Is there an engineer in the house?

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Jones
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Is there an engineer in the house?

Post by Jones » Mon May 12, 2008 8:28 am

If so, please help me with a calculation of moment, simple bending stress:

Purpose: Spreader Bar (mild steel)

Design: Two 4" x 5.4(lb/ft) C-channel facing opposite: ]-[

Gap: 1.5" (bolts regarded as shear property)

1" G5 bolts (3) located center and ends, housed in 1" i.d. pipe (spacers).

No cable/chains to be used above spreader... strictly moments to ends from center

Length between spreader ends: 8.0'

Area known on channel section: 1.59 sq.in.

Question: Given a safety factor of 5:1 (50,000psi base) @ 10Kpsi what can I expect for a working load on this spreader? I'd calculate the moment myself but I'm not skilled in working out the modulus and I'm finding the Machinery's Handbooks text to be a bit cumbersome. Not their fault, mine.

Any help with this MUCH appreciated!

- MJ
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FDMSIV
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Post by FDMSIV » Mon May 12, 2008 8:58 am

I am having trouble picturing what you are trying to do. If you are using this beam to hang a weight from, the load depends on how you are going to attach the ends of the beam

If the beam is going to be used in an application in which a compressive force is going to be used, there is a different animal to deal with altogether.


The needed information for calculating the section modulus is in a steel book, or from a vendor. I assume you have access to one or the other since you listed the cross sectional area.

If you could post a sketch of what you are trying to do I could help you with more details.

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Bill Edmundson
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Post by Bill Edmundson » Mon May 12, 2008 9:01 am

The section modulus for C4x5.4 is 1.93 Back to back would be 3.86.

At 10 ksi x 3.86 =38.6 "k

Moment is PL/4 = 38.6 P= 4x38.6/96 = 1.6k or 1600#

Bill
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Jones
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Post by Jones » Mon May 12, 2008 9:11 am

FDMSIV wrote:
If you could post a sketch of what you are trying to do I could help you with more details.
Sure:

Image
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Jones
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Post by Jones » Mon May 12, 2008 9:15 am

Bill Edmundson wrote:The section modulus for C4x5.4 is 1.93 Back to back would be 3.86.

At 10 ksi x 3.86 =38.6 "k

Moment is PL/4 = 38.6 P= 4x38.6/96 = 1.6k or 1600#

Bill
Bill, thanks :!:

Questions...

PL/4? I don't understand the formula, had I known what to "plug" in I could have done it. I'm guessing distance x moment? When I mentioned modulus, I thought I had to perform a Young's modulus.

Also, section modulus? My charts in Machinery's Handbook gave area, how did you convert to section modulus?
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FDMSIV
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Post by FDMSIV » Mon May 12, 2008 9:59 am

P is the load, L is the unsupported length, 4 is well you know

Stress = Moment / Section Modulus

Your allowable stress is 10 ksi, modulus is 3.86 in^3.

For your section at 8', the formula would be =>

(Yield strength of material / safety factor) * 3.86 * 4 * (1 / 96) * 1000

This would give you the working load in pounds

Section mod. is a property that depends only of the geometry/shape of the cross section of the beam.

One last thing, mild steel usually has a yield of 36 ksi (ship building mild steel), a safety factor of 5:1 would put you at an allowable stress of around 7.2 ksi with a working load of 1158#. 50 - 51 ksi is usually high strength material. You may want to double check the strength of your material before loading anything up.

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Bill Edmundson
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Post by Bill Edmundson » Mon May 12, 2008 10:21 am

P is you lifting force at the center. P/2 on each end. The distance from the end to the lift point is L/2. M= force x distance or P/2 x L/2 = PL/4

It's hard to believe that I made a living for almost 34 years on variations of one equation. Stress = P/A -+ MC/I.

Bill
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Jones
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Post by Jones » Mon May 12, 2008 12:55 pm

Thanks much for the responses.

I also was curious about the yield/ultimate strength because my manual (typical) gives the lowest carbon grade. It's A36, which used to be rated at 36K but is now rated @ 48K yield, 70K ultimate.

I'm not sure how that has happened, but several searches on google under "A36 steel yield strength" produced a number of testing results that reported the higher numbers.
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Cec
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Post by Cec » Mon May 12, 2008 1:08 pm

I consider myself a fairly intelligent person but right now I feel dumb as a post :(
Some mornings it just doesn't seem worth it to gnaw through the leather straps.

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leakcheck
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Post by leakcheck » Mon May 12, 2008 5:56 pm

I have quite alot I could add, but I think I will let these guys kinda struggle along for a little while by themselves...it sorta builds character, then when they need my help they will come to me on their knees !!

Steve :roll:

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Bill Edmundson
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Post by Bill Edmundson » Mon May 12, 2008 6:47 pm

:roll: :twisted:

Anyway, I think as more and more steel was recycled grade 50 steel got mixed with A36. The strength went up over time. It was sold for a while as "Multi-Cert". For all intents and purpose it is now 50 ksi steel.

But, if you want to be sure make sure you have Grade 50. I think it's A572 or A992.

Bill
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upspirate

Post by upspirate » Tue May 13, 2008 7:12 am

leakcheck wrote:I have quite alot I could add, but I think I will let these guys kinda struggle along for a little while by themselves...it sorta builds character, then when they need my help they will come to me on their knees !!

Steve :roll:
Steve,

I think they are doing pretty good on this one & don't really need our help!! :roll: :wink:

Jones
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Post by Jones » Tue May 13, 2008 8:29 pm

I used it today to lift the BoJest from a flatbed and place it on the 3800 trailer I'm building. Now I can get proper bunk angles since there's quite a bit of twist to get a good supporting fit.

Thanks for the help on the calculation!

MJ
Avoid Haste.

Brian Eager
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Post by Brian Eager » Wed May 14, 2008 4:24 am

These calculations are the type of thing that caused my daughter to switch majors from civil engineering to meteorology. I told her: "Smart choice, just think, you can be wrong half of the time, nobody is surprised, and you get to keep your job!"

MJ, congrats on the successful lift! Best wishes as you press on to launch.
Noah was a first-time boatbuilder

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leakcheck
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Post by leakcheck » Wed May 14, 2008 4:33 am

MJ did you take pictures ?? Could you post some hrere ???

PLEASE


Steve :lol:

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