Existing Composite Beam Check 2

[Lot2Learn]

I was orignally going to post this on the RISA Floor forum but I saw that that place was not very active, so I am posting here to get more help.

The question is about RISA Floor. Can I model an existing beam in this software? There are going to be some new loads on existing comp. beams and I need to check. Can I force the program to input the studs size and spacing thats on the drawings? I am not an expert in RISA Floor or comp beam in general.

And the second part of my question is: How difficult would it be to check an existing composite beam with hand calcs, I imagine next to impossible.

Thanks,

 

[rapt]

Lot2Learn,

The first thing you need to learn is that you need to be able to do it by hand and understand it all before you use someone elses software to "do it for you"! If you think it is next to impossible to do by hand, you should pass it over to someone who knows what he is doing.

If it was built more than 10 or so years ago, the original design was probably done by hand anyway.

 

[ToadJones]

Checking the original design by hand will not be impossible.
Take your time and learn how the design is done.
Plugging it into software for an answer will not benefit your career as much.
Later on, once you know the basics, the software will become a great tool.

 

[WillisV]

Checking composite beams by hand is not hard. Time consuming maybe, but not hard.

 

[Lion06]

For what it's worth, you'll get a pretty big increase in capacity using the 13th edition method (full moment capacity with the steel going plastic) compared to the old ASD green book approach that almost always was governed by the tensile bending stress in the steel (pre-composite stresses on bare section superimposed with the post-composite stresses).

Z/S for a typical WF ranges from about 1.1-1.2 (depending on the section), I've had it be as high as 1.8 for composite beams that I've checked (I've gone through similar checks for old composite beams - by hand).

 

[JoshPlumSE]

Checking an existing composite beam by hand should not be all that difficult. Especially if you are using some of the design aids in the AISC manual... see tables 3-20 and 3-21 of the AISC 13th edition manual.

Now, in RISAFloor it should be very simple to model an existing beam. When you initially draw in the beam, it will be viewed as a general wide flange beam and the program will want to optimize it.

However, you can quickly change this using the modify beams tool to be whatever beam you want it to be. Once you have solved the model you can use the "Design" button (located on the left hand side of the model view near the detail report button) to change the number of studs to be exactly what you want them to be.

I hope that helps!

 

[Lot2Learn]

Thanks you all for your help and also the concern about my professional development, I am not offended and actually this encourages me to dive into this subject more. Composite design is not my forte but my office set up does not afford me ready access to more experienced sources so hopefully I can post some of my calcs here for review, if someone has time.

 

[Lot2Learn]

I was wondering if anyone can take a look at the situation I have here (using 9th addition):

Existing W36x135 (fy=50) girder, with 45' trib on each side, and 30' span. 3/4" studs @ 1' oc.
hr (rib)=2", total depth of deck=6" therefore, Y2=4".
Actr=4"x(45'/4)/8=67.5......where 8 is my n value for f'c=4.

Now looking @ table on 2-264, my Str= 620 (aprox).

I think I got this right but what I don't understand is, where do the studs play into this.

I know this is too much to ask but if anyone can look at this real quick I'll be in debt. Thanks,

 

[Lot2Learn]

Correction: Girder spacing is 45' oc, therefore the total trib for my girder is 45'.

 

[JoshPlumSE]

Composite code checking in ASD 9th is actually more complex than in the newer LRFD codes IMHO.

The studs are used to calculate the your percent composite action. This is then used to calculate an effective moment of inertia and an effective section modulus. From these you can get deflection and extreme fiber stresses.

The code checking procedure is best described in one of the older versions of the Salmon & Johnson textbooks (the newer versions are probably all based on 13th edition code). There is also a summary of this in the RISAFloor Help file which I will attach for your reference.

 

[Lion06]

I agree with Josh. Str doesn't really mean anything anymore. Now it's all about the plastic moment capacity of the composite section, regardless of whether it's shored construction or not.