section property modifiers

[StructFemMan]

hellow all
this is my first thread in this forum.
Iam using sap2000 to model a reinforced concrete elevated water tank.
1. should I apply stiffness reduction factors (m11=m22=m12=.25) for slab sections -this will reduce the moments considerably hence reduce areas of reinfocing steel.

2.The geotechnical site report does not provide values for modulus of subgrade reaction,
what is the best approximate equation for calculating the modulus of subgrade reaction.

3- As a rule of thumb - for a low bearing capacity soil (1.5 kg/cm²) whis is correct to reduce differential settlement in a raft foundation :
increase raft plan dimension or increase raft thickness

 

[johnbridge231]

1. You can apply such factors. But in that case, you pressume that cracks will occur, so you will have to do a crack width calculation to validate that their size is acceptable as per the design code. o.25 factors seem to small to me, I would go lower than 0.70.
2. You can try 2 different soil moduli, one large and one small, for example 200000 kN/m3 and 8000 kN/m3. The small modulus will demand more reinforcement for loads except of temperature. In the case of the larger soil modulus, increased stresses may appear due to restrained temperature deformation (if thermal loads are used). In this case, you should apply horizontal soil springs as well (rule of thumb: horizontal soil spring stiffness=0.5 of the vertical one).
3. Among the two choices you provide, I would definitely choose the incease of the thickness of the reft.

Regards.

Analysis and Design of arbitrary cross sections
Reinforcement design to all major codes
Moment Curvature analysis

http://www.engissol.com/cross-section-analysis-design.html

 

[StructFemMan]

Dear Sir ,
Mr.johnbridge231
Fist of all thank you for your reply,I appreciate every word you wrote!
For point No 3) you said you will increase the thicness of the raft,but doing this will increase dead load,on the other hand if we increase the plan dimensions of the raft this will distribute the pressure of the structure on alarge area - hence small stresses and small settlement will be generated - is that correct?

 

[johnbridge231]

Dear StructFemMan,

ok if the problem are the soil stresses developed for static loads, then you should increase the raft dimensions in plan, but I think that this is not so common.
If the problem are the differential settlements, by increasing the thickness, you get better behavior.

Analysis and Design of arbitrary cross sections
Reinforcement design to all major codes
Moment Curvature analysis

http://www.engissol.com/cross-section-analysis-design.html