STEEL-CONCRETE MOMENT FRAME 1

[czar07]

hello,

Newbie here, practicing my design career.

I created a 2 story (recreational building) moment frame in which the ground floor columns are concrete and 1st floor beam and columns are steel (due to client requirements).
Plan dimension is 12mx39m
frame support = pinned
1st slab & roof = 150mm concrete on steel deck
limitation = 400mm deep concrete column along z-axis and 600mm wide along x-axis
queries;
1. Option 1 - 1st floor steel columns are moment released
- 1st floor Steel Girders will be on top of GF concrete columns and 1st floor steel columns will be on top steel girders, my problem is how to design connection of steel column on top of steel girder & the steel base plate of steel columns which i know that it is not the same in designing base plate resting on concrete.
- gf concrete column #123 requires 8616mm2
2. Option 2 - 1st floor Steel columns be fixed connected
- steel column will be on top of concrete columns and connection will be the same with normal steel column on pedestal, is this ok? all girders/beam will be connected on column flange/web, does this configuration be considered as FIXED?
- gf concrete column #123 requires 3072mm2 is more favorable for my concrete columns

I prefer option 2 but Im having a doubt with my assumed configuration of its connections.

Any comments or suggestions will be very helpful

 

[KootK]

What drives the requirement for concrete on the bottom? Corrosive environment? Traffic impact loads? Could the ground floor columns be concrete encased steel columns? That might simplify matters.

No matter what, I'd keep all of your girder to column connections as steel to steel connections for economy. It sounds as though you're already headed in that direction.

You could consider making the second story a moment frame and the first story a cantilever column system. That would work the ground floor columns pretty hard but eliminate the need for serious moment connections between steel and concrete.

If you decide to moment connect the steel columns to the concrete columns, I'd use anchor bolt chairs and possibly prestress the anchor bolts. While neither of these measures may be needed for strength, they will improve connection stiffness which is also important.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[czar07]

Kootk

thanks

in option 1, I released moments on my Girder's since it will be just on top of concrete column, no instability problem and as a results, gf concrete columns reinforcement were reduced.
is this ok? releasing moments on my girders, though deflection would be my next problem.

Our client doesn't want steel columns on the GF because of the frustration they have with their previous project.

About this Cantilever type column system, I'm not that familiar, Im just starting my design career.

 

[czar07]

Kootk

if Im going to use option 2, and that girders are connected to steel columns, will it be ok to design the connection of steel column to concrete column the same with common connection in a pedestal concrete and it will be pinned connection in my analysis.? though in my case is not a pedestal but its a column

 

[KootK]

For option two, I'd use a fixed connection between columns and design the connection that way. In practice, it's pretty difficult to detail a conventional connection to be rotationally flexible like a pin while not failing in a brittle manner when those rotations occur.

I'd still look into concrete encased steel columns for the ground floor. It will be much simpler for you to design and detail and might even be cheaper to build. The connection details and inspection requirements for the column moment connections may be onerous. So much so that it would offset the additional length of steel column required to use a concrete encased solution.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[czar07]

thanks KootK, I appreciate your expertise & suggestion

I really want to try encased columns but I'm on a time constraints and we are now going to proceed with concrete columns, and that model restraints & connection are my problem.

Any thoughts about this;
in option 1, steel column moment is released (will be on top of girder) and I released moments on my Girder's since it will be just on top of concrete column, no instability problem and as a results, gf concrete columns reinforcement were reduced.
is this ok? releasing moments on my girders, though deflection would be my next problem.

 

[KootK]

As I mentioned previously, I'm skeptical that you could construct a conventional column to column connection that would justify a pinned base assumption. So I wouldn't do that personally.

If you pin connect the steel framing to the concrete columns then, for one story, your lateral load resisting system will be cantilevered concrete columns. Your columns should require more reinforcement rather than less and your foundations will be more substantial to resist overturning. There may also be seismic design penalties depending on your jurisdiction.

And you most definitely will have a stability issue requiring evaluation. This system can be made to work but is by far the least stable of the options that you've considered. A moment frame over cantilevered columns is a tricky system and you can expect to spend a considerable amount of time designing it. And, as you've suggested, drift will be a challenge.

Structures that are simple are almost always cheaper to build, faster to design, and more reliable in service. For this reason, I think that you were on the right track with your original proposal of having a true, two story moment frame. And if your project is constrained for time, that's even more reason to pursue an encased column option. It would be faster to design and faster to erect.

