Do you calculate the amount of load a beam can take based on when the beam cracks?

  • #1
like if I was to say "this beam can take this much load" does that mean if you put any more load on it, it will crack? Sorry if this is a confusing question, help my understanding if you can.
 

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  • #2
stewartcs
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like if I was to say "this beam can take this much load" does that mean if you put any more load on it, it will crack? Sorry if this is a confusing question, help my understanding if you can.

No not generally. Normally the "amount of load" a beam can take depends on what criteria you are governed by. For example, if you have a regulation that states you cannot exceed 80% yield, then you are limited to that.

The yield point of the material is what you are concerned with generally, i.e. the point at which the material no longer remains elastic and starts to deform permanently.

So... "if you put any more load on it, it will permanently deform" is probably the statement you are looking for.

CS
 
  • #3
Astronuc
Staff Emeritus
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like if I was to say "this beam can take this much load" does that mean if you put any more load on it, it will crack? Sorry if this is a confusing question, help my understanding if you can.
As stewartcs mentioned there is usually a limit on the principal stress in the beam. The yield stress is the stress at which permanent (plastic) deformation begins and a beam will be permanently deform. The criterion may depend on the application and margin to permanent deformation. I've seen criteria as low as 2/3 of yield strength.

Normally beams are designed to operate in the elastic region of the stress/strain domain, i.e. less than yield.
 
  • #4
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like if I was to say "this beam can take this much load" does that mean if you put any more load on it, it will crack? Sorry if this is a confusing question, help my understanding if you can.

You may be also looking at working stresses in which a safety factor (commonly anywhere from 1.1 up to 5) is included.
 
  • #5
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I was wondering something along the same lines as the OP.

Say you have a steel beam which is a major support for any kind of structure, how do you know how much the beam can hold? Wouldn't direct testing on the beam itself lead to a weakening of the beam? And if you tested another beam of the same material and applied the results of this testing on the beam you intend to use, what is to say that the beam you choose to use doesn't have a crack or something which would weaken it? Is this where safety factors come into play?

Thanks
 
  • #6
FredGarvin
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I was wondering something along the same lines as the OP.

Say you have a steel beam which is a major support for any kind of structure, how do you know how much the beam can hold? Wouldn't direct testing on the beam itself lead to a weakening of the beam? And if you tested another beam of the same material and applied the results of this testing on the beam you intend to use, what is to say that the beam you choose to use doesn't have a crack or something which would weaken it? Is this where safety factors come into play?

Thanks
That's a good question. Obviously we can calculate the expected load carrying abilities of structures. Safety factors do play a part in allowing us to take care of all of those "unknowns" that creep up on you. However, when it comes down to it, you have to have something in place to ensure you have what you think you have. You can't do a destructive test on a beam that you want to use in your structure. That is where organizational specifications come into play. For example, in the US, most structural steel is governed by ASTM specifications. That spec will outline material constituents as well as most required parameters that have to be met for performance, i.e. minimum tensile strength, etc...

The manufacturers, in order to be able to say that their product meets that spec, has to do a lot of statistical analysis with a lot of testing. There are checks all along the way during manufacturing that help to ensure what the end product will be. At the end, if so desired, there will be a pedigree of sorts for that material that is signed by the head QC person for that manufacturer that outlines exactly what they are giving you.

After all is said and done, there are still two main groups of people that are still in place to make sure things are as they should be. Those groups are the quality control people on both ends of the project. You have final QC inspections at the point of manufacture and there will be QC inspections at the point of use. Then you also have the eyes of the people doing the actual installations.

I hope this helps.
 
  • #7
Pyrrhus
Homework Helper
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Other important factors include the materials (concrete, steel, ...) and the type of design (ASD or LRFD) used to build the structure.

The type of criteria you said "cracks" will not be valid for a beam in a reinforced concrete structure. These beams crack because of concrete's low tensile strength, thus cracks are quite common and must be taken in consideration. Also, depending on the type of structure the materials may be stressed over their elastic region, which is true for reinforced concrete members in most of the current structures.

Another factor is design philosophy, what other posters explained above one method used to design. It's called Allowable/Permissible Stress Design which consists of calculating the stresses resulting from the service loads (expected loads), and a factor of safety is applied. The other philosophy is Limit state design or Load and resistance factor design. In this one, the adjusted resistances (depending on dimension of the structural member and the material's strength) are compared to the expected loads factored. The last method is used more widely around the world.

Addedum: Safety factors are a way to handle the "unknowns" like FredGarvin states. In fields like Geotechnical Engineering, they can be upto 45! if needed. In the cases of foundation on rocks, the uncertainty of the state of rock (cracked or not), the stresses acting on the rock before the structure and the other stratums surrounding the rock, plus others make it hard to trust the ultimate bearing capacity value obtained through the tests and theoretical equations.
 
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