Beam sized by chart challenged
Beam sized by chart challenged
(OP)
My beam size is challenged by reviewing engineer. "check beam size results," he wrote.
I used EnerCalc to size a beam spanning 39.58 feet. The DL is just 90 PLF and the LLr is just 200 PLF.
Using AISC tables, I get the same result: W12x19
But using tables based upon span, I get a heavier beam. Do I need to satisfy both design methods? I have been designing according to DL+LLr and the moment at the connection to the column. Now I am being asked to meet another design standard.
Does the beam have to meet both design methods, or just one. It doesn't seem correct that I should check two or more design methods and size my beam according to the largest beam resulting.
I used EnerCalc to size a beam spanning 39.58 feet. The DL is just 90 PLF and the LLr is just 200 PLF.
Using AISC tables, I get the same result: W12x19
But using tables based upon span, I get a heavier beam. Do I need to satisfy both design methods? I have been designing according to DL+LLr and the moment at the connection to the column. Now I am being asked to meet another design standard.
Does the beam have to meet both design methods, or just one. It doesn't seem correct that I should check two or more design methods and size my beam according to the largest beam resulting.






RE: Beam sized by chart challenged
DaveAtkins
RE: Beam sized by chart challenged
RE: Beam sized by chart challenged
That specification (and the associated design tables, charts, etc.) can be utilized to design the beam.
I 100% agree with Dave and ChipB here - 12" for a 40' span....hmmm. As any good structural engineer knows - you check BOTH strength and serviceability.
RE: Beam sized by chart challenged
I am sorry. I couldn't think of a nicer way to say that.
RE: Beam sized by chart challenged
If you check the Sx tables, you do get the W12x19 that you talk about, but that is only designing for moment. You still have to check shear and deflection (and vibrations if applicable).
You can't just pull the size out of the table without checking other criteria.
RE: Beam sized by chart challenged
RE: Beam sized by chart challenged
However, there are many ag buildings in this area with 12x19 beams, and they have been standing for decades. So, I wanted to pass one through the county office.
-DD
RE: Beam sized by chart challenged
I checked the beam on Enercalc. Make sure you have all your parameters entered correctly. Under the uniform load tab, there is a check box to use the beam weight in the calcs. Make sure it is checked. Using 50ksi, I still got the beam was overstressed and the deflection was 4.6" or L/103. I've always kept this to L/360 on an itial design. If I'm checking an existing beam, I'll limit the LL deflection to L/360.
Lb: Unbraced Length. Distance between members framing perpendicular (usually) to your beam. There are other factors which get involved, however, it would be best for you to discover those under the direct guidance of your mentor. No, 271828, I'm not even thinking about Appendix 6. Right now, I haven't gotten my mind around the 13th edition. Required brace strength and required brace stiffness is blowing my mind.
RE: Beam sized by chart challenged
What the heck are you doing using a computer program to design a simply supported beam with a udl. It takes 10 minutes to design this totally by hand, including deflection, stresses and lateral torsional buckling checks (using charts).
RE: Beam sized by chart challenged
csd72: It takes 30secs to do it in Enercalc. However, it would be good to do it by hand for a bit until you can realize what the computer is spitting out has an error in it. Enercalc should put deflection in yellow if it is <L/360, and red if it is < L/240
RE: Beam sized by chart challenged
RE: Beam sized by chart challenged
I agree it's always prudent to run a manual calc when there's any question about the program.
BTW, ChipB, what's up with avoiding W10 & W12? I wish I had a quarter for every one of those I've used. Not 40' long of course, though.
RE: Beam sized by chart challenged
RE: Beam sized by chart challenged
It sounded worse?? Crap.
Nothing wrong with W10 or W12
Most of us have favorite sections. I have one with almost every group from W8 to W24, but W12x26 is my section of choice.
BTW, technically, you probably do have a quarter for every one you have used.
I did get a 13th Ed. Steel Manual from a fabricator. That was cool. I'm wondering if that would add up to a quarter for every one.
Also, do you understand this brace strength and stiffness? I tried to talk to the lecturer yesterday at the seminar, and his direct words were, "Good Luck"
RE: Beam sized by chart challenged
You're probably right about the quarters.
Understand brace strength and stiffness? That's a really funny one. I've spent A LOT of hours with that stuff and I think I've scoured every paper on the subject.
The answer: "NO," well maybe "Yes, but only for simple cases" would be better.
I think the problem is that the equations were made up for very simple cases and these don't translate worth a darn to general cases. It doesn't help that lots of the seminar examples are confusing and poorly chosen.
It also doesn't help that the real equations are in the 13th Ed. Commentary and the simplified versions (that'll get your brace killed lots of times if you try to use them...) are in the Spec.
Some of it is completely counter-intuitive also, to the point that I doubt the validity. For example, say you have a beam and are applying the lateral bracing stiffness equation. The more braces you have, the stiffer they all have to be. That's because it must buckle into a sinusoid with a shorter wavelength which is harder to do.
I was in a very famous steel professor's office not all that long ago and he was contemplating teaching that stuff in class. He looked like he was about to give up.
I'm kinda hoping somebody will take it upon themselves to re-invent this stuff. It can't be THAT hard.
Maybe we should build shell models of beams + braces, make the beam initial geometry out-of-plumb, then solve directly for the buckling load. That'll be in the 15th Ed. Spec., LOL.
RE: Beam sized by chart challenged
RE: Beam sized by chart challenged
An example of the problem I have with the current provisions is a beam-column. Does one compute the stiffness and force reqd for axial and flexure, then add them? Linear interaction? Squared interaction? One or the other?
Another example is a rigid frame beam conn to a composite slab. The bottom is unbraced now and they say we can't use IP for braced points. Try using the slab as a torsional brace and the web distortion term eats your lunch.
