RCC Flat Slab Over Steel Columns

[M.Arslan Umar]

AoA

Dear members, I have a few questions regarding the topic mentioned above. 

Is there possibility of resting an RCC flat slab over steel columns? Is there any research paper/article regarding that? What and how will the connection be made?  What will be the connection response against sesimic activity? 

Thanks and regards 

[Simple Structures]

If you are in Pakistan then i would be tempted to use "Precast Concrete Columns" rather than Steel Columns due to limited availability of heavy sections; Steel Sections are used for speed on construction.

Nevertheless: I have designed a steel column / concrete flat slab (plate) configuration previously. This is your best course of action:

1. Decide which "design code" you will use for the design, and follow its procedure for slab shear check; This is key;

2. The steel columns will require short cantilever sections coming out into slab;

3. Make sure you can feed reinforcement through the steel section or above and below it and get adequate concrete cover;

4. Check slab shear - you will require lines of shear (vertical) reinforcement within slab, successive perimeters, around the column;

Slab shear check around column is key! 

 

 

[UmarMakhzumi]

On 5/26/2020 at 2:11 AM, M.Arslan Umar said:

AoA

Dear members, I have a few questions regarding the topic mentioned above. 

Is there possibility of resting an RCC flat slab over steel columns? Is there any research paper/article regarding that? What and how will the connection be made?  What will be the connection response against sesimic activity? 

Thanks and regards 

The situation here is very similar to where foundations are placed on steel piles (although there is no continuity for piles to top like columns can have another column on top of slab). I can explain to you what is done for the pile connection. Normally a cap plate is used on top of the pile and dowels are welded to the cap plate that get embedded in the foundation concrete. The dowels can be designed for shear and tension forces (Moment is resolved into tension). I haven't done a configuration in slab but for piles, I have done a lot of these.

Regards,

Makhzumi.

[M.Arslan Umar]

On 6/1/2020 at 4:01 AM, UmarMakhzumi said:

The situation here is very similar to where foundations are placed on steel piles (although there is no continuity for piles to top like columns can have another column on top of slab). I can explain to you what is done for the pile connection. Normally a cap plate is used on top of the pile and dowels are welded to the cap plate that get embedded in the foundation concrete. The dowels can be designed for shear and tension forces (Moment is resolved into tension). I haven't done a configuration in slab but for piles, I have done a lot of these.

Regards,

Makhzumi.

AoA,

Sir can you please share the connection details of Steel pile and cap plate?

Regards

[M.Arslan Umar]

On 5/26/2020 at 4:41 PM, Simple Structures said:

If you are in Pakistan then i would be tempted to use "Precast Concrete Columns" rather than Steel Columns due to limited availability of heavy sections; Steel Sections are used for speed on construction.

Nevertheless: I have designed a steel column / concrete flat slab (plate) configuration previously. This is your best course of action:

1. Decide which "design code" you will use for the design, and follow its procedure for slab shear check; This is key;

2. The steel columns will require short cantilever sections coming out into slab;

3. Make sure you can feed reinforcement through the steel section or above and below it and get adequate concrete cover;

4. Check slab shear - you will require lines of shear (vertical) reinforcement within slab, successive perimeters, around the column;

Slab shear check around column is key! 

 

 

AoA

Can the steel plate (connected to column) replace the cantilever sections. If not, then which type of sections would be more suitable?   

[Fikri]

Quote

The situation here is very similar to where foundations are placed on steel piles (although there is no continuity for piles to top like columns can have another column on top of slab). I can explain to you what is done for the pile connection. Normally a cap plate is used on top of the pile and dowels are welded to the cap plate that get embedded in the foundation concrete. The dowels can be designed for shear and tension forces (Moment is resolved into tension). I haven't done a configuration in slab but for piles, I have done a lot of these.

Regards,

Makhzumi.

i've just read this topic and i started to search through google.
i found this paper .
https://www.mdpi.com/2076-3417/6/11/317/htm

this is interesting.
So, in your project, what is your consideration for choosing this typical of pile (steel piles).
 

[Simple Structures]

@M.Arslan Umar

I attach two sketches which I did over my tea break:

1. Shows the steel column supporting a flat slab (flat plate) with no down stand beams.

2. Shows steel column with welded steel cantilever 750mm long beams cast within slab depth - no down stand beams.

You can also carry the column above to the next slab level up.

 Structural Engineer - key design decisions

1. What design code am I designing to? ACI or Eurocode - follow their procedure for flat plate / flat slab design.

2. Make sue slab is deep enough to house steel sections.

3. The key thing with flat slab design to make sure there is shear links/stirrups in slab around the column - again follow code procedure!

Sadly, Pakistan does not have code; Many practicing Engineers follow American codes - different versions ACI 318-19; ACI 318-11 etc. I prefer to use the Eurocodes - easier to follow for practical design! Sadly, in Pakistan we are obsessed with US stuff - status thing I think.

Sorry about poor quality of sketches... done on the kitchen table on paper hand towels!

 

 

 

 

[Ayesha]

SimpleStructurs,

Good sketches. I will just add:

1) For Option 1, adding Nelson studs for embedded steel members would beneficial.

2) For Option 2, the dowels are generally aligned with column circumference for circular columns or on flanges for W columns so that the moment due to tension can directly transfer to column else plate needs to be designed for that force and based on the arrangement of dowels the column to beam connection can behave as pin.

 

[Arslan Umar]

On 6/13/2020 at 12:21 AM, Simple Structures said:

@M.Arslan Umar

I attach two sketches which I did over my tea break:

1. Shows the steel column supporting a flat slab (flat plate) with no down stand beams.

2. Shows steel column with welded steel cantilever 750mm long beams cast within slab depth - no down stand beams.

You can also carry the column above to the next slab level up.

