Steel Framed System Selection 5

[271828]

Hello all,

I have a question--more like a poll, because I know what I think. Say you are selecting the system for a steel-framed building. It needs a 1 or 2 hour fire rating and the columns can be spaced on a 30 ft square grid.

What system would you select? Slab thickness, fire protection, composite or not, etc.

 

[271828]

sd, I think that's too hard and fast to reflect reality. The last time I added it up, which admittedly was a few years ago, cambering cost about a beam size for beams in teh range that we're talking about. For one thing, these beams will have a camber of 0.75" which might be there from natural camber anyway, so it's free in a way. Also, the floor is likely to end up with a lot smaller concrete over-run if the beams and girders are cambered.

Here's my favorite: There's also the question of "spreadsheet" cost and real cost of the floor system. I have been in meetings with contractors, owners, and architects on jobs that I tried to make more practical by consolidating sizes, not cambering, making columns bigger to eliminate stiffeners, etc. On two occasions, I've had contractors slam me over the head in front of the important people: the owner and architect, with the fact that my system was x psf "heavier" than so-and-so engineer whose bldg he built last year. I've had two contractors praise our design when we cambered beams and girders in the 30' range and knocked x psf off what they were used to. BOTTOM LINE: anybody can compare psf whereas it takes some effort and extra thought to stop and realize that the design is "heavier" but it's actually *better* because of these practicality aspects. My conclusion was that it was safer from an EOR's standpoint to go lightest, within reason when it comes to members.

 

[slickdeals]

My conclusion was that it was safer from an EOR's standpoint to go lightest, within reason when it comes to members.

Although I don't vehemently disagree, I think a lighter building is not necessarily the cheaper one. I guess it would depend on the type and scale of a project. And you are always bound to get a comment "I could have designed it with lesser psf that you EOR".

I did check some numbers recently and to go from a W21x44 to a W21x50 was cheaper than cambering a W21x44. You are right in that we are talking about cambers in the order of 3/4" - 1", which we will more often than not get from mill camber.

 

[Lion06]

We often smooth beam sizes and up-size columns to get away from stiffeners and such. If a contractor made a stink about a comparable building being lighter, just tell him that you're saving more than the extra weight in labor. The real savings are in labor. Make it as simple to construct as possible with more weight and you'll get a less expensive building than if you give them the lightest building you can with camber, and column stiffeners/doubler plates all over the place.

I'd be surprised if you find any steel fabricator who will tell you differently.

 

[haynewp]

I used to use AISC's Parametric Bay Studies program to find the most economical layout for composite beams based on spacing, camber, studs, etc. They have redone the program and named it "Floor Framing v13.1", you might try it if you know you are going with composite beams for the floor.

http://www.aisc.org/content.aspx?id=2960

 

[271828]

Thanks haynewp. I've used that before and wrote my own version at one time. Very useful stuff.

StrlEIT, I agree with you about the real cost, but the situation is not quite so simple IMO. First off, in the meetings I'm talking about, there was no fabricator there. It was the GC who was looking at a single number of psf--a number everybody in there can grasp in 2 sec. In simplistic terms, 5% heavier psf *to them* mentally translates into "the stupid EOR just wasted 5% of $[total steel cost]!" The EOR gets to to sit there sounding like Mr. Whiney Pants with all his buts (*but* it's easier to build, *but* there will be less labor, ...).

It's just a bit of a vulnerable situation that's avoid with a least weight design. I dare say that no contractor would ever get into the design deeply enough to beat the EOR over the head with practicality issues during a typical owner/arch/contractor/consultant meeting. Even if they try to, *now* the burden is on them to articulate these issues during the meeting. All the EOR has to do is keep in his back pocket "Yeah, but last time you gave me a hard time when I did this and that to make the design more practical, but heavier!"

To be clear, though, I am a strong believer in some consolidation of beam sizes, design reactions, and especially with items like base plates (to avoid anchor rod placement errors). It's that extra step of not cambering, trying to get rid of stiffeners, etc. that I stopped trying to do because those run up the main steel psf.

