PDA

View Full Version : Beam sizing (60' freespan)



brandon75173
01-31-2008, 11:27 PM
I am about to put up our new shop. We are in the structural steel business (most of the time anyways). Anyhow, the shop is 60'x100'. The span of course going the 60' way. Columns at 25' O.C. In pre-eng. steel construction they fabricate most all of the beams and girts out of 1/4" base. A common way I have seen to span the 60' with pre-eng. is a non tapered beam, roughly 20" web, rising at 1.5-1.75/12". Now if you go to off the rack I-beams, I am thinking W16x26lb rising at 2/12" with a center beam tie (mini truss) going across roughly 20' out of something like W6x12/15. My question is if any of you guys have had experience running mill beams out like this vs. fabricated beams. Now I know that I can truss and kick this thing a few times and it can fail. Would much rather limit it to a one beam per side run. Dont feel like that is likely since there is no center bearing point. The beam is more likely to try and sag mid way up the beam because of the more weight in web/flange over the lighter fabricated counterpart.

Thanks

Brandon

scott9116
01-31-2008, 11:48 PM
I have no idea. All I know is that the last steel building I worked on I ended up roofing too. I was almost scared. If I hadn't welded the thing myself you never would have caught me walking around on it. It just looked too flimsy. It wobbled quite a bit with me on top of it too! I took a lot of mechinical engineering courses about 15yrs ago but it was smaller scale stuff (engines). I'd check on an architectual engineering site. I think most of the guys here just weld the parts together like we're told. If it were up to me the building would sink to the center of the earth due to it's own weight.

Rojodiablo
02-01-2008, 01:46 AM
If it were up to me the building would sink to the center of the earth due to it's own weight.

:laugh::laugh::laugh: Ain't that the truth!!! It's truss time, just because of that reason. I guess they make an I beam that is 60ft. long. But it is gonna cost more than the whole building, and it will weigh in the metric tons range. Look into a steel construction span book, and see what you can find. With a solid beam like that, it will need some very serious footings and supports to hold it up.

MoonRise
02-01-2008, 04:01 PM
60 ft clear span on 25 ft centers?

All these are standard A36 grade steel, flat roof (no pitch) because my quick tool can do that.

You're looking in the range of a W8x18 or a W10x12. For NO snow load or roof load or any other load. Just the beam going from one wall across 60 ft of air and then onto the other wall. Each beam would weigh 720 pounds for that length in a W10x12. About a 2 1/4 inch sag in the middle of the beam just from its own weight for the W10x12, 3 inch sag for the W8x18.

If you have to handle snow or roof or any other loads (like wind or seismic), and in the real world you do, then you are up in the range of a W24x131. And each one of those pups wiill weigh 7860 pounds. Each. About a 3 inch sag in the middle with roof loads.

Go to a W30x99 at 5940 lbs each, and your midspan loaded sag for those 60 ft beams will be about the same 3 inches.

But at 25 ft OC, you'll only need 5 for your 100 ft long building. :laugh:

Alan N
02-01-2008, 04:36 PM
Yup, it takes a lot more pounds of rolled steel than it does fabricated steel to do the same job. The PE manufacturers have their designs trimmed down to the bare minimums because the business is so competitive. By using haunched columns and tapered beams (I know there's a better term for it but it escapes me now) they are able to put the metal where it is needed to get the right section for the load anywhere along the run. If you use rolled sections you have to go with what is needed at the most heavily stressed point and carry that weight the whole length of the member.

You would probably be money ahead to go with a pre-engineered package and erect it yourself. They aren't hard to put up but they are different than a "real" building. I used to supervise for a company that did a lot pf PE structures, it was quite an experience coming on the heels of supervising a fab shop. That spaghetti steel just doesn't act like real metal. :)

Where are you located?

Alan N
02-01-2008, 04:42 PM
But at 25 ft OC, you'll only need 5 for your 100 ft long building. :laugh:

Actually you will only need three clearpsans, the end walls can be framed, unless you want to keep the option of adding more length later. In that case you would use a main frame and what would basically be a non-loaded wall in that location. You would still use columns to support the main frame to eliminate deflection or you would need to use a slip joint to connect endwall to frame to allow for deflection under design load. The columns are the better option.

brandon75173
02-01-2008, 09:04 PM
Aside from Alan, you guys obviously dont get it. The span is PITCHED to the center like this /\. I have put several up and retrofitted even more. Hack *** building kits like this are around $35K-$40K. I can fabricate and do all the work with our crew for around $20K. Now when you go to the W16X26 (which I will probably do) it will take a center and two diagonal kick points. That will without a doubt maintain a rigid span. I am in the DFW area with no snow load, and a dead load of the material only. The reason that you run 25' centers is because you have 10-12" Z-purlins that your r-panel attaches to. No need to go closer than that. In applications like this you need depth of the member, not weight. Weight does nothing but make it sag under its own load. With diagonal cable bracing, kickers off the purlins, and corner bracing its very rigid after the sheathing is installed.

Brandon

SundownIII
02-02-2008, 11:25 AM
Brandon,

You seem to have ALL the answers. Makes me wonder why you even came on the board in the first place.

