steel beam reinforcing for opening in 2 way slab

[psustruct]

I have an existing 2 way slab with drop panels. The bays are 28'x28'. We are installing steel beams (with adhesive anchor) to the underside of the slab to frame out a large opening. The opening will be cut in the column strip. The opening is 7 feet wide (so it will take out the portion of the column strip that is left of the gridline.)

Am I right in thinking that I should size the steel beams for the tributary load = (1/2*distance from column centerline to new steel beam)? Is this a conservative approach?

 

[JedClampett]

What are your beams framing to? If you're just anchoring to the slab, I wouldn't trust it.

 

[psustruct]

They are framing into other (new) steel beams which are framing into the existing concrete columns.

 

[TXStructural]

Look at deflections. It is likely that you will either be supporting the slab entirely on the steel (if the steel has less deflections than slab), or the steel is deflecting such that the load will be mostly on the slab until it fails. Either model the entire system, or model the steel as a spring support for the slab, using the entire steel structure to determine the spring constant.

Since the hole will be cutting the column strip reinforcing, you lose some continuity in the column strips for adjacent slabs beyond columns. The slabs on each side of the column strip with the hole also lose continuity, so the design assumptions for these no longer hold. This is a large hole.

Look at ACI 318 Chapter 13 (specifically 13.4 - Openings in slab systems). 13.4.2.3 allows holes in column-middle areas that interrupt not more than 1/4 of the columns strip reinforcing. You are severing 1/2 of it. This makes the behavior different and requires use of finite element or other similar method.

If you can slide the hole over or turn it so that 1/4 of the columns trip is taken and the remainder is in the middle-middle, you could use FRP or a method to install an equivalent amount of steel to replace the cut bars. Carry the repair far enough to engage the cut steel by lap/development length beyond the opening. This allows the slab to continue to work (almost) as designed.

Remember that the slab may also be integral to the lateral system, and disturbing slab action could change this.

 

[BAretired]

Please provide a plan view. Thanks.

BA

 

[gobsmacked]

psustruct: The loads will not be attracted to the beams unless the beams have some support of their own. Think of yield line theory or Hillerborg for the remaining U-shaped slab, and you will see that the beams arguably carry no load. With a small hole (say, < 10% of the span) in reinforced concrete, I was taught to make the edge steel for the opening at least equal to the reinforcement displaced by the opening. That is a very different principle to the one you are suggesting. But this is not a small opening. Will anyone ever use the opening to install a staircase, and put a load on the beam that way?

 

[csd72]

a few comments:

Have you considered fire in the design of your adhesive anchors. This is a factor that is quite often overlooked in these types of designs.

I had a similar situation in a building a few years ago with 2 different holes, I managed to get away without steel supports on most sides by:

1. analaysing the before and after moment diagrams for the relevant column and middle strips a checking that there was adequate reinforcement and that any additional deflections due to lack of continuity were negligible.

2. In one case I added a renforced concrete wall along one edge to provide the moment continuity.

What I find best in these situations is to give 2 or 3 preliminary options including moving the hole to a better location and get the client to decide which option they would prefer. Usually the have no idea of the cost implications of their decisions and will be steered towards the cheapest option.

 

[enko]

How are the new steel beams connected to the existing concrete?
I would check first if that connection is adequate for the new load path.
After that, the steel design for the new layout should be easy including the tributary load you mentioned.

Please attach a sketch

 

[dcarr82775]

I wouldn't simply use tributary areas. Do an equivalent frame analysis that accounts for the missing concrete and added steel. You need to take into account the EI of the slab v the EI of the steel. In my experience the steel is more flexible than the concrete so it doesn't see as much load as you might hope.

Also, as TX noted, your hole will likely screw up the continuity of the slab and potentially require strengthening of adjacent spans.

 

[a2mfk]

Its been a while since I have done anything with flat plate, but they can be pretty tricky, especially when it comes to modifications. I agree with others, you really have to do a complete analysis of the entire slab with the openings. It cannot be simplified into trib areas like you would a one-way slab/deck type system, since the column strips are acting as continuous girders with negative reinforcement over the columns, etc. You may have bending stresses (or reversals) in places now that the slab was not designed for...

TX and CSD have given you the procedures.

 

[Brad805]

If you use SAP or some other 3D software you might want to consider a finite element analysis. If the slab is not huge, it is not a big deal to do. Connect the beams to the slab using rigid links and you can figure out the load proportioning with few assumptions. The beam EI is critical.

By the way, another option to reinforce the existing slab might be to use some Carbon Fibers like Sika has. Just a thought. If the project is large enough they will do a lot for you to convince you to present the concept to the owner. Eventhough the carbons are expensive, the savings to complete the install is typically significant and the owner does not loose a ton of headroom.

Brad

 

[psustruct]

Thank you, everyone, for your input. I agree with what you are saying. I figured there was much more to it. I just don't have much intuition for how the slab will react when the opening is cut.

My project manager told me to just take tributary loads and size the steel beams that way. He doesn't even want me to check to see how the slab will react with this opening.

 

[hokie66]

Your 'project manager' is a dill. You have been given good advice here.

 

[a2mfk]

I thought dill was a pickle, at least in the US. Hokie are you an Aussie? I have heard of dillweed but only on beavis and butthead.

Are you aware the "Hokies" are a college football team here? Virginia Tech.. My school Florida State plays them tomorrow. Their mascot is a turkey. <Shrug>

Good weekend boys!

 

[hokie66]

a2mfk,
Probably a colloquialism, I don't know where I picked it up, but to me 'dill' is synonymous with 'moron', so you get the drift.

You are new here, so you wouldn't know that I live in Australia (Brisbane), but yes indeed, I am a Hokie. And that mascot is now called the HokieBird. In case you are interested...

http://www.hokiesports.com/hokiebird/about.html

May the best team win. On second thought, GO HOKIES!

 

[TXStructural]

I've heard "dillweed" in lieu of "moron" for at least 30 years, so it is probably a regionalism gone international.

Think of the slab as a piece of relatively brittle, thin material. It is under bending stresses as it crosses the columns strip (which is really just a thin beam of a sort.) The columns strip in turn is the beam supporting the slab and giving continuity to the adjacent column strips.
If you weaken the columns strip by 1/2, you end up weakening not just the slab, but also the "beam" system connecting the columns. Where the slab cannot take the load (your stiffness is reduced along the lines of the hole), the columns may try to take over for the lost continuity. Some width of slab will try to offset for the loss and moments will redistribute, but you get the idea. Any redistribution will place stresses on members already at their design capacity.

Draw an elevation of the "frame" along the column line. Now, make the hole in the columns strip (1/2 strength, 1/2 stiffness) and see how you think it will act. Does your solution solve that action?

Draw a section in the perpendicular direction, again looking at the hole, and see if the fix solves those issues.

You can affix the steel to the slab in an attempt to replace continuity. I've designed such a hole (smaller in relation to the slab spans) using thru-bolts, flat bars and channels. It was never done, because of complexity, they decided that they didn't need holes after all.

My favorite design used a box of concrete beams around the hole; it was solidly connected to the slab (above the slab) to provide required flexural and shear capacity and stiffness, with a steel beam below one end extending to columns as needed for increased capacity.

 

[csd72]

In addition to the above info. here is a good reference:

http://www.concrete.org/members/ci2807mota.pdf

I forgot to mention a third way that you can approach this.

If the hole is the right size and at the right location it is possible to have a hole without any reinforcement.This only really works if there is flexibility.