Sometimes it takes a strong hand on the part of the structural consultant to guide the client towards smart decisions that benefit all parties. If there's room for that kind of negotiation, that's where we add the most value in my opinion.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[czar07]

thank you so much kootK.. I appreciate your patience answering my questions, very helpful

 

[KootK]

You're most welcome czar. Once you pick your path, I'd be happy to help you work through the details, even if it's not the path that I would have chosen.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[czar07]

Im glad that you are willing to help me with this, thanks again kootK

the final structure will be the original concrete gf column and steel frame for first floor, as insisted by the contractor (happens to be a close friend of our client) for the reason that it will be faster & no need for encasing steel sections.

Since concrete gf columns are to be used, Im thinking of making all framing to be conventional concrete (not Prestress). But what im doubtful is that girder span is 11.5m and that this will be a multipurpose gym (LL=3.6Kpa), gf will be a fitness gym and 1st floor will be for table tennis & billiards. With your experience, Is it possible for a conventional concrete beam to span 11.5m with bay spacing of 5m o.c. without having a problem with flexural cracks (deflection)? if yes, could you advise me with this....thanks in advance

 

[jayrod12]

How deep is your beam.

you can make anything work if you have enough depth. If you can fit a beam that is a metre or so deep then I don't think serviceability would be an issue.

But generally a metre deep beam is frowned upon by the architects and clients.

 

[KootK]

I second Jayrod's post. You might need camber and a vibration check but, with sufficient depth, you should be able to make a go of it.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[czar07]

Jayrod12, Kootk

my beam depth limit is 700mm

Thanks, I'm working on it

 

[jayrod12]

I would bet you can get something to work with that kind of depth, it might be awkwardly wide with more reinforcing than normal but it's doable.

However serviceability (deflection and cracking) will be need to be a design consideration. You may want to leave yourself some extra capacity when doing your deflection calcs using Ieffective.

And as Koot noted above, at 11.5m span and only 700 deep vibration is likely to rear it's head, especially for the gym portion, maybe not so much for the table tennis or billiards. Although the billiard tables will let you know right away if there are deflection issues.

 

[czar07]

jayrod12 & kootk

why is it that in my structure, moments along global z-axis is bigger at upper floor than moments in the lower floor? and as a results, more reinforcements are required for the upper floors, Also
I change my column from 400x400mm to 600x400mm (gf to first floor)just to increase the moment of inertia along global z-axis by 50% but its requiring more reinforcement than the original 400x400mm columns.

please enlighten me
attached here is my std file,

thanks

 

[KootK]

That result is to be expected. At the floor levels, the moment coming in from the floor system is split between two columns (above and below). At the roof, similar moments must be resisted by only one column (below). Additionally, the upper columns carry less axial load which tends to increase the demand for flexural reinforcement. Increasing the size of your columns makes them stiffer relative to other framing elements and causes them to attract more moment still.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[czar07]

thanks kootk

but how can i solve this or how can i reduce the steel requirements if i can't make my columns more stiffer

 

[KootK]

Some options:

1) Stiffen your beams. This will draw moment away from your columns. Insufficient beam stiffness may well be the primary source of your problem.

2) I have worked with colleagues in the past who will apply a stiffness reduction factor, on the order of 0.5, to exterior columns such that they will draw less moment. The idea is that columns carrying light axial loads will crack early in the loading history and their stiffnesses will drop dramatically. Unfortunately, I don't know of a reference that I could point you to to substantiate any of this.

3) Increasing the in plane dimension of your columns should eventually make them reinforcable. I think.

4) If you could talk your contractor into some post tensioning, you could use balancing loads / drape to eliminate much of your dead load moment.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[czar07]

for:
2.) I used 0.7reduction but for crack only, how could reducing stiffness be helpful in reducing steel requirements, especially for spans without intermediate columns?
3.) Not applicable, my columns are limited to 400mm

How about precast slab along 11.45m (oneway distribution)?

 

[KootK]

how could reducing stiffness be helpful in reducing steel requirements, especially for spans without intermediate columns?

The moment coming into a joint is doled out to the various intersecting, rigidly connected members in proportion to member stiffnesses. If you soften your columns, they will shed moment to the beams which become comparatively stiffer.

How about precast slab along 11.45m (oneway distribution)?

Sure. That would reduce the moments coming into the columns.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.