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Old 04-03-2007, 11:09 PM   #1
MSGMIATA
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Default 18' SPAN WITH 3'' I beam

Hi there,
Need some help with this. I have a two car garage with a width of 18' feet. The ceiling is 12' feet high. I'm in need of storage space so I wanted to make sort of like an attic at the entrance of the garage. My plan is to run the 3'' inch I beam from wall to wall across the width of the garage about 4' feet from the entrance of the garage. I figured it would have to support no more than 600 pounds. This weight would be shared by an attachment point attached to the header at the entrance to the garage and spread over the width of the garage. I went to a steel place and saw the 3'' inch I beam put it between two elevated points and stepped on it. there was probably about half in deflection. I weigh 230 lbs. and I was standing right in the middle of the beam. I figured that since the weight will be spread and shared with the other holding point, that I can get away with doing it this way. Also if that deflection is too much, can I have the beam cut slightly longer and have it flex up during installation, so that when the weight is put on it, it will flex downward putting pressure on the walls and preventing it from sagging. I'm I totally off the wall here or will this work. Thanks for any help.
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Old 04-03-2007, 11:22 PM   #2
wilbilt
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Default Re: 18' SPAN WITH 3'' I beam

The capacity of wide-flange beams is generally determined by the allowable deflection rather than the ultimate failure point.

The wiki has some links to formulas for determining deflection:
http://en.wikipedia.org/wiki/Beam_(structure)

I would think that the deflection you indicated in your chosen beam is within allowable limits based on the planned capacity and support.
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Old 04-04-2007, 10:34 AM   #3
Ironcrow
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Default Re: 18' SPAN WITH 3'' I beam

Quote:
Originally Posted by MSGMIATA
...if that deflection is too much, can I have the beam cut slightly longer and have it flex up during installation, so that when the weight is put on it, it will flex downward putting pressure on the walls and preventing it from sagging. I'm I totally off the wall here or will this work. Thanks for any help.
No! IF you could attach it securely to the walls with the beam in this deflected condition - the lateral load transfered to the walls would be very great. You'd have creaking lumber and cracking drywall etc. The walls are not intended for this load.

As a matter of fact, no matter how or if you try to bend the beam, you should not solidly attach it at both walls. Bolt it at one wall and land it on a pad on the other side to allow the length of the beam to float.

This beam would probablt work, but I'd be concerned with the deflection under load. 1/2 inch with you is 1 inch with your storage items. I'd support the beam in the middle, re-inforce the beam, or just get a slightly larger beam.
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Old 04-04-2007, 01:53 PM   #4
CraigFL
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Default Re: 18' SPAN WITH 3'' I beam

A few comments about your stated ideas...

1. I-Beams are called efficient sections because they give you the greatest strength in a small space. Unfortunately, because of this , they have a weakness in what is called "lateral torsional buckling". Very significant is the length to depth ratio which in your case is very large. What this means is that before you could reach the allowable deflection (or stress) for the beam, it would buckle and fail. You can see this by standing in the middle of the 18' and watch what happens(you may need to have extra weight to see it). The beam probably won't just deflect straight down but will twist, move to the side and throw you off. This will happen suddenly and may cause a major problem with your setup if it does. Typical ways to prevent this include lateral spacers running at right angles to the beam or decking firmly attached to the beam.

2. Fixing the ends of any support beam will allow it to carry more load at the expense of transferring more load to the structure at the ends.

3. Reverse cambering the beam as you noted is a good way to keep the beam more level under load but not easy to do unless you have the right equipment. Typically, one would use a torch to heat one flange of the beam or use stagger wels on the top flange to allow it to take a curve when it is cooled.
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Old 04-04-2007, 09:03 PM   #5
bobbyd
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Default Re: 18' SPAN WITH 3'' I beam

Based on the information you gave, I come up with a total deflection at the center of the beam of 0.38" or 3/8". I won't bore you with the reasoning or the calcs unless requested.

I done the same thing in my attached garage above the door using only 2x4 lumber. However, I supported it at third points using chain hung from eyebolts into a spandrel 4x4 resting on the top of the bottom chords of the roof trusses (not directly into the bottom of the truss). So the back and ends are simpy 2x4's lagged into the wall studs and the front is suspended from the chains in two spots which really don't get in the way. The overall span in the front is divided into about three 7' long sections. Because of the height of my garage I was able to do this twice, one above the other sharing the same single piece of chain.

I loaded her to the gills and there is no evidence of deflection at any points. All the load on the walls is veritcal and since the shelves are only about 3' deep, the concentrated load on the trusses provides little bending stress, about the same as a 30 pound weight hung from the middle of the truss.
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Old 04-05-2007, 03:54 AM   #6
Kong
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Default Re: 18' SPAN WITH 3'' I beam

Talk to an engineer or architect.

Kong
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