiar it is like the Machineries Handbook, similar in size and scope. So I found in that book a chart and formula. For the I Beam that I specified, at the length 86" I used the formula 1695AD over L where 1695 is some derived number for a steel I Beam (16000 lbs per square inch allowable fibre stress for steel), A is the sectional area of the beam in square inches. D is the depth of the beam in inches, and finally L is the distance between supports in feet. An important note to make would be that my original calculations produced an answer of 182.73 lbs because I used "inches" in the length value, a simple error of assuming that all units need to stay the same, but incorrect. So after running the formula again this is what I get:

A = 2.21 square inches
D = 4.0 inches
L = 7 feet
Answer is 14,983.8/7 = 2140.54 greatest safe load in pounds, load in the middle.

References are Machineries Handbook 16th edition page 378 and Marks Handbook 3rd edition 8th impression page 442.

If somebody could just check my calculations are sound I would appreciate it!

Thanks everyone.

Derek


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[RJSakowski]

One error that I see is your length. If you have a 7'2" beam supported on concrete columns, the length will be shorter by the amount resting on the columns. I would think you would want to use the minimum distance between the columns as your length.

What is the definition of safe load? Maximum deflection, beam failure? In any case, you won't have to worry about moving the surface plate.

 

[Moderatemixed]

The surface plate weighs 350 lbs. With my calculations I should be fine. As for "safe load" I was just reading the title of the chart in the book. I am not sure what the definition of safe load is. Further, you are right about the span being less than the 7 foot calculation but I thought that if I use 7 feet the more conservative the answer will be. Still hoping that someone will confirm that my numbers are ok. Cheers!


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[f350ca]

The formulas I use generate the deflection and loads in the beam for a particular load rather than give maximum load. Generally you have a load you want to design a beam to carry.
For your 4 inch 7.7 pound per foot I beam with an 86 inch span the 400 pound surface plate would deflect it 0.0292 inches at mid span and generate a bending stress of 2840 psi. Structural steel should have a yield strength of 36,000 psi.
In most cases its the deflection you want to design around. In your case if the beam deflects too much the load will want to roll to the centre then you have to push it up the slope to the other end.

Greg

 

[Moderatemixed]

So if I am reading into what you are saying Greg there is no danger of the beam collapsing and the beam will bend about 300 thousandths of an inch. 300 thousandths of an inch over 42 inches is not too bad at all. Thanks!!!


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[JPigg55]

Does the formula account for the unsupported weight of the beam itself ??? Just asking.
In any case with the beam in the basement, I'd thing you could attach one or more supports along the span connected to your floor joists to increasing the load capability.

 

[Dave Paine]

Gregs calculations state 30 thou deflection.

 

[Moderatemixed]

You are right Dave. Even better! And apparently I need to review the basics. Thanks.


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[Eddyde]

It might be interesting to somehow rig a dial indicator at the top middle of the beam and measure the actual deflection under load, just to see how close the calculations match real conditions.