 Structural Engineer - key design decisions

1. What design code am I designing to? ACI or Eurocode - follow their procedure for flat plate / flat slab design.

2. Make sue slab is deep enough to house steel sections.

3. The key thing with flat slab design to make sure there is shear links/stirrups in slab around the column - again follow code procedure!

Sadly, Pakistan does not have code; Many practicing Engineers follow American codes - different versions ACI 318-19; ACI 318-11 etc. I prefer to use the Eurocodes - easier to follow for practical design! Sadly, in Pakistan we are obsessed with US stuff - status thing I think.

Sorry about poor quality of sketches... done on the kitchen table on paper hand towels!

 

 

 

 

AoA 

Thanks a lot for sketches and explanation. I have now few queries regarding its implementation via ETABS. Actually I want to design a 30ftx30ft flat slab (Post Tensioned)  with steel columns on four edges only(Two story) .  My queries are:

1) Is this type of modeling possible in ETABS with connection details shown in sketches. I know the shear reinforcement using shear studs could be provided, but as per this design how the connection will be modeled and analyzed?

2) If I dont model the cantiliver sections and just draw slab over steel columns,  will this connection be ok.  Even if not,  can I use the forces (shear and moment) after analysis to manually design the connection as shown in sketch?

3) If I model HSS section as columns and provide I Sections as cantilever, and then draw slab over it,  is this modeling technically ok? 

Thanks

[Waqar Saleem]

Salam Arslan,

I don't think that type of modeling is possible in ETABS especially the connection one, PT slab could be modeled in ETABS, you can have Composite beam and column design so you can define composite sections but you will have to check the connection manually after taking forces from ETABS.

 

 

Regards

[Arslan Umar]

3 minutes ago, waqar saleem said:

Salam Arslan,

I don't think that type of modeling is possible in ETABS especially the connection one, PT slab could be modeled in ETABS, you will have to check the connection manually after taking forces from ETABS.

 

Regards

In that case,  the connection will be designed based on the forces (Moment and shear) in column or from the cantiliver sections. 

Secondly in Etabs if I draw these cantiliver sections with the column,  the punching shear is not calculated(N/C) during slab design.  Is this ok? Will punching shear check be neglected in that case? 

Regards 

[Waqar Saleem]

in case of composite beams at connection i dont think there will be punching.

[Simple Structures]

I think we are mixing a lot of things up here.... one must first feel comfortable designing a structure by hand, then rely on software. I will carry on with my previous numbering so you see the whole picture.

4. You must ask yourself, could I design a flat slab concrete structure by hand by determining moment and shear coefficients for continuous slab case for vertical gravity loading. This will give you both shear and moment value. Each CODE has rules for splitting the slab into "column strips" and slab strips" to determine width of slab for each hand calc. Decide which code you are going to use and follow its "design rules" for "flat slabs" / "flat plates".

5. With flat slab structure (no down stand beams) you will always require shear reinforcement in successive perimeters in the slab around the column. The exception would be if your column was say very large in plan size, or you thicken the slab around the column. Again, follow code approach, you should be able to do quick check by hand!

6. Next, how is the lateral stability of the structure achieved - shear walls or lift shaft? This is key as all lateral loads will go to these vertical elements via the floor slab diaphragm, limiting lateral load moments on the column at junction with slab. In earthquake regions i would plan lateral stability system first!

7. PT slabs require specialist PT tendons and grouts, quality control etc. They are often used to "thin" the slab section. They are expensive and require specialist installation. If you have all that (not sure if its economical for two storey building slab, unless it’s very heavily loaded, and has large plan area) then I would now even pursue this option. 

8. Instead of PT slab, I would consider down stand beams to cut down slab span. There are various slab types which will span 9m (30'), or a coffered slab (designed same way as flat slab).

9. For your analysis, do not put in the 750mm long cantilevers, and design slab shear around the column.

10. Get the code, then the code handbook, then design. ETABS it seems to me have done a very good selling job in PK!

Design is in three steps: Crawling (college and uni studies), then walking (understand all structural concepts and design concepts by hand) and then running (rely on software only when you can judge if the output is in the "correct ballpark", by looking at the results! 

a. Will span say 7-8m (24'), approx 300mm thick,no beams

b. Will span 10m (30'), 600 o/all thick, beams within depth

 

Or a (c) Traditional beam and slab system will suffice.

Or (d) if you local expertise available, then use PT slabs (the tendons need to be protected during building life from any damage, say someone drilling a hole into the tendon during building life, and also special techniques are required to demolish PT buildings.

 

 

 

[Simple Structures]

PS: I would model the connection same way you would a concrete column/slab connection and then "detail" the connection by hand to achieve the connection behaviour assumed in the analysis.

[UmarMakhzumi]

On 6/22/2020 at 10:53 PM, Simple Structures said:

PS: I would model the connection same way you would a concrete column/slab connection and then "detail" the connection by hand to achieve the connection behaviour assumed in the analysis.

That is correct. You just model slab to column like normally you would model. Analyse your ETABS model, get maximum shear and moment for the worst cases that would be max tension and max shear. Now you have to provide a mechanism to transfer that moment and shear from the slab to the column.

Which you can do by using weldable rebars. Provide an end plate to column. See the sketch. Sizes where shown are just for example as I have no idea what your loads are.

**Update**

I also edited my sketch to show concrete breakout that you will need to check per ACI Code. It is also a good idea to provide shear ties for that, but you will need to develop Ldh for ties too. Also added circle on weld symbol to show all around welding.

The break out line is show incorrect, as it should start from reinforcing bar,not end of plate. Sorry but hand markups using mouse are tough.