RC stair support problem

[mar2805]

Hi folks!
Please see pictures attched.
The owner of the house wants to have a new entrance to the 1st floor that will be created with an RC stairs.
The planed solution is to transform the exsisting window of the 1st floor into entrance door.
RC stairs need to have an support at the 1st floor level.
How to solve this without casting a new RC frame under the upper end of the stairs?
I need to stick the steel bars into the exsisting slab that will provide continuity for the reinforcement....but how?
Any othe sugestion is more then welcome.
Thank you

 

[mar2805]

RC bars pictures

 

[TehMightyEngineer]

I've hung stairs using angle iron with embeded, anchoring studs into a precast concrete stairway. However after trying this method I don't recommend it for a number of reasons. Your method you show with the rebar is best.

I'd drill and epoxy some bars as you showed in your second post and analyze the strength as dowels per ACI 318 app. D. Should give you enough strength for the loads you'll need and not be too difficult to install.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[mar2805]

I never worked with epoxy :-(
The bars shoul be anchored at least 0.6 meters into exsisting slab.
Should this lenght be also achived with epoxy?

Another problem that I run when using this solution is that my lintel below newly created support doesnt have enough rebar to support the new vertical load thats created for the stair loads...

 

[TehMightyEngineer]

That will be hard to do with a drill to get them that far into the slab. What size bar are you using? You may be able to use a shorter length and have sufficient strength with the epoxy, or you might try using smaller bars more closely spaced.

As for the existing lintel you're going to have some problem with that. Reinforcement of existing concrete can be difficult. Easiest solution is to add a post under the area where the stairs connect to the lintel.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[KootK]

I never worked with epoxy sad

These situations pretty much demand it. Hilti has a new software offering dealing with this that you may find useful: Link

The bars shoul be anchored at least 0.6 meters into exsisting slab

That degree of embedment is not likely to be necessary. 600 mm would be pretty onerous for the contractor. It would also probably cause spalling in the concrete around the new rebar.

Another problem that I run when using this solution is that my lintel below newly created support doesnt have enough rebar to support the new vertical load thats created for the stair loads...

Some options depending on what your options are:

1) If you can call the support at the bottom of the stairs both a horizontal and vertical support, you can assume that the stair only applies a horizontal load to the beam at the top.

2) Install a new steel beam beneath the existing beam if there is space available.

3) Cast a few inches of new concrete and some new bottom steel (make it composite) below the existing beam if there is space available.

4) Attach steel channels to the sides of the existing beam for reinforcement.



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.

 

[mar2805]

@TehMightyEngineer
I will try using the HILTI software and see if smaller bars more closly spaced could be anchored into exsisiting concrete.
BUT as KootK mentioned the problem is that 60cm of hole driling into exsisting concrete slab could couse problems and if not impossible.
I dont have any knowledge in epoxy usage but I think I remebered that anchorage lenghts are much smaller when using epoxy
Correct?

"Easiest solution is to add a post under the area where the stairs connect to the lintel"

You mean to add a column under the lintel wich will in turn create an two bay lintel (beam with an support in the middle)?

Thanx a lot. WIll try it.

This is something taht has been confusing me also.
Since the stairs are cast after, it is clear that the stairs would probably create some king of horizontal force on the exsisiting lintel.
As you mentioned I would say that the lintel will probably have an horizontal reaction also...stair pushing it in horizontal direction since the lower support of the stairs is cast at the same time as the stairs.
I might be wrong.

This might be an easy solution but didnt we say that the stairs would create an horizontal force on the lintel!?
Dont see the point in using the steel beam under the lintel then...


Yes there is space available for this.
But would this 2 part beam behave as one single unit?

KootK coule you please explain this procedure a bit better.
Thank you.

 

[Leftwow]

This may be a dumb question, why make the bottom of stairs so rigid?

 

[TehMightyEngineer]

I dont have any knowledge in epoxy usage but I think I remebered that anchorage lenghts are much smaller when using epoxy
Correct?

Anchorage lengths are whatever you want, you don't need to develop the full strength of the bar with epoxy anchors. For your application you'll likely be controlled by concrete breakout of the top lintel or the stairway before you exceeded the epoxy anchorage strength. So, you'll likely find that your embedment length doesn't make a different after a point.

You mean to add a column under the lintel wich will in turn create an two bay lintel (beam with an support in the middle)?

Yes, the area behind the stairs is probably useless space anyway but I don't know what your architectural constraints are. KootK gave some other good options for reinforcement of the lintel but are more time consuming to design and build.

Thanx a lot. WIll try it.

Powers and Simpson also offers design software. They're all a little quirky unless you're very familiar with ACI 318 appendix D or the Eurocode equivalent.

Most of your other questions rely on an understanding of shear flow and how to design built-up members to act monolythically. If you're not familiar with the method you could learn it but I would caution against applying the principles without a full understanding of it.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[nonplussed]

My take on the situation:
Firstly, try and convince the owner to use light weight steel or timber stairs (my favourite option ). If the existing lintel is inadequate to support the extra load then obviously they will need a column under or an extra steel beam to support the load.

As for your rebar detail:
Make the soffit (bottom) of the stair concrete flush with the base of the lintel (so where the base of the lintel is, continue the concrete right until it intersects the stair). Drill and epoxy bars 300mm CENTRAL into the lintel concrete. It needs to be central so that you won't accidentally drill through the existing lintel top rebar. You only need 300mm or even less embedment since this connection doesn't need to be moment transfer, it can be pinned. So the rebar just needs to be a shear transfer hinge. It can be pinned because the lintel should be supporting the full weight of the stair anyway. You would only need a moment connection if the lintel wasn't there and you needed to transfer the load completely to the left end of the slab.

