Bridge Crane designs and ideas...38' span

[Egon]

NO you mean that a bunch of forum math shouldn't be relied upon for designing gantry cranes?

Knowledge is a wonderful thing! Knowledge of what you don't know about is even better!

I have heard that an infinitely thin and deep beam would theoretically support prodigious weights but would not be able to support itself in real life?

 

[Streetcar]

Knowledge is a wonderful thing! Knowledge of what you don't know about is even better!

I have heard ears that an infinitely thin and deep beam would theoretically support prodigious weights but would not be able to support itself in real life?

The infinitely thin and deep beam could support infinite weight if braced at infinitely small intervals

 

[Egon]

The infinitely thin and deep beam could support infinite weight if braced at infinitely small intervals

Bracing wasn't in my statement.

 

[LD1]

Knowledge is a wonderful thing! Knowledge of what you don't know about is even better!

I agree with this, and perhaps others can share their knowledge, instead of making blanket statements.

MANY MANY bridge crane tables out there. Here are just a few I have referenced during this thread.

https://www.cmworks.com/Public/11833/Sngl Grdr Underhung Crane Brdg Kits-113533-71.pdf
http://www.harringtonhoists.com/tech_support/edocs/EDOC 0367 rev02.pdf

None of the beams are braced unless specified. The problem with charts is.....for a given load or span....there may only be 1 beam listed but in reality, any of a dozen beams may work.

Using Mace's w8x31 beam wanting to span 20'.......BOTH charts show a LIGHTER W8x21 beam being good for 1-ton. The harrington chart then jumps to a W10x39 for 4-ton, and the CM chart jumps to W10x26 for 1.5 ton then W10x30 for 2-ton.

So...why the difference right there, 1 calls out for a W10x30, the other for a W10x39?

In either case, The 8x31 beam going to be LESS prone to top fange buckling since it is wider and shorter.

NO I am not a certified structural or civil engineer. I hate how often it gets mentioned....consult and engineer. If I was building a space shuttle...I would. This is a simple span bridge beam. ITs not rocket science, just numbers and formulas. I am just as capable of plugging numbers into the formulas. And am able to verify my results, with many of the charts out there like I posted using various scenerios, weights, and spans. As well as using Strucalc program Which looks at all sorts of other variables. And for a 38' span like my beam, using a wide flange and 5:1 safety....the buckling never pops up.

Perhaps If I were designing a floor or something not dynamic like a crane, I would allow for more than 7200psi bending stress, and then euler buckling would come into play. But limiting bending stress to no more than 7200psi....not a concern with alot of wide flange beams.

So, as far as I am concerned, this thread dont have much left to offer, other than the few that will still pop in and leave their blanket statement of doubt, then offer nothing as far as advise, formulas, or anything to help anyone gain more knowledge.

I have no use for people who are the type to tell you something wont work but refuse to tell you why or explain as if their knowledge is sacred. Not saying anyone here is like that, but feeling like we are getting there.

 

[LD1]

Wait...has anybody checked any of these solutions for Euler Buckling ??? The unbraced length of the top flange is critical in determining the allowable stress in a member induced to bending. Many beam tables only calculate the allowable load based on a braced top flange. Make sure the beam design accounts for the allowable stress reduction due to the total unbraced length of the top flange, including Euler Buckling. I can guess from experience (Registered Structural Engineer for 35 years) that a 6" flange will have a very low allowable bending stress at spans over 12'.

Along the lines of my previous post.....

Any knowledge or wisdom you are willing to share? Or are you just going to leave it as is?

FWIW, all the reading, learning, calculating, reading more.....I havent heard the term euler buckling until now. So I had to look that one up.

Euler buckling doesnt apply to HORIZONTAL beams. Euler buckling is a concern for COLUMNS.

And making a comment like "I can guess from experience (Registered Structural Engineer for 35 years) that a 6" flange will have a very low allowable bending stress at spans over 12'." Taken out of context makes no sense and dont mean a thing.

A 6" flange on a 6" or 8" beam is pretty substantial. Like a W8x28 or W6x20.....Would have to get to FAR longer than 12' to even worry about top flange buckling.

Now a 6" flange on a 21" beam, like a W21x44....(or about ANY of the S-type beams)....then yea, you need to account for TF buckling and either add a channel cap or something.

 

[Egon]

[video]http://thechronicleherald.ca/sites/default/files/imagecache/ch_article_main_image/articles/B97436230Z.120150316192917000GM18STDU.21.jpg[/video]

Simple beam design would indicate the beam is restrained/supported at each end. The tables are just that, Tables. When entering real world design there will be many other factors which must be checked as the beam is chosen. Each condition defaults to another condition even to the support design. The beam seat must also be determined. Temperature conditions in case of say fire and other items of which my knowledge is deficient.

