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Hi, First of all kudos to the administrators for taking the initiative of providing this platform. I am new here and i hope to contribute and gain maximum from this portal. I am designing a B+G+2 RCC Building with a foot print of 30x40(ft). Owing to the architectural requirements columns have been provided along the edges only giving a span of 30ft c/c in x dir while 14ft in y direction. The Allowable bearing capacity is 0.8 tsf for raft. There is a basement retaining wall 8ft high while the story height is 12ft. So, the RCC wall is only for soil retention. Initially, i tried to provide isolated footings but the bearing pressure was well above the allowable. So, i resorted to raft foundation. I didn't model the RCC wall in SAFE to check the punching shear; which eventually lead to a raft thickness requirement of 27 inches. My questions are: 1) Is there a way to optimize the raft by NOT providing the 27" thickness all over the area? Like, the area in vicinity of columns can have 27" thick while the remaining raft can have lesser thickness? If there is a way, please provide some details of doing it in SAFE. Considering the size of this project, it may not save much but i need to learn this technique of modeling the raft with variable thickness. 2) There is always a failure of bearing pressure when columns are located along the edges of raft. Yet it's quite a common practice to provide columns along the edges of raft. How such rafts are designed? I am facing the same problem here. Thanks

1) i want toAssign soil springs as i want to do the analysis in ETABS so what should be the value of Area springs for .6tone/sqft baring capacity ?? Raft Area = 120ft x 35 ft 2)There is always a Punching failure when columns are located along the edges of raft. Yet it's quite a common practice to provide columns along the edges of raft. How such rafts are designed? I am facing the same problem here. (fig-2) thanks

Dear all I want to discuss an issue recently encountered at site. Designer had given details of raft with #6 @ 6" o.c reinforcement BW in top and bottom meshes but contractor instead provided#4 @ 7" o.c. the raft and 1st story columns have been casted. On doing its capacity analysis the raft with provided reinforcement was not adequate to support to super structure. Now I'm thinking of doing retrofitting by providing an overlay of 9 to 12 inches. I wanted to know about shear keys/dowels for the connection between old and new slab. Can anyone provide any helping material for it? Thanks

assalamualaikum sepakistan usersi wanna ask what are the considerations of using this ribraft foundation thing?what are the advantages and disadvantages?what are some load applied on it?how do we design the slab and the beam?thank you.

Dear All, I have recently designed Raft foundation for 6 story building in SAFE. I have 4 feet offset area on site and have already spread the raft upto 4 feet offset edges. The SBC is 17000 lb per sq.ft. I created uplift load cases according ACI-318-08 as shown in the attached picture. After analysis I observe that the uplift pressure is negative but the magnitude exceeds SBC. I watch many tutorials where negative results are considerd safe but I am still confuse whether is it Ok or I should reduce the result. Please match the SAFE results with the mentioned SBC and answer this topic. Thanks in advance.

Assalam-u-alikum for modeling raft foundation in etabs the supports for raft is point spring or area springs support?How i will assign value to a spring?

Assalamu Aalikum, Hi, I am a new member. I have a plot measuring 30*70 feet. Most of houses being built here (in Rawalpindi) have masonry foundations (a pyramid of bricks on PCC or sometimes the PCC layer has three to four iron rods of size 3/8 or 4/8 in this "PCC" layer. I want to make a raft foundation instead of masonry foundation but the cost seems to be high. I want to ask if I make a raft 1 feet thick and put a single mesh of center to center distance 12 inch. Even this will cost more but if it gives more strength then I want to go for this. Would such foundation more durable and strong?

Dear SE Pakistan Members, I am trying to model pits in Raft in CSi SAFE. PIT Bottom slab is depressed 2m below the Top of RAFT. I have following concerns which I would be glad if anyone can address. 1- What is the criteria/condition to analyze raft slab at one level (continuous) and pit/ raft on different level. 2- What is the appropriate method to utilize in SAFE to model RAFT with Pits. I came across option of Vertical offset in SAFE and the other is to model wall at periphery and model slab at bottom. Generally, we model same raft thickness below PIT depressed at 45 degree angle . For this reason, I always modeled PIT on same level as that of RAFT. (See Attached Pic for RAFT Plan & Section of PITS).

Salam Alaykom All, I am modeling a refugee camp on Etabs. I have over 20 buildings to be modeled, each 5 share the same raft foundation , and each raft foundation is no more than 3 meters away from the other. For simplicity I modeled all 20 buildings in the same Etabs Model to Calculate the forces on the raft foundations. My question is : 1_ Will the Raft Foundations be Affected by each other , knowing that the buildings they support arent connected in any way?

I need help regarding MAT footing design in safe. Please read questions mentioned below and comment if any extra step or correction need to be done. This MAT footing is for 4 storey building in zone 2A and bearing capacity would be more than 1.25 ton/sft. 1) Does we need to increase the bearing capacity by 33% as mentioned in NILSON book for soil pressure calculation or not if we are taking basic load combinations of UBC including earthquake loads not the alternative load combinations. 2) If we want to take reduced live load for footing then is there any way to export in form of reactions from Etabs to safe. In my opinion Etabs does not give Reduced live load reaction, Correct me if I am wrong. 3) When we use the basic load combinations which include earthquake load then there is very localized uplift (due to earthquake combo) when we check soil pressure. is it normal or something wrong with my reactions. If it is normal then we always used non-linear basic combinations to check soil pressure and non-linear design combinations for design of raft? 4) Does it required to check crack width and settlement in case of raft or not. If required then explain in few words. 5) At boundary of raft where there are shear walls/lift core, soil pressure is more than the bearing capacity in these localized regions. what we need to overcome this situation if we cannot increase the area of raft at that side. any additional check which you think important for design then write in comment your help would be appreciated Regards

Dear members, As salamu alikum .To resist 700 kips total load in the column,if compressive strength of concrete 4 ksi,fy=60 ksi,to satisfy punching shear in raft,how much thickness is need?

Aoa dear S.Es Wat is the criteria for selection of a particular type of a foundation especially with reference to an RCC building? e.g A RAFT or SINGLE FOOTING?

AoA everyone ! I want to know that is there any minimum & maximum reinforcement ratios for section of Raft footing ? Any Code or Reference. Also if you can provide your steel quantity consumed to Rraft concrete quantity ratios. Regards

salaam.. hope so all will be fine i m the civil structure engineer using etabs or sap for designing but i would like to prefer designing by hand calculations.. actually i need the hand calculation for mat or raft design if any one have the calculations regarding these please email or upload it.. i will be very thankful to you..thanks

What could be the purpose of reducing raft stiffness by 60% (m11=m22=m12 modifiers of 0.4) in SAFE Model?

DEARS; IS IT POSSIBLE THAT TWO ADJACENT RAFT FOUNDATIONS TO BE ON DEIFFERENT LEVELS AS 1 METER DIFFERENCE IN LEVEL??. I THINK NO... IF YOU ACCEPT WITH ME WHAT IS POSSIBLE SOLUTIONS? THANKS

Salam there is a heavy raft of 6feet depth .Reinforcement is provided at top and bottom.is there any requirement of reinforcement in the middle of this T/B reinforcement for temperature/shrinkage control of this mass concrete.if there is any reference in any code kindly mention Regards

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

when we have raft foundation, is it necessary to draw it as we draw slab or just select all the joints at base level and turning them to fixed support will generate the effect of raft in analysis?