RAFT FOUNDATION

The procedure for raft design is as follow

• For design of raft foundation use inverted floor/flat slab approch. Calculate the loads coming on the raft for Gravity load and buoyant load exerted by water separately.Consider maximum load for design out of both loads.Then calculate the moments from Moment distribution method. Calculate thickness of raft required.Check that thickness for Punching shear criteria. Check raft settlement also.

You can use FEM also.The result by FEM will be compactible only whenthe assumption made for soil stiffness are correct one.

If it doesn't find suitable go for Piled raft foundation.This is most efficient and economical type of foundationfor your case.

Whether we have to add base pressure (from gravity loads) with uplift pressure

• Check for buoyancy in case of uplift...pl compare the total upward pressure versus total dead wt of raft and ensure a factor of safety of at least 1.25, preferably 1.5...so that the raft is not washed away during construction itself, or else u may have to carry out continuous dewatering until basement roof has been cast too and the counterwt is adequate to achieve this factor of safety.

• The uplift pr is to be considered while performing the overall check for buoyancy alone, while designing the RCC of the raft we do not add it to the base pressure generated due to dead and live loads.

I want to model the founding soil

• Effect of Soil Structure Interaction on High Rise Building founded on Soft Soil subject to Seismic Forces".

• We have SAP 2000 and STAAD 2004 available in our college. Is any one of you have idea @ How to model the founding soil & how to define its Dynamic.

• You can either user soil springs to model Winkler type foundation or else you can also use 3DSOLID elements to model soil.

Option2• The actual problem is too complicated. However, in practice the soil is sometimes replaced

by a set of springs - springs in all 6 DOF if you are modelling the superstructure by beam elements.The expressions used are based on studies on Machine Foundations.If you are in Mumbai, the BARC and IIT-Bombay libraries have excellentpublications on Soil Structure Interaction.Not only soil springs but soil damping is also to be considered andmodal damping must be derived.

• It is not an easy problem even with gross assumptions of soil as spring-dashpot system. There is a controversy associated with how much soil mass to be lumped or not. Just blindly using packaged software, will lead you nowhere.

• You must read literature and note the assumptions.This is not to discourage you but to inform you that if you are doingresearch, you must readw what has been attempted so far.

•On the contray, if you are designing, anything goes. There is a draftCode [may be already finalised] on industrial structures which you mayuse for design but not for research.

Raft Footing

• Treating a raft foundation as rigid is not only wrong but is also dangerous.

The correct way to design rafts is to treat them as beams on an elastic foundation, by means of the following methods:

(i) Miklos Hetyeni's closed solution (ii) Finite grid method (J.E. Bowles) (iii) Finite element method.

• Raft footing on soft, low BC soil is normally designed as beams on elastic foundation. But for hard, high BC soil, soil deformations are normally minimal and do not impact on the behavior of raft footing under load. In such a case, raft footing can be considered as "rigid". I've heard, but neither practised, nor read in the literature, of a raft footing on a hard rock strata being treated as an elastic plate. Is a raft necessary on Hard Soil ? I thought Raft / Piles are used when soil is iffy to support footings of columns in that case, raft may have to be designed as a plate resting on flexible soil

• RIGIDITY OF RAFT MEANS RIGIDTY OF CONCRETE ELEMENT and not the Hardness or Softness of supporting Soil.

• For same Vertical Loads (Dl + LL ) and OVERTURNING MOMENT (M) a Very Thick Concrete (say 11 Ft.) will have no flexure stresses (Bending Moment) in Concrete and a nominal thickness ( 3 ft.) will bent like a beam bending and will have flexure stresses in concrete.

• Thick Concrete ( 11 ft.) will rotate like a Rigid Plate on Soil Springs with no flexure Stresses.

• For Vertical Loads only Shear Stresses in Concrete no Flexure Stresses. •

It is the Rigidity or Thickness of Concrete Element and not the stiffness of the Soil, Soft Soil or Hard Soil is the issue in such analysis.

• If you have a Concrete footing sitting on Rock Surface you will have only Direct Bearing Stresses and no flexure stresses. agree that in several cases, even on hard soil, a raft type footing may result as default. in case of elevated circular water towers, we use annular circular raft. also, wherever the columns are closely spaced, the individual footings may overlap resulting in strip footings for design purposes, on hard soil, mostly such footings are deemed to be fixed at base

• If one does not model the foundation & soil but considers only the superstructure over the foundation, then forces on foundation & soil, could be worked out based on static check.