Even the professionals as shown above make mistakes in calculation or construction handling.

 

[ultrarunner]

When I worked at the Fab shop we had two large overhead cranes... they had cogged gears with a chain fall to move one end of the shop to the other which was about 200' for the one in the back.

When I built my dump trailer it was child's play to flip it over or turn it to get the perfect angle for mig welds.

Anyway, the shop had a fire and the next day while the bricks were still hot... the beautiful cranes were heaps of tangled spaghetti.... all the aluminum plates on the mills and lathes were puddles on the floor... as was my tool box and tools...

Steel turns to putty with heat and then melts...

 

[Arc weld]

Sort of on the topic of beams and bending, a contractor in Edmonton did a $4,000,000 %$#@! up on a new bridge. They are paying $11,500/day in penalties and put the project about a year behind schedule. People with common sense knew what happened as soon as they saw the pictures but of course the city isn't going to say anything because it's under investigation. 3 month's later they concluded what everybody thought. The contractor installing massive (narrow flange) I beams for a bridge and were putting the beams up 1 at a time and installing all the cross braces as they went. Then I think because they were trying to beat a storm or the wind or something got in a hurry and put the last 3 beams up without putting all of the bracing on each beam before installing the next one. The beams buckled under their own weight. The contractor had to bring in a HUGE crane (1200 ton I think), that there only 2 in the province, to take the entire bridge structure down in 1 piece. One of the local structural shops was able to heat straighten the beams which is a miracle in itself. They got the beams and structure back in place but now it's too cold to pour the concrete. The engineers and the city didn't want to take a chance on trying to keep the concrete above 5 deg. C using heaters and blankets. If something went wrong it would be an even bigger problem as the concrete wouldn't have the required strength. These weren't columns and were definitely horizontal beams so I'm not sure if this is an example of Euler buckling or another type of buckling but it a perfect example of why beams load charts by themselves may not be sufficient when designed weight carrying beams.

 

[LD1]

These weren't columns and were definitely horizontal beams so I'm not sure if this is an example of Euler buckling or another type of buckling but it a perfect example of why beams load charts by themselves may not be sufficient when designed weight carrying beams.

Everything I see refers to it as lateral - torsional buckling. NOT euler buckling.

And yes, it must be accounted for. But very difficult to figure on something as dynamic as a bridge crane.

I am not looking at simple beam span charts. I am looking that charts put out by bridge crane companies. Places that sell end trucks, hoists, wheels, etc. Those charts show their recommend beam sizes. If top flange bracing is required, they specify what size channel cap to use.

IF you look at any of these charts, none of the beams all tall skinny beams. The ones that are like the S-type, start specifying a channel cap starting around 28' spans. And none of the wide flange beams start specifying a cap til ~50' spans.

Today though, I am going to pick up my beam. Bout 1.5hr drive. Using only tape measure information provided by the owner, I have concluded it is either a W18x60, W18x65, or W18x71.

Harrington hoists chart tells me the 18x60 is good for 2-ton at 40' span and the 18x71 is good for 3-ton. (The 18x65 will likely fall around 2.5-ton) But my span will be shorter too. More like 37 or 38'. Weather the beam is 60, 65, or 71# dont much matter. It will do what I need with a comfortable safety factor, and the price is good at 15 cents a pound.

 

[LD1]

These weren't columns and were definitely horizontal beams so I'm not sure if this is an example of Euler buckling or another type of buckling but it a perfect example of why beams load charts by themselves may not be sufficient when designed weight carrying beams.

Everything I see refers to it as lateral - torsional buckling. NOT euler buckling.

And yes, it must be accounted for. But very difficult to figure on something as dynamic as a bridge crane.

I am not looking at simple beam span charts. I am looking that charts put out by bridge crane companies. Places that sell end trucks, hoists, wheels, etc. Those charts show their recommend beam sizes. If top flange bracing is required, they specify what size channel cap to use.

IF you look at any of these charts, none of the beams all tall skinny beams. The ones that are like the S-type, start specifying a channel cap starting around 28' spans. And none of the wide flange beams start specifying a cap til ~50' spans.

Today though, I am going to pick up my beam. Bout 1.5hr drive. Using only tape measure information provided by the owner, I have concluded it is either a W18x60, W18x65, or W18x71.

Harrington hoists chart tells me the 18x60 is good for 2-ton at 40' span and the 18x71 is good for 3-ton. (The 18x65 will likely fall around 2.5-ton) But my span will be shorter too. More like 37 or 38'. Weather the beam is 60, 65, or 71# dont much matter. It will do what I need with a comfortable safety factor, and the price is good at 15 cents a pound.