Yet anothe example is using a shear tab coming into the side of a girder as a girder torsional brace. The web distortion depth should be just the distance from the shear tab to the flange, but the Spec. equations clearly show it as the entire h and it kills your brace if you try to do it that way.
Anybody reading this is bored by now, so I'll get off my soap box!
RE: Beam sized by chart challenged
Mf = ql^2/8... and 0.000624MsL^2/I for delta
Dik
RE: Beam sized by chart challenged
RE: Beam sized by chart challenged
whoa.....
RE: Beam sized by chart challenged
Came up with a W21X62. A mentor at that time noted the importance of unbraced length and was provided as required.
The project involved checking the 12"x12" existing wood column to carry the additional load.
I think it is nice that you can step back and look at things from a better perspective. The learning curve gets steep in cases like this.
RE: Beam sized by chart challenged
I am using the unbraced length on the compression flange - this is the purlin spacing, bolted to an angle and the angle welded to the top flange. I also have always used the autocalc for beam weight.
The difference in our results is the "end fixity" option. Your results are from the 'pin-pin' option, correct?
I used the "fixed-fixed" option and designed my connections according to the greater moment forces produced.
RE: Beam sized by chart challenged
Yes, I used pin-pin.
Careful:
1) In a fix-fix scenario, your bottom flange is going to go into compression about 8.5 feet out from the support.
2) Load cases with wind uplift are going to place the bottom flange into compression.
Purlins aren't going to help you in these situations.
RE: Beam sized by chart challenged
RE: Beam sized by chart challenged
RE: Beam sized by chart challenged
Very few situations in reality are truly fixed. If you are relying on a connection to an adjacent beam or column then the flexibility of these members will reduce the flexibility. Any reduction in end fixity will increase your moment and deflection at mid span.
In order to find the real end fixities you will need to carry out a second order analysis.
Please dont get insulted by people trying to help you, we cant tell if you are a guru senior engineer or a student. People will assume you know nothing if that is what your post indicates, if you do not want to be judged wrongly then word your post carefully.
RE: Beam sized by chart challenged
Basically, I was told: "You can say this is a pinned connection and design the other members to take the loads. The column beneath the pin is then axial only. Design the connection to standard construction practices. If you design the connection as fixed, then calculate the welds, plates and bolts according to AISC, UBC, etcetera.
So, csd72, can you reference a good article/book on second order analysis to determine fixity?? Appreciated.
RE: Beam sized by chart challenged
RE: Beam sized by chart challenged
I am reading the second order analysis requirements in the 2005 Steel Construction Manual. We have always checked slenderness effects. Also, I use an omega-O factor from UBC table 16-N when designing connections.
I will study up on Appendix 7 also. I like to use the software, but I want to know very well the principles and assumptions behind what is produced.
The added forces in second order analysis seem quite small (e.g. ".42 percent of gravity dead load added to lateral forces"). The structures I'm doing calcs for are ag buildings without walls. This means a greatly reduced uplift (Cq=0.7 vice 1.3 in UBC). The unbraced penalty for the compression flange in uplift usually is minimal.
RE: Beam sized by chart challenged
I am interested in your beam calculation of 8.5 feet from the connection will be lower flange compression.
I have been using a "rule of thumb" of 20% of the distance from the haunch to the ridge connection. This 40' out-to-out building has a total haunch to ridge span of 20'. Using the thumb rule, I get approx. 4' in compression.
Is there a more exact method you're using? Thanks. -DD
RE: Beam sized by chart challenged
By comparison, your selection of a W12x19 gives a span-to-depth ratio of 39.
RE: Beam sized by chart challenged
The more exact method is to use structural analysis software. The 8.5 ft is correct for fully fixed end supports. This distance will reduce if the supports are modelled to better simulate the actual structure, ie. by including support columns in the model.
RE: Beam sized by chart challenged
When you have tension in the members such as uplift on a portal frame, the tension tends to straighten out the members reducing the deflections and out of straightness. The net result is that the tension actually reduces the moments (by a very small amount less than 1%) so first order is more conservative for this case.
Check the code, I am sure it says that if compression is less than a certain amount then you can use the first order method. Tension being negative compression is less than any amount of compression.
regards
csd
RE: Beam sized by chart challenged
You do NOT need a second order analysis to take into account the relative stiffness between the beam and the supporting columns. You just need to do an indeterminate analysis on RAM Advanse. Model everything correctly, using the correct member sizes, and you will get accurate results.
I still think deflection will be a problem.
DaveAtkins
RE: Beam sized by chart challenged
I didnt mean for the beam, I meant for the columns. The assumption is that if the beams weren't properly analysed then neither were the columns.
In this day and age I dont see the point of doing a first order analysis for a frame when basically all packages are capable of doing a second order.
But as I mentioned in todays post, if uplift is the critical case then it is not going to make any difference.
csd
RE: Beam sized by chart challenged
8.5 feet:
Based on fully fixed supports with uniform loading. Look on pg 3-216, 13th Ed of the Steel Manual, top of the page. Moment diagram's inflection point is 0.211(l). (Hey 271828, did you notice I OPENED the new book?)
If you are haunching your beam at the ridge, is it fixed or pinned at the ridge? Defining this condition is going to affect the design of the beam as well as your columns. This would be best to be modeled as others have suggested.
I haven't messed with RAM since 1997 when the company I was working for wanted me to do a side by side comparison with STAAD. I was much more familar with STAAD at the time, and the time for building the model was drastically different with RAMSteel winning out. Haven't used it since.
For a quick 2D analysis, I use a free program called FastFrame. I recommend downloading it from enercalc.com
Chip