Here's another question I have. I've meant to look into this for a long time, but never got around to it.

Say the EOR makes the design more practical than normal, with much less labor required. I assume that a fabricator studies the plans and computes a material price and then estimates some factor for labor, connections, and stiffeners, say 10% more (or whatever--no clue what this is nowadays)--then make his bid. What's to stop the fabricator from keeping his labor factor high, as if the EOR made the design far less practical, as in lots of column stiffeners, camber, etc.? Nobody would ever be able to figure out that he did that. *WHO* got the money savings there? I've wondered for a while if the EOR ends up getting suckered into making more money for the fabricator at the cost of the owner.

The only thing stopping them, that I can think of, is competition. Then again, I could see them all just using a labor factor corresponding to a less practical design and not cutting it closer when the labor/connections/stiffeners/etc will be cheaper than normal.

I'd like to read some commentary from some of you guys on this question because I honestly don't know.

 

[WillisV]

271828 - I agree that all owners etc. generally see is the bottom line psf number. I have never had a contractor say "well its a bit heavier than the last building, but it sure is easier to build so we'll give you a savings mr. owner." Just doesn't happen - they want to know psf and multiply it by their costs (at least most of the steel fabricators I've talked to).

Now that being said, I still design to an easy to construct albeit heavier structure for other reasons. For one, the steel fabricator and erector will appreciate it, be more willing to work with you in the future, and will generate less RFIs etc. So though you don't see any pure revenue from it, you certainly spend less time and perhaps get the next job because of it.

 

[Lion06]

Just out curiousity, isn't the minimum length for cambering greater than 30'?

 

[WillisV]

Though 30' is a pretty good rule of thumb economy-wise, the jig itself can generally accommodate beams down to about 24' (pins located about 20-22' apart plus a little beyond the pins to work with).

 

[271828]

WillisV, I agree with your last paragraph. That's why I would do some things, but not others. I was much more sensitive to doing things that bumped up *main steel* psf.

For instance, we determined at some point that camber was worth about a beam size, as in W16x26 cambered = W16x31 not cambered, so I'd go with the 26 every time. Sure, they *really* cost the same, but the 26 looks to everyone but the fabricator like it costs a lot less. As a bonus, I think it was really less expensive due to less concrete over-run.

I found that good drawings and promptly and apologetically fixing my own errors went farther than anything toward endearing us to the fabricator, erector, contractor, etc.

I do remember getting a good compliment for extreme consolidation of base plate sizes on one job. It was a big job and I think w had two sizes total, a 2.5x30x30 for all of the rigid frames and 1.5x18x28 for all of the gravity columns. The lack of anchor rod errors really got their attention. Then again, if I'm not mistaken, the psf of base plates doesn't go into the main steel bid.

Anyway, I know it's gaming the system, LOL.

 

[271828]

Oops, that's 1.5x18x18 for the gravity columns!

 

[haynewp]

I have gotten questioned before on a higher weight building where I made consistent size members, avoided stiffeners and things like that. I think I had brought this up on this forum a few years back because I had been reading a lot of articles about doing this that were put out in Modern Steel Construction magazine and a seminar I went to about this kind of thing.

But I have also felt kind of bitter about doing this after I got questioned on this about 6 years ago and the guy even called my boss and was asking why our building weighed X more per square foot than the last one this commercial developer did a couple of years before that was very similar. Getting into the explanation about saving labor and consistent sizes and blah blah didn't really penetrate his head very well.

 

[271828]

You articulated exactly what I'm talking about. I don't think an EOR ever gets criticized when he has the psf on his side.

"...I had been reading a lot of articles about doing this that were put out in Modern Steel Construction magazine and a seminar I went to about this kind of thing."

I wonder how many of those MSC and seminar statements come from steel fabricators or other folks who stand to directly benefit! LOL