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Thread: I-beam ratings
05-05-2004, 07:15 PM #1
How are I-beams rated as far as weight capacity when you mount a hoist and trolley? Mine spans 30 feet,is 4 inches top and bottom,and 15 inches in between.As best I can tell the overall thickness is 3/8 to 1/2 inch thick.It is a tri-post setup.I have a 5 ton hoist on it.So far the heavyest thing I've lifted is my 7800lb. mill.I was extremely careful as all the locals said the beam wouldn't hold up.Actaually I was more worried with the trolley.Everything held up fine,The beam didn't budge,and the trolley did it's job.The hoist wasn't a worry to begin with,at least not for me.These folks I have got me to wondering now though.
05-05-2004, 07:55 PM #2
I think you are pushing it Ray. I found a manufacturer of I Beam gantry cranes. A 23' 6" clear span using an I Beam with an 18" web is rated at 3 tons.
05-05-2004, 07:57 PM #3
I pulled a 27' beam out of a crane system, and it is marked with a capacity of 3000 pounds. Very similiar in size to the one your discribing.
We used 10" ones at work a while back and they held up 3000 pounds time after time, all day long.
David from jax
05-05-2004, 08:08 PM #4
My steel handbook lists a 15 x 5 Am Std 26# beam for 30 ft point load as rated for 10,100 lb.
Failure mode in long skinny beams like this not supported to resist rotation is upper flange rotation and bucking. It doesn't creak alarmingly and slowly bend like it does in the movies - it simply collapses before you can react.
05-05-2004, 09:09 PM #5
We recently built a custom bridge crane at work. We load rated everything according to structural ratings. One problem...Cranes have a completely different rating system than structures do. We ended up adding a set of legs at the midpoint. Our crane is 22' long with a 14' bridge and 4' long trolleys. The "I" beams were 6 x 8 with 1/2" web thickness. We only wanted a 1/2 ton rating and we had to add the middle legs. Structuraly we were more than all set but according to crane spec's we were slightly under with out the 3rd legs.
05-06-2004, 12:07 AM #6
Is this a free standing beam supported on the ends or part of the building roof structure?
If this beam is part of the roof, you better subtract the roof live load from your beam rating. Forest is right about any notice before collapse. You may have a little between the bending moment and plasticity. At the upper end, it can go in the blink of an eye--Not in the middle, where most people watch for bending, but towards the ends.
05-06-2004, 12:24 AM #7J Tiers Guest
The depth of the web and the thickness and width of the flanges control the rating. And point loads are different from distributed loads.
Since the top flange is in compression, it can collapse as noted. It becomes unstable with a certain load (compression load), in waht is probably a similar fashion to a column.
The crane rating is probably based on a substantial de-rating, possibly 2 or three times less than the elastic limit as a structure, with another de-rating for movable cranes due to swinging loads and on any crane for braked loads when lowering.
You may have been well into the de-ratings, but it held up, so you "won" this time. Not quite 4 tons.
That sounds like about a 35 lb per foot beam, you can look up the spans for distributed and point loads for various dimensions and weights of beam. Steel is "40 lb plate" if 1" thick , as that is 40.5 lb per square foot. You can figure the web as thinner than teh flanges, maybe 2/3 or 1/2.
05-06-2004, 02:09 AM #8
The "3/8 or 1/2" thick" covers a wide range. For the simple vertical load you could be right at the top of the "safe and reliable range" or as much as 2X beyond it depending on exactly what beam style it is.
The real problem however is the side deflection. With even a 1 ton load your beam is unstable with respect to side deflection and buckling. If the setup is as I picture it from the description, I don't understand why it didn't buckle and collapse with your 7800 lb lift.
A tall skinny beam like that 4" flange is meant to be attached to a stiff membrane along its length, such as the floor deck that it supports, to keep the top from buckling.
Just ballpark, something with an 8" flange width is more in proportion. And more likely a beam in the 80 lb/ft range not 40 lb.
I'm unable and/or unwilling to be more specific. All the raw data and equations are in Machinery's, though it can be tricky to figure out which of the many equations is the correct one for the situation.
05-06-2004, 08:28 AM #9
The only safe way to determine the capacity of your beam is to have a Professional Engineer inspect it and calculate the load capacity. He will take into acount the actual installed conditions that nobody here on the forum would be capable of doing without seeing it. It's not worth getting someone hurt or killed trying to do this yourself.
05-06-2004, 09:57 AM #10J Tiers Guest
In general, the PE will look it up in a table anyhow.....the building code is quite definite...you can look it up too, if no paper is needed.
For a "special usage" it can of course be calculated....by a PE if you need a paper trail, or by you if you simply need it to stand up.
05-06-2004, 10:26 AM #11
I sat down and tried to do some number crunching on this.
1. If the beam is A-36 and is structuraly rated, it will have an identification on it.
This is shape, size, & weight per for. Yours should be something like S15*42.9. If you don't have that rating, it's "wonder" steel and all of the calculations are out the window.
