How Can RC Two-Way Solid Slab on Beams Be Analysed with Higher Accuracy? 4

[Huck85]

Hi, everyone

I am a structural engineer graduated 2 years ago. Currently I'm working in construction management; however I like structural design (my major) and I am planning to pursue practicing after completing my current project.

I'm trying to analyze/design a reinforced concrete solid slab on beams flooring system for a single family residence (please check the attached drawing). As you can see, there are one-way and two-way slabs (Non-uniform loads) and the spans are different in each direction. Therefore, ACI Direct Design Method can't be applied considering its limitations.

I really wanted to understand the moment distribution and behavior of two-way slabs at various situations (different loads, spans, boundary conditions), so I referred to the ACI Code and read many books and material about the subject but they briefly talk about two-way slab action and then immediately explain the DDM and EFM methods and their limitations (Don't they realize that architects and owners don't care about those limitations during planning?). I just couldn't find a comprehensive explanation source for two-way slabs behavior and moment distribution that considers the various situations.

Lately I found a design method called Marcus' method that can be used for analyzing and designing two-way slabs with different spans and can be applied for the attached building plan? but I don't know how accurate Marcus Method is.

The questions include
1- Could you please tell me how to analyze and design two-way slabs on beams with non-uniform loads and spans (any sources I could read)? Is there something I am missing about this subject?

2- How accurate is Marcus' Method?

3- I just started studying Finite Element Analysis after hearing of its power. Still not finished with it yet but it seems to me that it depends on the assumptions and inputs one makes(0.0). What is the most accurate FEA model that best represents RC slabs-on-beams? Is there anything I should carefully pay attention to during slab modeling using FEA from your experience?

Your help is appreciated
Thanks in Advance

A. Huckamee

 

[Huck85]

The design is posted PDF file. I finished it months ago. It wasn't as complicated as i thought it would be. I guess the difficulty i found at the begining of this analysis is because i wasn't familiar in dealing with unusual layouts and the combination of one-way and two-way actions in one single system. Actually it was much easier than i thought it be.



I used slab thickness of 15 cm. 10-mm and 12-mm bars for the slab reinforcement. However, the owner wanted to use 12-mm and 14-mm bars instead of the 10-mm and 12-mm bars respectively. Apparently, a contractor had told him to use larger diameters to aviod quick corrosion of the reinforcement bars in case it ocurre!!!

Just didn't know what to say to that!

 

[TXStructural]

Dik, It was in Huck85's post. I guess my point was that there is no reason to try the three bars when it is prohibited by code. If the test "worked", the temptation would be to go ahead with a build that violates the standard and would likely not perform correctly.

 

[BAretired]

Huck85,

I think you should have a structural engineer review your design. It does not look quite right to me.

BA

 

[TXStructural]

The bar sizes you mention are European sizes, and those design codes and construction practices are significantly different from those in the US.
In the US, we no longer use truss bars, because producing and installing them is not economical here. Standard practice here is separate top and bottom bars. It results in slightly more rebar used, but the savings in fabrication, handling and placement cost can be substantial. About 75% of our installed cost of a structural concrete frame is formwork and labor, so uniformity in formwork and rebar layout usually results in overall savings.

Unless this slab was subject to corrosion from salts and moisture cycling, corrosion would not be an issue. I cannot imagine this would be an issue unless the house is above water or wet soils. Many contractors simply prefer slightly larger bars because they result in fewer ties and fewer supports. Larger bars also hold shape better under rough handling, and allow workers to step between them before the concrete is placed. Properly installed, even small diameter welded wire reinforcement (mesh) performs very well, so this is a red herring in most cases.

 

[rowingengineer]

Following on from Ba comment;
I do wonder why for all walls you show top reo bar one?
I do wonder what reo you will have at the stair location, I also wonder if these are concrete or steel stairs and if any extra detailing is required here?
I hope there is an experienced design drafter on this project as details are lacking and the only detail is fairly hard to follow as I would have thought there was a slab somewhere in the beam.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[dik]

If corrosion is an issue, then your slabs are minimal, and changing the bar size up will help, but is not a solution... just extends the time a tad... There are other issues, concrete cover, concrete, etc.

Dik

 

[ukengineer58]

Am i right in saying that you have no design experience, no one is reviewing your design however it is being built?x

 

[msquared48]

Yea. It's called "fast tracking", allowing the structure to fall down sooner.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[Huck85]

Dik,
The main reinforcement is what the slabs need to resist the loads imposed on them. Moreover note the slab with curved corner is reinforced in both directions. My analysis for the flooring system (by using Hillerborg Strip Method , Marcus Method and FEA then the ACI formulas) shows that all the slabs only need minimum reinforcement rho=.002 except the curved 5x5 m slab, there should be 5 Bars no.4 per meter. Dick what's not right with design except the usual details?

TX, you're absolutely correct. Both bent bar system and straight bar system are still around in my locality however the bent bar system (truss) is being replaced by the straight bar system. The contractor had told us that it's getting less and less that he builds slabs design of truss reinforcement and actually offered to construct the slabs with straight bar system with much lower cost. However, TX don't you find the truss system is stronger and better considering the benefits resulting from its continuity.

for anyone not liking the drafter. guys that is not meant as a complete workshop drawings.
It just illustrates my preliminary analysis of the slab and it only shows the main reinforcement. Unfortunely it's the only file I have right now in my iPhone which the device I am using to browse this forum these days. I don't have the final structural drawings they're in my computer but I'll post them as soon as I get back home from Germany. The design had been reviewed by a structral engineering professor and he improved it with adding more details that was lacking in the design based on ACI Code Details and Detailing of Concrete Reinforcement, however he approved the main reinforcement. As I mentioned in my first post. I work in construction management and the senior engineers at work refuse to take on any project lacking complete detailed workshop drawings it's definitely refused.

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Guys I am reaally surpised that even though I used 6 inch thick slab with a RC density of 155 pcf and Live load of 40-lb/ft2, yet the analyses resulted in minimum reinforcement requirement (rho=.002) in all the slabs except the 16.5 by 16.5 ft slab . I was wondering if that can be interpreted* as the concrete is capable on its own of resisting the tensile stresses resluting from those small moments developed in the slabs?

* I DO KNOW CONCRETE TENSILE STRENGTH IS COMPLETELY NEGLECTED AND CONCRETE NEEDS STEEL TO HELP IT DEAL WITH THE SHRINKAGE, CRACKS AND SUDDEN HIGH LOADS.

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Regards
A. Huckamee

 

[ukengineer58]

Min steel is to ensure ductile failure. without it the dlab would just explode. never rely on concrete tensile resistance in a suspended situation. sometimes acceptable for footings etc