Raft Design For A 2 Basement+G+24 Story Building

[G_Farooq]

I am designing a 2B+G+24 story building. I made the model in Etabs, and ran it, then exported the base reaction to safe for footing design. It has central core walls. When I ran safe (v8), the only version available with the office. It gave me a bearing pressure of about 14 ksf as opposed to allowable of 6 ksf, I increased the thickness from 4 ft to 9 ft gradually but pressure distribution remained same & value only deceased to around 11 ksf. Then I added foundation beams (tied up all columns provided them under all core walls) and assigned line support to them (Kv x width of beam). This run gave me a value of 5.6 ksf bearing pressure which is safe. But I have some questions. I calculated the total area required from total load of the building and its coming out almost equal to the building plot area available. For, punching requirement under single column I at least require a 5.5 ft deep raft thickness.

1. Can I model a raft like this with beams (12" x 72")  under all columns (42" x 42") & (36x72") under core walls (24" thick); not inverted beams but beams with top flushed with raft top

3. when we define slab properties do I have to tick the " thick plate" option or not. What is the difference?

4. What is difference between slab types options? slab? footing? mat?

2. Do the slab elements between these beams will have a soil support also assigned to them?

3. Is this type of foundation safe for this high building? or do we have to go for piles or pile/raft interaction? Which would you recommend considering that top of raft is at -19 ft & ground water table is at -11 ft.

4. How to I assign wall supports in safe v8. By placing beams of above property on my raft slab & assigning a wall/line support to them (Kv x width of beam) or Assigning null property to beams and use safe v8 option in which it calculate support stiffness from its size. Kv is the vertical sub-grade modulus. 

 

5. How do we detail thick rafts. Do we provide mid layer of steel. if so can somebody point a reference book or source on how to calculate it. 

 

Confused

[UmarMakhzumi]

I will try to answer some of your questions:

 

Make sure that when you model your shear/ core wall in ETABS, you have meshed it well, so that the wall has enough well distributed support along the length. Further reading..

 

1. Can I model a raft like this with beams (12" x 72")  under all columns (42" x 42") & (36x72") under core walls (24" thick); not inverted beams but beams with top flushed with raft top

Yes, you can; raft is an inverted slab!

 

2. Do the slab elements between these beams will have a soil support also assigned to them?

Yes they would. Assign them. Further reading...

 

3. when we define slab properties do I have to tick the " thick plate" option or not. What is the difference?

 

Generically, thick Plate should account for shear deformations in your raft/ mat; Increase your moment and shear. I would suggest you to look into SAFE manual to see the exact difference. If in doubt, keep it checked.

 

4. What is difference between slab types options? slab? footing? mat?

Is your question from a software point of view or general? 

 

4. How to I assign wall supports in safe v8. By placing beams of above property on my raft slab & assigning a wall/line support to them (Kv x width of beam) or Assigning null property to beams and use safe v8 option in which it calculate support stiffness from its size. Kv is the vertical sub-grade modulus. 

I wouldn't bother assigning any supports to beams. Would just assign area springs to slabs and run it. 

 

5. How do we detail thick rafts. Do we provide mid layer of steel. if so can somebody point a reference book or source on how to calculate it.

There is no hard and fast rule and things depend on how thick your mat is.  Mid layer can be provided for very thick mats. I have also provided say #5 @ 12" c/c if the mat is very deep for shrinkage control. Make sure you provide skin reinforcement on the sides of the mat and horizontal perimeter bars around it. 

 

You will always get areas of high stress under high loads. You can neglect some of the high concentrations by judgement. Also, your mesh size will effect your results and avg stress values.

 

Good Luck!

[G_Farooq]

Thank you very much for your reply. I have rechecked my Etabs model & tried different shear wall configurations and then checked the raft on safe (flat slab with drop panels, beams were confusing me). My base pressure results remain in the range from 7 to 10 ksf. Then a senior in my office told me to draw a layer of lean concrete under my raft plate, slightly bigger than it, with 6" thick & no design. The draw my raft slab, over it, then my drop panel or thickened sections over it. Select all 3 slab properties & assign soil support to them. Pick up column points, right click & assign column sizes to the loads. And my base pressur comes down to 4.5 ksf.

 

I can understand the logic of assigning soil support to all 3 as they are overlapping each & sfae will take then which is uppermost & thicker, so assign soil support to all. Next by providing column sizes we are ensuring that safe take into account the rigid zone for column in base pressure, punching & bending. But what is the logic of providing the lean layer? How does it control the base pressure?

 

Second can we ignore the reinforcement peaks in slab strips under column & core wall edge points? When I calculate reinforcement from strip moments manually the value is much lower than the one reported. How to resolve this?

