RISA Eq. H1-1b controlling

[sklev]

I am modeling a two span W8X40 beam in RISA that has simple supports and a distributed load only. I did the hand calculations and found that the beam was fine in yielding and LTB, but when I check it in RISA, the unity bending check is bad. RISA says the beam fails by Eq. H1-1b. I have tried to figure this out, but how can Eq. H1-1b control if my flexure checks are okay and I have no axial loads?

 

[Ron]

Check your boundary conditions. I suspect you have an odd restraint that is giving a screwy answer. RISA is very picky with regard to restraint conditions and it is easy to have them conflict using the standard check boxes.

 

[sklev]

I'm pretty sure I have my boundary conditions correct. I have taken a RISA course and feel pretty confident there. I also feel fairly confident in my unbraced lengths. Could it be anything else?

 

[slickdeals]

Look at the axial forces in the model. You should have no axial forces. If you do, then there is something screwy as Ron suggests.

We are Virginia Tech
Go HOKIES

 

[sklev]

I don't have axial forces in my model. That is why I am confused.

 

[PMR06]

Do you get the same shear & moment diagram? I've seen things as simple as the wrong material throwing off the self weight.

Can you post the file?

 

[sklev]

Here's the file. I am focusing on the beam in the middle with two spans.

I used RISA to give me my shear and moments with which I checked the code. If I'm using the same moments as RISA and following everything in the code correctly, we should get the same results for code check. I suppose I am confused about why they are using Chapter H at all if there is no axial force. I used Chapter F. I'm afraid there is something fundamental that I am missing.

The flexure was as far as I got before I got stumped, I haven't checked shear yet, but I do realize that it says it fails in shear also.

 

[JAE]

What unbraced length are you using for LTB check? 4 ft?

 

[sklev]

14 ft. I thought the longest length controlled. Am I backwards here?

 

[JAE]

You have a 1.6 "load factor" in your combinations.

But if you use the moment that you get from the moment diagram (which appears to be 167.2 ft-kips at the intermediate support), the ASD moment capacity for an A992 W8x40 beam is listed at 99 ft-kips for ASD at Lb = 4 ft. and at 88.5 ft-kips for Lb = 14 ft. (all with Cb = 1.0)

These are from the spagetti charts - page 3-127 in the 13th Edition AISC.

RISA reports that M/[Ω] = 99.3 ft-kips with Lb = 18 ft and Cb = 1.29.

With Lb = 4 ft, RISA reports M/[Ω] = 99.3 with Cb = 1.0

It looks to jive with AISC to me.

 

[271828]

It's been a few years since I've used RISA, so I don't remember how much output it gives for the design checks.

What is RISA using for the following?

Mu
LTB Lb
phiMn
Pu
Flexural buckling (KL)x and (KL)y
Torsional buckling (KL)z (does it check this?)
phiPn

You typed that you did _hand calcs_ and know that the beam is fine in Y and LTB. That's not the same as "...if my flexure checks [inserted: presumably in the program] are okay" because RISA might have a different phiMn than your hand calc.

Does it report if Ch. F is satisfied or does it go straight to Ch. H even if there is no axial load. The latter makes good sense, BTW.

You say "I have no axial loads." That's not the same as stating that "RISA reported Pu=0 for these members." Does the _model_ have axial loads for those members in the output?

You said that you're fairly confident in the unbraced lengths. That means nothing LOL. What does RISA report that it used for Lb, (KL)x, and (KL)y?

From "I thought the longest length controlled." you're making me think that you really don't know what RISA is doing, LOL. The default is Lb=L for each member. It can be changed to something else, though.

What exact is reported for Mu, Lb, phiMn, Pu, (KL)x, (KL)y, (KL)z, and phiPn? You should be able to match every one of these values with manual calcs.

 

[cessna98j]

Did you check to ensure that you assigned A572 50 ksi steel for your W8 in RISA? It typically defaults to A36 for everything unless you change it manually.

 

[sklev]

JAE -

The 1.6 is in the load factor for my combinations because I am using LRFD. I am multiplying it by the self weight and treating it like a dead load.

I hadn't looked at the charts in Table 3-10, and even though I am using LRFD, it appears I am under capacity. I must have made a mistake in my calculations somewhere. One mistake that I can see right away is that I didn't adjust the Cb for a different unbraced length; I just calculated it using the entire length of the beam.

271828 -

I understand that RISA uses L for Lb as a default. I inputed what I felt was appropriate for Lcompbot, and left the rest as I thought they were unbraced.

Perhaps I don't know what RISA is doing. That's why I am trying to get help in understanding where I went wrong.

It appears I need to go over what I had done to match these values with what RISA gives me.

cessna98j -

I did change my steel value.

Thanks everyone for your help. I'll dig in deeper and figure out with my manual calculations where I went wrong, since it appears the problem is with my understanding.

 

[JAE]

sklev, Your RISA model was using ASD as the design approach - you need to be consistent.

Check the global parameters and change the steel code to LRFD.

 

[sklev]

JAE - Yes. Thank you. I checked that. It was something that I thaught I had done, but apparently I hadn't set it to default. I also was going off the assumption that A992 steel was 60 ksi. I see now that it is 50 ksi.

Once again, I appreciate the positive feedback. I am getting to the bottom of this and things are meshing now. It has been helpful to get multiple viewpoints.