That's what I would do anyway. There's probably a thousand different ways.

 

[KootK]

This might be an easy solution but didnt we say that the stairs would create an horizontal force on the lintel!?

The lateral thrust will go directly into the floor diaphgragm and not the lintel. As such, the lintel ought not be designed for that load.

But would this 2 part beam behave as one single unit?

Making the beams composite would be your task as the designer. Drilling in some dowels that cross the new/existing concrete would probably be the way to go.

KootK could you please explain this procedure a bit better.

You bolt a channel to the side(s) of the lintel and allow said channels to share in resisting the load applied to the lintel. Again, you need space to accomplish this.

This may be a dumb question, why make the bottom of stairs so rigid?

In this way, one can consider the bottom of the stair pin supported and the top of the stair horizontal roller supported. In this scenario, there would be no vertical load imposed by the stair on the lintel.

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.

 

[mar2805]

Cant do that since the there is the window below that wide almost as the stairs alone.

nonplussed if you could make a small sketch of this I would be very grateful.

You mean to have a system that would behave like this picture attached?
This is the problem that confuses me.
Suppose that I dont make bar anchores into the egsisting concrete at all.
I just finish all of my stair reinforcement at the stairs end and cast the new concrete that will now be touching the old slab and the lintel.
So there is NO reinforcement connection at all!
Wouldnt the whole system now behave as if you had an system like marked in the attached picture?

 

[nonplussed]

Wouldnt the whole system now behave as if you had an system like marked in the attached picture?

Theoretically your model is correct, but it is practically very dangerous. If there is any horizontal movement in the supports at all then the stair will collapse. What if there was an earthquake, or if a builder modified the top slab in the future without fully understanding the importance of the lateral restraint. Is the top slab even capable of resisting the horizontal load without deflection horizontally... many potential problems.

Just stiffen the lintel somehow. The architect is being silly and stereotypical in thinking they can modify the building and add load without repercussions.

 

[KootK]

Wouldnt the whole system now behave as if you had an system like marked in the attached picture?

Yes, that is exactly right and that would really be the goal of the strategy. The way that I see it, your stair has four supports and not one of them is really a pin or a roller. Rather, they're springs of varying stiffness as follows:

1) Bottom vertical spring support: stiff.
2) Bottom horizontal spring support: less stiff but depends.
3) Top vertical spring support: less stiff but depends.
4) Top horizontal spring support: stiff.

I think that it's prudent to take realistic stock of your support stiffnesses and account for those stiffnesses in your design accordingly. It's very likely that your stair will deliver a horizontal thrust into your floor diaphragm. Plan accordingly. And, until you get some real horizontal movement at the bottom of the stair, it's unlikely that your lintel will see much vertical load at all. In highrise concrete buildings, you often find that stair landing supports are really quite skimpy. This generally works out okay because of the same principle. It also makes thin, stairs that are dog-legged in elevation work.

but it is practically very dangerous. If there is any horizontal movement in the supports at all then the stair will collapse.

I understand the sentiment but disagree to some extent. For the stair to actually collapse, enough horizontal movement would need to occur to allow the stair to slide past the lintel/floor diaphragm. That's quite a bit of movement. I'm not saying that I'd forgo any vertical connection between stair and lintel of course. But I might just use that as belt and suspenders. And like I said, no matter how much we stiffen the lintel, there's a very real possibility of serious thrust in the floor diaphragm.

Frankly, my preferred method here probably would be lintel stiffening simply because I wouldn't want to have to mess around with the foundations to make them suitably rigid horizontally. And that's definitely what I'd do were this all new build. That being said, I feel that the mark of success in renovation scenarios is the ability to creatively mobilize unconventional load paths in order to achieve architectural goals and save owner $$$. It's tough to beat a "do nothing" reinforcement strategy. When you manage it, it's a win and a feather in your cap.











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.

 

[mar2805]

I cant agree on this.
If the stairs are design for the ultimate bending moment as an simple beam and reinforced to that moment the stairs wont break BUT they could "fall down".
But as KookT said we are talking BIG horizontal movements for this scenario to happen.
I think even if soil gets pushed 2 inches in the horitontal direction, you wont see many problems except small vertical deflection in the upper part of the stairs.
Let not forget that passive earth pressure are always greater then active.
Soil gets compressed here.

But whats the point of conecting reinforcement of the new stairs with the egsisting slab?

I can imagine reinforcement activating only if there comes to horizontal movement in stair foundation wich in return will create vertical deflection in the upper part of the slab.
Same thing goes if the stair foundation settles.
Only then I can understand that there will be an vertical load transfer to the exsisting lintel.
As you mention, pinned connections get activacted!

WITHOUT ANY HORIZONTAL DEFLECTION IN THE STAIRS, THERE WONT BE ANY VERTICAL LOADS ON THE LINTEL.

Will this ever happen in reality?
I think probably never, maybe if your Building your foundations on rocks.

 

[KootK]

But whats the point of conecting reinforcement of the new stairs with the egsisting slab?

This may sound contradictory coming from me but, essentially, I'd connect the stair to the slab in acknowledgement of all of the same concerns that nonplused expressed. I agree, the most likely "problem" would be the development of a vertical offset between the top of the stair and the top of the floor slab. And that would represent a rather problematic serviceability issue.

Will this ever happen in reality? smile. I think probably never, maybe if your Building your foundations on rocks.

It may well require foundation alternatives not common on residential projects. Some alternatives would include:

1) Using a foundation beneath the stair with something akin to a shear key.
2) Using CIP concrete piles with lateral load resistance.
3) Using a couple of battered screw piles beneath the bottom of the stair.
4) Casting a grade beam to connect the bottom of the stair to something else nearby that would restrain horizontal motion.

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.