[UmarMakhzumi]

Thank you very much for your reply. I have rechecked my Etabs model & tried different shear wall configurations and then checked the raft on safe (flat slab with drop panels, beams were confusing me). My base pressure results remain in the range from 7 to 10 ksf. Then a senior in my office told me to draw a layer of lean concrete under my raft plate, slightly bigger than it, with 6" thick & no design. The draw my raft slab, over it, then my drop panel or thickened sections over it. Select all 3 slab properties & assign soil support to them. Pick up column points, right click & assign column sizes to the loads. And my base pressur comes down to 4.5 ksf.

I can understand the logic of assigning soil support to all 3 as they are overlapping each & sfae will take then which is uppermost & thicker, so assign soil support to all. Next by providing column sizes we are ensuring that safe take into account the rigid zone for column in base pressure, punching & bending. But what is the logic of providing the lean layer? How does it control the base pressure?

Second can we ignore the reinforcement peaks in slab strips under column & core wall edge points? When I calculate reinforcement from strip moments manually the value is much lower than the one reported. How to resolve this?

I personally don't agree to what your senior in office told you. Here are some things that you should consider:

1) You should challenge your results because area for pressure distribution remains same as before. Ask yourself that why did your results improve?

2) (update later)

3) Lean concrete has no structural use and shouldn't be modelled in SAFE v8. If your slab/ raft/ mat was 8" inch, I would accept a 6" concrete layer for pressure distribution but considering the thickness of lean concrete layer to be <<< than that of mat, I wouldn't consider it.

4) Please watch CSI WATCH and LEARN Series for SAFE v8 to see how to model drop panels in SAFE.

5) Do consider cracked section properties for your mat/ raft foundation flexural and shear design. It would be 0.25*I

6) Plot your moment contours for all load combinations. Select a reinforcement in slab and check your section capacity for maximum moment. Visually compare the contours with the max capacity that you calculated manually. That would allow you to assess if your foundation capacity is meeting the demand.

For general information, when you increase your foundation thickness, your pressure distribution contour peak drops, and the reason is that you raft/ mat becomes more rigid and load distribution becomes more even.

Thanks.

[WR1]

In addition to what Makhzumi has said, I would like to add that:-

 

1) I am not sure if you have to apply soil properties to all of the mat layers, but what I would prefer is to draw drop panels (thick portions of the raft) and then draw the rest of raft (no overlapping). Then as your senior told you to draw thick slabs (stiff property) fro drawing columns over column points in SAFE. But remember do not apply soil properties to these stiff elements. Make thickness of these elements as 1m.

 

2) You will get concentration of soil pressure and moments near supports (near columns). But do not go for the maximum peak pressure or moments.

For bearing pressure, take the contour value just at the face of the column (stiff elements you just have drawn in step 1). Do not read contour values at the edges of elements rather look for the value in the center.

 

3) For moments and reinforcement design, design for 1m wide strip and average the moments for 1m width. You can do this by applying design strips in SAFE in x and y direction of width = 0.5m and then replicating them every 1m to cover all of your raft. Then after running the analysis you can put in the numbers of bars or dia of bars and the intended spacing of bars to check what reinforcement you require.

 

Do let us know if you find more problems.

[Waqar Saleem]

how to decide weather raft/mat should be flexible or rigid?

[UmarMakhzumi]

how to decide weather raft/mat should be flexible or rigid?

 

You don't have any other choice but to assume a rigid raft if you are doing it manually. It would be way too complicated to solve manually if you consider a flexible mat. If you are using a computer software, you can consider both options to see the effects on your results.

[Muhammad Imran Zafar]

In order to assess the rigidity of mat please refer to ACI 336.2 R88

http://civilwares.free.fr/ACI/MCP04/3362r_88.pdf

[UmarMakhzumi]

In order to assess the rigidity of mat please refer to ACI 336.2 R88

http://civilwares.free.fr/ACI/MCP04/3362r_88.pdf

 

Thanks for posting such a good reference.

 

Section 5.2 lays the criteria for how to determine if the footing should be termed as rigid or flexible. However, under 6.1, the document states 

Mats may be designed and analysed as either rigid bodies or as flexible plates supported by an elastic foundation (the soil). A combination analysis is common in practise. An exact theoretical design of a mat as a plate on an elastic foundation can be made; however, a number of factors rapidly reduce the exactness to a combination of approximations.

 

 

It pretty much comes to judgement.

[Muhammad Imran Zafar]

Bowel has also indicated this reference and has discussed in detail for mats. It helps in assessing the behavior of raft. You are right and for experienced persons judgement should be given due weightage