Steps in structural slab 2

[canstructural]

Hi,

Please see the attached sketch and comment on acceptability of the rebar details at step location. All suggestions are wellcome. I need to know if someone has practically done any other detail for large spans and for steps larger than slab thickness.

 

[paddingtongreen]

swazimatt, being an "old-timer", I would use moment distribution on a 1'-0" wide strip. Any method for statically indeterminate structures will do.

In short, I would put a support under the step and work out the moments and reactions, then I would remove the support and load the step with a load, the magnitude of the reaction I just removed. Then I would add the results of the two cases. All this on paper, of course.

Timing has a lot to do with the outcome of a rain dance.

 

[canstructural]

asixth,

That step is about 50 ft long, so no possibility of treating it as a beam and no possibility of column too in that line. It's architectural requiement here, i already fight for this early to avoid this situation.
Thanks.

 

[paddingtongreen]

canstructural, if the slab is supported at either end of the 50', you have the complication of two way action on the side strips, and the step will try to behave as a beam.

Timing has a lot to do with the outcome of a rain dance.

 

[canstructural]

paddingtongreen,
There is nothing there to support that step. All galzed curtain walls below with deflection track.

 

[hokie66]

canstructural,

I wouldn't build a sawtooth stair like that without continuous support at the sides.

As to your slab fold, I have seen a lots of them built with a detail similar to your Option 2, without any problems to my knowledge. However, research has shown that these joints are not as efficient as the straight slab, particularly for the opening joints, which in your case would be the lefthand part. European practice dictates the use of 'hairpin' bars, and Nilsson in 1972 conducted tests showing about 80% efficiency with that arrangement. Further tests showed that 100% efficiency could be obtained with transverse stirrups and tension diagonals.

 

[haynewp]

I was going to mention that also, the diagonal bar adds considerable efficiency to the opening joint. The efficiency of the joint is also a function of the amount of reinforcing percentage you are using on either side of the joint. MacGregor has plots on this taken from the Nilsson reference. The closing joint is also not 100% efficient either.

 

[dhengr]

Canstructural:
Run the important pieces of reinf’g. stl. past the reentrant (inside) corners and embed and develop them in at the far face, and around a bar running parallel to the step. Maybe add a few extra pieces of reinf’g. stl. over the adjacent col./bm. lines to beef them up a bit (some two way slab action) and allow the slab to cantilever, somewhat, to the up turned or down turned discontinuity. Good luck on your stepped slab. If you don’t mind too strenuously, I would like to steal your thread for a moment.

Paddingtongreen:
Boy, you must be an old guy, like me. You actually know what moment distribution is and how to do those calcs.? Maybe that’s how we gained some intuitive understanding of how structures really work, deflect and react. Not that we always got it right on the first try though. Some of the younger engineers don’t have that same intuition, as early in their careers, because the computer doesn’t require that same thought process. Of course, with the computer we can change a few sizes and variables and rerun it in less time than it took us to readjust our slide rules.

Hokkie66:
In another thread, you mentioned that Prof. Richard Barker, drummed your early concrete design into your head during several college courses. I took my first concrete design course from him in about fall or winter of 1963, but that spring he left the Univ. of Minn., and apparently went directly to Virginia Tech. He was really a good teacher and a nice guy, I had some contact with him a couple of times after he left here. I don’t think he was a full Prof. here, and the course I took from him was an elementary conc. design course for architecture students. That’s what I thought I wanted to do with my life until I took his course and similar elementary courses in steel and timber. Seemed that I was better at and liked structures more than arch’r. I never could figure out how the make it canti. 300' without some structure, but that’s what our arch. profs. wanted in our arch. designs.

 

[hokie66]

dhengr,

Yes, UM's loss was VaTech's gain. I took my first course taught by him in 1964, so your memory would be correct.

http://www.cee.vt.edu/index.php?do=...=2&id=17&pid=8ec9004ffe6042b02648b6ad127b329b

 

[canstructural]

Hookie,

Anyways what your concerns are? No supports are allowed at sides of steps to make a beam spanning 50ft.

dhengr,
Attached sketch for what you meant, somtimes interpreting words are dangerous.

 

[hokie66]

canstructural,

I did not suggest making the slab fold a beam. That was someone else. My comment about sawtooth stairs was just that I wouldn't do it. You introduced the idea of sawtooth stairs to the discussion, and there is no logical reason to build sawtooth stairs spanning parallel to the flight.

 

[canstructural]

Anyways that chart represented the situation, sometimes you have to face based on your situtation. You are not allowed all the times to make slab cantilever from support.

 

[hokie66]

I didn't say anything about cantilevering stair flights. But as to the chart and details you provided, I wouldn't put much stock in that reference. Where did it come from? What is wrong with making the soffit a flat plane?

 

[slickdeals]

GO HOKIES

 

[paddingtongreen]

I decided that I would go back to the outline and start over. First thing I realised is that there will a day joint that needs a roughened surface. The bars and splices needed reworking accordingly. It is bad practise to put bends in long bars unless absolutely necessary, so I put them in the dowels. I've lost my feel for rebar size, and I believe that minimum lap lengths have increased since I retired. I didn't try for a cage because Of the uncertainty of the rebar sizes and the consequent minimum bend radii.

Let's make this a new starting point and offer some improvements on this for canstructural.

It's funny, I "speak" English English and American English; I just came from an English site and kept on using English spelling when I got here.

Timing has a lot to do with the outcome of a rain dance.

 

[paddingtongreen]

Oooops.
I forgot the diagonal bars, I usually add #5@12" at re-entrant corners in tension, always provided there is room.

That would be #16@300, but if "canstructural" indicates Canada, it would be 15M@300

Timing has a lot to do with the outcome of a rain dance.

 

[hokie66]

I don't think most builders today would want a construction joint at this location unless it otherwise fit their programme. The top form can be supported.

 

[paddingtongreen]

I'll bow to your more recent experience, reluctantly. That dam will be fifty feet across, a super structure to support it seems like an awful lot of work to avoid a day joint. Then I wonder about zero slump concrete because that step must be placed in one go or there will be dried (rather than cured) concrete sticking to the upper bars if they try to just place the bottom 10" or 12" and then come back after it sets. They are not permitted to place the concrete and then move it horizontally under the dam, but if they do, I would expect some voids on the tension side of that joint from lack of compaction.

Anyway, one "Z" bar can be used instead of two "L" bars if the joint is eliminated.

Just thinking about it, the fast track guys I worked with would have set the rebar for the lower slab and the dowels for the step and would be finishing off the rebar in the upper slab while the concrete was placed in the lower slab.

Okay rant over.

BTW, hokie66, you spelled programme English style after I went on about switching back and forth.

Timing has a lot to do with the outcome of a rain dance.

 

[hokie66]

Yeah, just because I now live in Australia, and stay confused about spelling most of the time.