Where was it that you got your PE certs?

60' clear span is not something I would erect without having a PE's stamp on the drawings.

By the way, I am a PE in Civil Engineering, but I don't provide consulting for free. You're being much too simplistic in your approach to the question at hand. If you really are in the steel fabrication business, I can't believe that your company doesn't have a structural engineer (on staff or retainer) who would be in a better position to answer your question.

Just my .02

gnm109
02-02-2008, 11:31 AM
I'm no structural engineer but I know when I need one. If for no other reason than protection from future liability, You should hire an engineer who will do the necessary calculations and certify his/her work.

But you already know this.....don't you? :nono:

brandon75173
02-02-2008, 04:46 PM
Brandon,

You seem to have ALL the answers. Makes me wonder why you even came on the board in the first place.

Where was it that you got your PE certs?

60' clear span is not something I would erect without having a PE's stamp on the drawings.

By the way, I am a PE in Civil Engineering, but I don't provide consulting for free. You're being much too simplistic in your approach to the question at hand. If you really are in the steel fabrication business, I can't believe that your company doesn't have a structural engineer (on staff or retainer) who would be in a better position to answer your question.

Just my .02

1. Yes I can 100% design a member that will do the trick.
2. Yes I have beam tables and stru-calc
3. Yes it isnt that big of a deal
4. You are a civil engineer so your stamp wouldnt hold any weight......

The original post was brought up to ask if anyone had experience with a non fabricated beam that made the span. Thats the jist of it. I know of atleast 3 fabricated shapes/weights to make it work. Trussing has another several designs that work as well. My intention is to not overbuild if able, thats it, pretty easy. Your reply is typical of someone that does not see how this type of construction works in a real world practical sense. Do you think that someone building a shop that is 60'X100' has an engineer on staff? If so you are naive. Please disregard this post and go on about your merry way.

SundownIII
02-02-2008, 06:27 PM
Brandon,

In response. An internet chat forum is the last place I would seek guidance for an engineering question such as the one you're looking for answers to.

With regard to professional certification, you are right, I could not sign off on your drawings. Certification is by state. I'm registered in VA, MD, and NC.

The reason you should seek "professional engineer" advice is so that your structure will be safe for others without being "overbuilt".

I don't know how things are in Texas, but around these parts, you'd play hell getting a building permit for that structure without a "Professional Engineer's" review.

Just my .02 (Given for free.:))

MoonRise
02-04-2008, 03:26 PM
A 60'x100' building is not in the realm of 'homeowner' and is thus outside the scope of most of the small-scale project building code rules. That building is a large commercial building, and thus will usually have to meet a different set of code rules and requirements.

And without a PE or licensed architect review and certification and stamp, such a structure shouldn't get a building permit. And it wouldn't, around here. Or in many other areas of the US.

You asked for "a non fabricated beam that made the span." In case my reply wasn't quite clear enough, the answer is that a non-fabricated beam or non-trussed design for a 60' clear-span is going to be HUGE. Huge, huge, huge. Pitching the roof slightly will change the beam size slightly, but not by a huge amount.

And if you or your company are going around making and selling 60'x100' buildings without proper design review (by a PE or licensed architect with the appropriate licenses for your locale, and the liability insurance behind them), then I sure hope you and your company have big liability insurance policies. Because all the liability for the design of that structure just landed on you.

My $0.02 as well. For free.

And as Alan pointed out, if you build the end walls with suport columns then you would only need 3 clear-span monster beams across the middle of the building. But in my defense, then you don't have clear span on the ends, and the design criteria for desired column spacing for the end walls (for doorway sizing or equipment transit) was not specified.

And actually Brandon, in my book, a 60'x100' clear-span building IS a big deal. That is a lot of steel and a lot of area and a lot of structure. Wind loads, snow loads, seismic loads, etc, etc.

And people 'in the trades' who try to claim "Well, this is the way we do it, those silly building code requirements and fussy engineering requirements are just getting in the way of our productivity/speed/profit/whatever. And we work in the real world, and those suits just sit in their airconditioned offices and push a pencil around. Yada yada yada ..." Like "why do we have to use 5000psi concrete? That stuff is too hard to work, and sets too fast, and 3000 psi is what we use for everything it it is just fine (so far)", or "Why do we have to run fasteners every 9 inches OC in the field and 6 inches on the edges of a shear panel, heck just every 12 inches will be enough", etc, etc.

Why are you supposed to do it "to code" or "to print"? Because that is what those 'suits' calculated is needed and what they are expecting and payed for!

Recent hurricanes in Florida and other southern states pointed out that half-a$$ed nailing on sheathing is pretty vulnerable to wind damage. As soon as a seam starts to peel back, the structure fails.

Sub-strength concrete fails when the design should have been just fine with the correct materials.

A seemingly 'minor' change in a support changes the entire strength of a structure and it fails, catastrophically, with multiple deaths (1981 Kansas City Hyatt regency Hotel disaster, 14 dead and over 200 injured). http://en.wikipedia.org/wiki/Hyatt_Regency_walkway_collapse

Etc, etc, etc.