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Beam sized by chart challenged
5

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.

RE: Beam sized by chart challenged

Whoa!  Did you check deflection?  If this is a simple span, there is 4.26" of deflection (obviously unacceptable).

DaveAtkins

RE: Beam sized by chart challenged

This beam is lightly loaded however, rule of thumb is, half of whatever your beam span is in feet (39.5/2 =19.75), is your beam depth in inches.  In this instance, I would start with a W18x35, provided it is laterally braced.  I haven't checked it though.  Check your deflection.  On long spans, that tends to control.

RE: Beam sized by chart challenged

You need to use the applicable building code in the area the work is done.  The code will most likely reference AISC (in the US) or some other steel specification relative to the native country.

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 don't want to sound disrespectful, but is there anyone in your office that you can go to for supervision with this problem?  The question seems to indicate that you're in over your head.

I am sorry.  I couldn't think of a nicer way to say that.

RE: Beam sized by chart challenged

First, as DaveAtkins points out, the deflection for your beam is over 4" (assuming simple span)!!!  
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

What is your unbraced length, Lb?

RE: Beam sized by chart challenged

(OP)
Thanks to all, it is clear. I typically spec W16x36 on buildings with these same dimensions.

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 think a better way of saying what 271828 said is, you need a little bit of hand holding right now, and really, this question indicates your mentor needs to provide more guidance.

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

Unlike 271828, I mean to sound disrespectful.

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

Oh yeah, I don't have any Steel Manuals in front of me, but most of us structurals stay out of the first grouping of beam sizes for the beam sections. i.e. W12x16, W12x19, W12x22.  These used to be called "junior" beams.  W16x36 is a good choice. It gets you out of that classification. For some reason, architects seem to love them.

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

Geez, ChipB, I think your version sounds worse!!!

RE: Beam sized by chart challenged

csd72, I'd probably use the program too.  Mainly because I can save the file and then modify it later when the architect makes it longer by 2'.

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

Check Cb in the enercalc program. Some programs default to a rather liberal value instead of using 1.00. It gets on my nerves.

RE: Beam sized by chart challenged

271828:
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

ChipB, yeah, it sounded worse to me.  You ogre, LOL.

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

Solomon and Johnson,"Design of Steel Structures" discussed bracing beams used for bracing need to be able to withstand 2% of compressive force in the beam they are bracing.  They also stated this was conservative.  If I remember correctly.  It's been a few years since I read it.  However, I did calculate the compressive force as they did, for the maximum allowable moment in the beam, not actual, and selected beams that could withstand 2% of that force in axial, and whatever bending it may have due to loading.  That was long before the 13th Edition was out.  

RE: Beam sized by chart challenged

Yeah, the 2% thing has been around a while.  I think its origin is the same as the bracing provisions--a Winter paper from way back?

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

csd72... 10 minutes? only if it takes 8 minutes to find a steel book!

Mf = ql^2/8... and 0.000624MsL^2/I for delta

Dik

RE: Beam sized by chart challenged

Some of us work in residential and W10 is mostly what I use because you can hide it in joist space (2x12 or 11 7/8 I-joists).  

RE: Beam sized by chart challenged

a 40 foot span?  with a 12x19?

whoa.....

RE: Beam sized by chart challenged

A few years ago, I designed a steel beam for a 40 foot span, the owner wanted a few columns gone since the space was going to be used for concert hall in a nice heavy timber building.

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

(OP)
ChipB,

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

Ahh, so we are talking about a roof beam.

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

To brace the bottom flange, use "fly bracing", or whatever you call inclined braces from the bottom flange up to the purlins.

RE: Beam sized by chart challenged

I would also be sure that you can actually obtain something close to a fix-fix end condition.  Just designing the connections isn't good enough if the member it is framing into doesn't have much rotational stiffness.

RE: Beam sized by chart challenged

SructuralEIT covered what I was going to say.

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

(OP)
I appreciate all the input. If anyone has not yet ascertained, my knowledge shortcoming was this: I have been told by two of the six engineers that I have worked with that a column-beam connection is simply what you say it is and how you design it. The idea of performing a second order analysis was what I needed.

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

I wouldnt do it by hand, it would take you a week. Your employer should have a computer package to carry out this anlysis.

RE: Beam sized by chart challenged

(OP)
We are using RAM Advanse.

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

(OP)
ChipB,

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

Using a rule of thumb span-to-depth ratio of 20 to 25, a 12 metre (39.58 ft) span will require a beam in the order of 480 to 600 mm (18 to 24") deep.

By comparison, your selection of a W12x19 gives a span-to-depth ratio of 39.

RE: Beam sized by chart challenged

DairyDesigner
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

If uplift is your critical case then second order analysis may not be necessary. Second order only gives a difference when members are in compression - mainly in portal columns but also in portal beams.

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

I do not agree with csd72.

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

Dave,

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

DD:
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

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