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[amitavabh]

Dear Bhavin,

Thank you for your mail. I have some items to discuss further, which are as following.

1. Support: I prefer spring support at nodes of base raft plate elements.

2. During Uplift pressure:

a) To know Stability against uplift a simple hand calculation may be used and at that time effect of spring support shall not be taken.

b) For uplift load case the local bending/forces of the base raft may be obtained from Staad using following idea:

Provide spring support around the edge of the raft, in line with the vertical tank walls to give a near site effect. Use proportionate influence area of raft to calculate each spring support.

When the base raft bends, it will mainly bend within its periphery beneath the heavy walls.

3. Additional Load cases: I had discussed about the loadcases that one shall take to analyse underground tanks. However, another 2 load cases shall be taken into consideration for analysis of underground tanks. They are:

a) Dynamic increment of soil pressure during seismic activity.
b) Sloushing effect of liquid inside the tank during the seismic activity. The liquid will change place during such movement like it does in rivers, ponds and Oceans during seismic activity.

However this kind of analysis is done for very important tank types. Say tanks that will contain radio active material, hazerdous liquids detrimental to underground and over ground environment etc.

Regards

wrote:
-------------------------
Message From bhavinpatel[AT]vms...
Subject: Underground Tank Design
Date: 13/09/07
Time: 11:11:23
-------------------------
Dear Mr.Bhattacharjee,

When we model Base raft in STAAD, there are two issues regarding
supports.
1. Which kind of support should we provide. Plate Mat
Elastic Mat Spring Support. (I personally feel the plate mat option
gives better results when raft is modeled with plate element.)
2. When central portion of base raft is subjected to uplift
pressure in tank is empty condition, The spring support will resist that
upward deflection according to their stiffness which is not the actual
case with soil.

Regards
Bhavin
_________

a_bhattacharjee[AT]yah... wrote:
Dear Jagjit,

For designing underground tanks you can use following methods:

1. Use plate element and model it in Staad or other software. Provide
soil spring support for base raft.

Calculate soil, water and surcharge pressure on wall. Calculate uplift
due to water on raft. Use tank empty + soil around tank present
condition. Then use full + soil absent around tank during test condition
for worst load cases. If there is chember in the tanks then use
alternate chember filled/empty etc. Then analyse.

Take output forces of plates. Select maximum force of "plate center
stress summery" among the plates of each wall. Add torsional moment to
the moment of both direction seperately. Then Design for this moment of
each wall. Then take out bottom and top slabs. Calculate and check the
crack width of walls/slabs against code values.

Use IS 3370 and IS 456 for refering to codal provision.

2. Calculate forces on tanks as told above. then use Roark's table to
find out moment coefficients and then design for those forces. Check
crack width.

Regards
Amitava Bhattacharjee
Kolkata

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