×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Steel Framed System Selection
5

Steel Framed System Selection

Steel Framed System Selection

(OP)
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.

RE: Steel Framed System Selection

I don't have my USD catalog in front of me, but it would likely be 3" composite deck with 3.5" of concrete (or 4.5", I can't remember off the top of my head what is needed for two hour rating with no fireproofing - we never fireproof deck unless it needs some ridiculous rating.  I haven't seen it done in our office yet.).  

It would also most likely be composite (though this would depend on the loading and deflection requirements) and the % composite would be in the 70% range.  That can get you out of some holes when the architect changes something late in the game.  All you have to do is add studs to the beam instead of bumping up the size of the steel.

RE: Steel Framed System Selection

2" unprotected deck plus 3 1/4" SLW concrete with composite beams at 7.5 ft on center, unshored.  

RE: Steel Framed System Selection

We use LWC to get the fire rating while minimizing slab thickness.  So our thicknesses would either be 5 1/4" or 6 1/4", depending on metal deck depth (either 2" or 3"). That is usually driven by allowable spans for concrete placement, and I'll use the 2-span condition in the deck manuals and select 2 or 3 inch based on the grid spacing.  

Agree with StrlEIT, never fireproof the deck.

I also always design as composite.  


 

RE: Steel Framed System Selection

(OP)
Thanks guys.

Those concur with my experience and opinions also.  I just had a disagreement with a good friend about this.  His opinion is that it's silly to go composite for such small bay sizes.  

I, and everybody in the firms I worked for in two major cities, would use either a 5.25" lightweight concrete slab on 2" deck, sprayed beams, unsprayed deck, D916 system or the corresponding normal weight concrete system.  The beams and girders usually end up about W16x26 or 31 and W21x50 or 62, respectively.  Beams spaced at either 7.5' or 10'.  Beams and gdrs often cambered 3/4".   

RE: Steel Framed System Selection

(OP)
By 5.25" slab, I mean 5.25" total, so 3.25" solid.

RE: Steel Framed System Selection

I'd probably go 6 1/4 LW composite with the 3" deck and beams spaced at 10'.   

RE: Steel Framed System Selection

Costs of NW concrete, LW concrete and fireproofing will vary all over the country. Therefore, if you do not know which way to go in your part of the country from previous experience, here is what I would suggest:

I would estimate the cost to see which of the following options is cheaper:

Based upon 2 hour floor requirement:
1) 3 1/4" Lightweight over deck
2) 4 1/2" Normal weight over deck
3) 2 1/2" Normal weight over deck plus fireproofing

In our area, at one time #1 was cheaper but more recently #3 is cheaper. You might also want to consider the use of the building, whether it will undergo extensive future remodeling, etc.

RE: Steel Framed System Selection

(OP)
jike, what kind of beams?

RE: Steel Framed System Selection

I would generally use composite beams for offices, hospitals, schools, etc. but may avoid composite if industrial with lots of floor holes (either now or in the future).

RE: Steel Framed System Selection

(OP)
OK.  Thanks everybody.

RE: Steel Framed System Selection

In my location, I would select 2.5" + 1.5" Normal weight concrete on 1.5" x 22 gauge deck with open web steel joists spaced at 5' or 6' centers.  Beams would be non composite. But as stated before, economy of structure varies across the continent.  

Best regards,

BA

RE: Steel Framed System Selection

(OP)
for a 2 hour fire rating?

RE: Steel Framed System Selection

If you have an "open concept" type of office building, you will want to run a transient vibration analysis before you lock yourself into a slab thickness. Sometimes, I would add an inch of concrete to dampen the floor movement for certain spans.

RE: Steel Framed System Selection

The OP doesn't state the overal size of the building.

Whether to use composite or non-composite beams can vary somewhat depending on how many studs you anticipate using.

An owner in a steel erection firm (who formerly worked as a structural engineer doing office buildings, schools, hospitals, etc)) has told me several times that if you do not have approximately 1,000 studs on the job, it is probably cheaper to go with non-composite beams and pay for the extra steel. Setting up the stud welding machine, and testing it at each site, and doing test studs, etc is a surprisingly expensive item, and its is his judgement that your don't overcome this expensive on very small projects that need just a few hundred studs.  

 

RE: Steel Framed System Selection

(OP)
Assume a decent sized building.

RE: Steel Framed System Selection

Assume a 1 hour fire rating?

Best regards,

BA

RE: Steel Framed System Selection

@271828,
It must be noted that you would be better off going with composite system, but DO NOT CAMBER.

RE: Steel Framed System Selection

(OP)
Why the blanket statement to not camber?

BA: 1 or 2 hour rating.

RE: Steel Framed System Selection

For a 30' span, the cost of cambering a beam will exceed the cost of making the beam slightly heavier to meet the same deflection criteria and you will get more value for money than cambering.. For spans longer than 40-45', there will be a savings from cambering.

RE: Steel Framed System Selection

(OP)
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.

RE: Steel Framed System Selection

Quote:

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.

RE: Steel Framed System Selection

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.

RE: Steel Framed System Selection

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

 

RE: Steel Framed System Selection

(OP)
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.

RE: Steel Framed System Selection

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.   

RE: Steel Framed System Selection

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

RE: Steel Framed System Selection

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).   

RE: Steel Framed System Selection

(OP)
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.

RE: Steel Framed System Selection

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

RE: Steel Framed System Selection

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.

 

RE: Steel Framed System Selection

(OP)
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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources