# Moment of Inertia for S Type I Beam

• posted
I have an I beam that is S type with tapering legs that is A36 steel.
The width is 3.5" and the height is 6". The web is .5" thick and the
ends of the legs start out at .25" thick and taper to .5" thick at the
web. I need the Moment of Inertia for this beam. I figured it using
Machinery's Handbook at I=26.49. I just want to make sure this is
correct. I also figured that with both ends of this beam simply
supported with a beam length of 38" and a single load point exactly in
the middle of the beam of 20 tons the Deflection at Load would be
3.42x10 to the fifth inches. That sounds a little low for that kind of
load. Can anyone help me out? Thanks, Steve
• posted
Naw! That was elongation, not deflection. Looked right to me.
BS
• posted
Assuming S6x17.25 L = 3' P = 40,000 lbs I get .051" deflection shear value is OK can't check web crippling because you don't give length of end bearing.
• posted
I am curious how this beam will "bend like a pretzel"? How will it bend like a pretzel if the deflection at load is only .05"? I understand what the yield of mild steel is and that it is mostly regarded to be 36,000 psi. However most mild steels are now rated at around 47,900 psi from what I have been reading in various resources and journals. Is this incorrect? This Beam is going to be used for the top of a hydraulic press and will be supported by 2- 2.5"x2.5" .25 wall square tubing. I can always make it less wide to make the beam more rigid and could also use a 10 ton jack instead. I crunched the numbers on these and they came in quite a bit under the yield point. Any more help is greatly appreciated. Thanks, Steve
• posted
I am curious how this beam will "bend like a pretzel"? How will it bend like a pretzel if the deflection at load is only .05"? I understand what the yield of mild steel is and that it is mostly regarded to be 36,000 psi. However most mild steels are now rated at around 47,900 psi from what I have been reading in various resources and journals. Is this incorrect? This Beam is going to be used for the top of a hydraulic press and will be supported by 2- 2.5"x2.5" .25 wall square tubing. I can always make it less wide to make the beam more rigid and could also use a 10 ton jack instead. I crunched the numbers on these and they came in quite a bit under the yield point. Any more help is greatly appreciated. Thanks, Steve
• posted
In a short beam like that, deflection is not usually a consideration. I used the formula stress= (weight x length)/(4 x Z), and came up with 43,000 psi, definitely too high.
stress = (40,000 pounds x 38 in)/ (4 x 8.77 cu in)
Z =I / distance to extreme point, (around 3 inches)
Another consideration is how well spread out the pressure is. If the support is really a point, the beam could kind of fold up. If the load is spread out over several inches of beam, that is not likely to occur.
You need a heavier beam to get the stress to 12,000 psi or less. In an S-beam, a 12 inch beam would be close, rather than a 6 inch beam.
Richard
Steve wrote:
• posted
Well, the allowable fiber bending stress for A-36 has been 24,000psi in building frames for the last 35 yrs, as far as I know. That assumes adequate lateral support of the compression flange, which is questionable in an H-frame hydraulic press. Allowable stresses and safety factors for machine designers may be different. Enough time has been spent on Steve's press. He'll have to eyeball the design.

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