Flat Slab Edge column connections
Flat Slab Edge column connections
(OP)
Hi Guys
Simple question: When doing a flat slab design, do you design the edge column to slab connections as rigid/fixed or simply supported? I have spoken to people with differing opinions on this. Do you really need the hogging reinforcement? If designed as simple support, the top rebar should yield, the concrete crack and the bending moment transfer to midspan as for a simple beam.
The same question for cases where you have RC transfer beams carrying massive loads fixed to columns. Can you design this as simply supported to get rid of the congestion of rebar at the supports?
Lastly, where flat slabs tie into walls, again, does this need to be designed as a rigid connection? I would think not.
Please let me know your opinions.
Regards
Seb
Simple question: When doing a flat slab design, do you design the edge column to slab connections as rigid/fixed or simply supported? I have spoken to people with differing opinions on this. Do you really need the hogging reinforcement? If designed as simple support, the top rebar should yield, the concrete crack and the bending moment transfer to midspan as for a simple beam.
The same question for cases where you have RC transfer beams carrying massive loads fixed to columns. Can you design this as simply supported to get rid of the congestion of rebar at the supports?
Lastly, where flat slabs tie into walls, again, does this need to be designed as a rigid connection? I would think not.
Please let me know your opinions.
Regards
Seb






RE: Flat Slab Edge column connections
When you rely in an entirely separate lateral resisting system, say shearwalls or braced frames, I find more acceptable to model as hinges the supports at exterior columns, specially in not highrise buildings where the lateral displacement may be reduced with such systems and so any effects of the notional hinge small. In spite of that here in Spain has been common (for framed, not particularly flat plate, buildings) to reduce much the moment at exterior supports respect that produced by elastic fixity. Again the visual effect in common dwelling buildings is nil, and you see there more "crack" due to the separate concreting of column and floor than mechanical cracking from overstress.
Respect transfer beams with important loads, yes, it is common to make use of hinges or reduced fixity at ends to ease some constructive details. You may have also cases with elastic fixity and well, as I have practiced and seen them have not resulted as bad as one could at first sight think. Taking unto account continuity also helps the lateral resisting system, for here, the buildings not being very tall, definitions of lateral resisting systems as separate entities are not as differentiated as in the US, and everything works together. I also have designed hinges for ends of steel multispan transfer beams to deliver forces to concrete through an anchor plate backed by an array of studs in columns, and bearing in sockets in walls, and again, no problem. The big stiffness of the transfer beams has much to say on this since it forces not much the respective housings other than in the devised manner. It is in the same manner that a shearwall protects weaker beams with ends designed as hinges, by diminishing the displacement excursions.
Yor last question is maybe where we may produce a more tolerant design, since we have plenty of support. You may design the node as simply supported, you, from your questions, are aware of the cracking issues, so it is a matter of preference and, for dwellings at least, minimum geometrical rebar should control the issue to limited proportion. In any case since the trend is to mandate quite significant minimum rebar anywhere and specially where tension appears, I have sometimes (much times) used elastic fixity atop RC walls, and I remember once it was to the surprise of some insurance reviewing party that was criticizing bottom rebar of the end spans without thinking I have done such thing.
Maybe after this account of my experience, the bottom line is that you can take almost the whole gamut of fixities and still have satisfactory buildings; that some may sastisfy some particular code or preference is an altogether different thing.
RE: Flat Slab Edge column connections
Flat slab always re-distribute the stresses
in some cases where flat slab deflection is critical , i assumed fixed connection with column/wall and should design the column for this moment and reinforce the slab as well.
For RCC beam framing to column ,if no negative steel is provided beam would crack at that pint and hinge/pinned connection would form,this should be considered as hinge in lateral force resisting system as well
RE: Flat Slab Edge column connections
2. If you are going to redistribute from negative moment to positive moment you will need to thicken the slab to account for the extra deflections.
RE: Flat Slab Edge column connections
I have also seen in the post-tensioning manual that a column can be assumed to be cracked and beam designed for positive moment. However, they recommend using closely spaced ties at the top of the column. I will try to scan and post that page from the PTI Manual.
RE: Flat Slab Edge column connections
RE: Flat Slab Edge column connections
For the original question:
If you "let" the slab crack with little or no top steel, I'd be worried about shear transfer - a vertical tension crack due to top moment bending sure seems to me to reduce shear capacity significantly.
RE: Flat Slab Edge column connections
By letting the column crack at the location, you are reducing the joint stiffness and not affecting the slab shear transfer. I am away from the office and will try to scan the page from the PTI manual that makes specific reference to a situation such as this.
RE: Flat Slab Edge column connections
RE: Flat Slab Edge column connections
I was responding to the comments above that talked about minimizing (or leaving out!) negative SLAB reinforcement.
Suggesting a crack in the slab at the column, not in the column itself.
RE: Flat Slab Edge column connections
RE: Flat Slab Edge column connections
For a general flat slab, when doing edge column design, would you just allow for nominal eccentricity of 20mm as per BS8110 and use this moment? This is prior to doing a full scale FEM analysis and finding out what the actual moment transferred to the column is. I am only doing preliminary design.
If say I have 2000kN on edge and internal columns, can I design the columns for the axial load plus 0.02x2000 = 40kNm?
When checking punching shear at edge columns I do increase the shear applied by factors as recommended in the BS.
RE: Flat Slab Edge column connections
For you latest question as to preliminary design, you need to be very careful here, If you size you columns for axial load only and you're working on a talk building, you column will shrinkage and creep further than your core/shear walls. You want to design your columns and shear walls for strength but then also ensure the average stress is the similar.
I also recommend reading :
Manual for the design of concrete building structures to Eurocode 2
FAQ507-1574: Rules of thumb (I am in the process of updating the FAQ's at the moment so take a look in a few days and the info should be better)
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