•The external forces applied on the superstructure are ONLY vertical - distributed over space for dead and live loads. This will result in vertical reactive loads on the foundation & soil. The CG of applied vertical loads will coincide with the CG of reactive loads. A similar argument holds good for horizontal loads. The horizontal loads applied at a height will result in horizontal reaction and as upward & downward reactive forces due to moment caused by the lever arm of horizontal force.

• Please note the unit for fixed but is KN/m or Ton/m etc ...wherein u have to multiply subgrade reaction with inflence area and if u go for elastic/plate mat than u have to give only the subgrade reaction value which may be Kn/sqm/m etc...Ton/sqm/m... so u can go for either of this three which suits u..

This is from STAAD help menu..whenever u have any doubt go to STAAD help menu...u have a search option wherein u can get ur doubts clarified...I have just written small abstact from STAAD help menu...for detail refer STAAD help...

The ELASTIC FOOTING option : If you want to specify the influence area of a joint yourself and have STAAD simply multiply the area you specified by the sub-grade modulus, use the FOOTING option. Situations where this may be appropriate are such as when a spread footing is located beneath a joint where you want to specify a spring support.

The ELASTIC MAT option : If you want to have STAAD calculate the influence area for the joint (instead of you specifying an area yourself) and use that area along with the sub-grade modulus to determine the spring stiffness value, use the MAT option. Situations where this may be appropriate are such as when a slab is on soil and carries the weight of the structure above. You may have modeled the entire slab as finite elements and wish to generate spring supports at the nodes of the elements.

The PLATE MAT option : Similar to the Elastic Mat except for the method used to compute the influence area for the joints. If your mat consists of plate elements and all of the influence areas are incorporated in the plate areas, then this option is preferable

•Generally in the industry we use to use Plate mats with spring supports defined by giving modulus of subgrade reactions.

I am still not clear about the requirements of elastic mat and plate mat. In which particular case we have use elastic mat and in which case we have to use plate mat.

•I for one has done the following for RAFT/MAT foundation design: A) Use the STAAD/PRO Mat design for Mx and My only ignoring Mxy (Twisting Moment. But make sure : 1) Thickness Check is based on largest value of Factored Load Combinations SQX and SQY of all elements. I add 3 inch extra in thickness for bottom cover.

2) Reinforcement Top Layer and Bottom layer of Mat is based on Mx and My but is not less than ACI 318-11 section 15.10.4 (Minimum Reinforcing Steel). This section requires that we provide at top as well as bottom layer in both direction As (Minimum Reinforcing) = 0.0018XBH (ACI 318-11 section 7.12.2.1)/Ft. of Mat. Where B = 12" and H = thickness (Gross Section) of Mat. This ACI 318-11 section 15.10.4 is so conservative that 90% of the Plate Elements design in STAAD/PRO for actual Mx and My will give you less calculated reinforcement area/ft. If you provide minimum area of reinforcement/ft. = 0.0018bh at top and bottom in both direction, you have plenty of reinforcement in Mat to ignore WOOD ARMER equation for Twisting Moment Mxy.

• In continuation, the efficacy and feasibility of the following methods may also be explored: 1. Lime stabilisation for the full depth of 5m 2. Provision of stone columns, which may be cost effective compared to RCC piles

• The load to be transferred by your structure is meagre. Even then, I feel it is not advisable to place the foundation (even floating type) over the BC soil as it will considerably swell when wet and shrink when dry, thereby causing a lot of possible movement & tilt for it. Can you replace the BC soil for a 3mx3m area , 5m deep ? Otherwise, is some 5m deep socketed piling possible ?

• Hard Soil is at 5m below the Ground Level .Please suggest alternate solution

• In continuation, the efficacy and feasibility of the following methods may also be explored: 1. Lime stabilisation for the full depth of 5m 2. Provision of stone columns, which may be cost effective compared to RCC piles

• The load to be transferred by your structure is meagre. Even then, I feel it is not advisable to place the foundation (even floating type) over the BC soil as it will considerably swell when wet and shrink when dry, thereby causing a lot of possible movement & tilt for it. Can you replace the BC soil for a 3mx3m area , 5m deep ? Otherwise, is some 5m deep socketed piling possible ?

• Hard Soil is at 5m below the Ground Level .Please suggest alternate solution

• Usually, differential settlement is not a problem in raft foundation. Hence this type of foundation is adopted when two or more types of soils are encountered within the area of building. Of course, rafts are also used when the SBC is poor and the footings of columns merge.