2. The number that I obtained using the above beam size said that you are within the limits of the beam (if it's A-36). The number that Forest gives does not have a safety factor involved. The number that I came up with only gives you a SF of 2. IMO, not enough. All of our overhead stuff must have a SF of 3 or more. You must account for "shock loads" & "side Loads" like a chain slipping or not having the load directly under the beam.
3. If this beam is supporting part of you building, disregard everything that was written above and do like MBensema said and get an engineer or architect.
4. Without seeing your situation, it's hard to do the correct math (this is my disclaimer). But, from what you have said here, I would rate the beam at 2 1/2 tons.
(this is free advise and is worth just what you paid for it)
05-06-2004, 11:16 AM #12
J Tiers, the difference between a PE and this post is the PE knows what is important when making the calculations (not just looking things up in a table) and after inspecting it can use the proper correction factors. Everyone here is just making guesses based on what Ray has posted. Ray is clearly over his head with this, he doesn't even know what beam he has, and should get someone onsite that can give him an accurate calculation.
05-06-2004, 12:40 PM #13
MBensema, I would agree that it is really best to properly analyze the beam and supports.
What Forrest is saying is right on the money. Look at the Ixx and Iyy values for a S15x42.5 beam (not what Ray has but similar)...these are geometrical measures of the stiffness of a given cross-section (and I am quoting from Machinery's Handbook). Ixx is 447 and Iyy is 14.4 so you can see that the beam is a relative noodle right/left (looking down the long length of the beam, web vertical) as opposed to up-down.
Any misalignment of the load with the web of the beam (due to beam installation, side-loading with a hoist, twist in the metal, whatever) starts to challenge the beam right-left and will accentuate the twist as the beam deflects which further misaligns the load and you can see above a certain threshold, you have got major problems.
Safety factor is a must when designing to take into account all kinds of little details and inaccuracies which can't be calculated and analyzed. It also protects against crazy stuff that happens along the way like high winds, accidentally jerking the load (shock loads can be 10-20x static loads witness the WTC). Any PE who designs or rates a structure will certainly include a factor of 2-10 in his or her calculations.
The tri-post Ray mentions is something that might reduce the 30ft clear span but again without more direct info, recommendations aren't a good idea.
05-06-2004, 02:18 PM #14J Tiers GuestJ Tiers, the difference between a PE and this post is the PE knows what is important when making the calculations (not just looking things up in a table) and after inspecting it can use the proper correction factors. Everyone here is just making guesses based on what Ray has posted. Ray is clearly over his head with this, he doesn't even know what beam he has, and should get someone onsite that can give him an accurate calculation.
Yes, a PE is best, although anyone who has taken Statics should be able to figure it out from first principles if they have to. I just had a PE do some structure work for me at the house.
But, the point load and distributed load limits for any standard beam are well known and tabulated. The tables usually have various conditions, such as braced and on what centers, etc, etc.
If there is a bent flange, a saw cut mostly through it, it's resting on a base of 2 x 4 framing on 24" centers, etc, etc, that clearly changes matters.
The first law of doing any engineering is to know whether you know what you are doing. You may know it very well, you may be a bit doubtful, or you may be clueless.
Two of those possibilities have problems. The other one leads to errors of carelessness if one is not methodical.
[This message has been edited by J Tiers (edited 05-06-2004).]
05-06-2004, 05:26 PM #15
So basically what everyone is saying is,that I was damn lucky the whole nine yards didn't come crashing down.Take the 5 ton hoist off and stick with the 3 ton and don't exceed roughly 2 tons,right?
[This message has been edited by ray french (edited 05-06-2004).]
05-07-2004, 02:47 AM #16jpfalt Guest
I am a PE in Oregon and like many have said in this thread, there is no way to know without some specifics.
When looking at it you can start with the upper beam, but the way it is attached at the ends can reduce it's capacity by up to a factor of 8. The way the trolley wheels contact the beam can also reduce the capacity significantly. The legs on the ends also deserve some serious looking into as the length, angle of loading and cross bracing make all the difference in the world. Even the type and placement of bolts can make or break it.
Because of the D****d lawyers, I can't give you an answer in your state, but for what you are doing, a reasonable person would test it over the length of the upper beam and then never use it above half the test load.
05-07-2004, 02:58 AM #17
05-08-2004, 03:22 AM #18Cass Guest
Wallace makes gantry cranes in a variety of sizes and they have a table specifying the I beams for various spans and capacity. They show a reinforced 18" I beam to handle 5 tons with a 30 ft. span. Here is a link: http://www.wallacecranes.com/triste3.htm
I notice in the original post you state that it is a tri-post set up. Does this mean you have some supports along the 30 ft. ?
05-08-2004, 01:39 PM #19lc Guest
Forrest, thanks for the "immediate fail" info. Been wondering about that on the one I use at work.
05-08-2004, 02:25 PM #20
BTW, putting two I beams side by side tied together on the top and bottom flanges with the trolley running on the outer flanges of both more than doubles the capacity of each beam. That makes a box section with torsional strength that a single beam doesn't have.