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[dubeyratnesh]

hello friends
i want to design a rectangular clarifier on soil having sbc less than 4 t/m2.
what process i can workout find best design for such structure. structure is in seismic zone bhuj .also water table is  1 m below at ground level.

[vikram.jeet]

Just a lay- man's view

Not aware of functioning of clarifier of this type . Had designed Clarriflocculators in water treatment plants  / primary & secondary clarifiers in sewage treatment , which are large sized open to sky structures ,circular in plan and walls are subject to hoop forces predominantly.

Rectangular clarifier as a structure is not problematic , but SBC of 4 T/m2 is very low.  In case ud loading is less than this , one can think to rest on soil otherwise under reamed piles is an option to think. As seen at net , PVC tubes groups are provided but still not clear about it.
Also it may be seen that at what depth of ground this SBC is available . Also whether it is net or gross SBC. If net SBC , it can be enhanced to gross SBC .

[dubeyratnesh]

yes sir ,regarding as it is in low sbc area pile foundation is advised by Geotech engineer.
pile raft can be good for this structure.
further i want to know analysis process of pile raft?
for a tank like structure. can i modal it in staad pro?

[vikram.jeet]

Piled raft

It is a flat slab and can be easily designed  as per  IS 456 in internal portion where loading , I suppose,  be UDL.

Only at periphery the piles (minimum two rows along walls) provided take care of wall moments in form of additional +_ loads as result of moments in addition to wall  loads.

For simplicity of detailing of reinforcement, column strip reinf be provided as top mesh , as also bottom mesh based on column strip moments . Otherwise detailing can be divided into column and middle strips if spacing of piles is more. For smaller pile spacings ,  a uniform meshing is site friendly and may not affect economics.

[dubeyratnesh]

[vikram.jeet]

Central portion sump

If sump size is small  the raft will bend down and at sump depth to form base slab of sump .  The reinforcement of flat slabs can be detailed  in continuity  of sump walls and sump  base slab.Sump base  slab to be also checked as independent slab and higher of two reinf to be provided at sump  .
If sump size is substantial , it is better to provide piles along sump walls and also in the dono in grids. But all depend on sump size.

Since water table is high and it may further rise during monsoon , the tank empty condition with uplift pressure need also be checked on provided reinforcement.

The views expressed herein are very generic and advice of
experts dealing such structures design be taken for clear understanding of functioning and str behaviour

[dubeyratnesh]

dear sir
as you suggested piles on periphery of tank wall  
how can we come through with no of piles? load taken by raft ?
for pile raft would pile take all the load of raft for design purpose? in staad pro how effect of pile can be included?

[vikram.jeet]

Piles are required supporting entire raft , in a  square grids both ways , BUT at periphery since wall moments from tank walls are there , two rows of piles along tank walls will take care of moments in form of additional +_ loading on  these peripheral piles.

[vikram.jeet]

Modelling in staad

Since illiterate I.e. not proficient in CAD designs ,just a layman's view :
Pl model piles as pinned supports for raft.
Model  raft as tank base slab  along with modelling of walls

Apply inside loadings

Also see tank empty case with outside loadings

Hope other members may guide u more explicitly , vividly and more candidly..

[dubeyratnesh]

dear sir can any one suggest modeling of pile raft in staad pro and its design