Factor of safety reinforced cement concrete.
Partial factor factor of safety of concrete & rebar is 1.5 & 1.15 respectively. Use of
factor of safety for both materials I have got in compression member.But same I
have not got in flexural member.What will be combined F.O.S for reinforced
cement concrete material.
Combined partial FOS for RCC
I think it must be proportional to the stressed area of each component (conc &
steel) used.
For flexure
(stressed area of conc*1.5+ Total area of steel*1.15) / (stressed conc .area + steel
area)
For Shear/ pure compression
( Total Area of conc*1.5+ Total area of respective steel*1.15) / ( conc .area +
respective steel area)
For bending compression
Lies in between These values are nearing 1.50 .
In the limit state design of Structures, we consider partial safety factors for the
loads and materials as specified in the IS 456-2000 and design accordingly.
a. Is it possible to determine the "factor of safety" or "Fafety factor" for
such Structure designed as per the limit sate design.
b. Is there any reference material available to determine so.
c. Can we relate "factor of safety or safety factor" to "partial safety factors"
considered for the loads in the analysis.
As per Limit states design
phi (Sn) >= Gamma(Qd), where
phi= strength reduction factor applied to nominal strength Sn
Gama is the load factor applied to code specified loads
Considering DL or LL, Gamma is 1.5.
(1/Phi) or Gamam as per IS 800 is 1.1 and as per IS 456 is 1.5
Hence the least factor of safety for DL or LL will be 1.5 x 1.1 for Steel Structures
or 1.5 x 1.5 for concrete structures.
Actually the issue is "Can we relate "factor of safety or safety factor" to "partial
safety factors" considered for the loads in the analysis. ( whether 1.5 or 1.1
depending upon the material)
In Limit states design we adopt different load and material safety factors to get
consistent safety under different conditions. These factors have been derived using
probability theory.
In Limit State Design, we use Partial safety Factor (PSF) for Loads and for Stresses
both (1.5 loads, 1.5 for conce and 1.15 for steel). This is consdering the probability
of Expected variations in a value of loads and stresses (a Statistical approach).
After analysis of the section we further perform the design. In Design if we get say
80sqmm as required steel in RCC, where as the Ast(minimum) required is 100sqmm
and for practical reason if we provide say 2 rebars of 10mm dia (i.e. 157sqmm Ast)
then the actual factor of safety is more that the PSF.
Suppose, in a building out of all Columns say 40% columns have FS higher than PSF.
And also a few beams have FS higher than that of the PSF.
In such case we consider the Lowest FS as the Limiting FS for that entire
Structure. But Limit State method do not answer how to find the actual FS of
individual members and the correct value of FS for entire Structure, as a whole.
As per Is : 1893 Mentioned
0.9 DL + 1.5 EL combination for sliding and overturning.
But while designing pedestal Huge amount of tension is coming.So,for anchor bolt
design and pedestal which is to be consider or not.Pls clarrify anybody.
A] For stability check on foundations
Under Normal condition
Factor of safety against Overturning =2.0
Factor of safety against Sliding =1.5
Above criteria required for :
Retaining structures and
All Structures subject to horizontal loads (other than wind/EQ)
Overhanging members needing embedment length at back
like a cantilever beam on BK wall (overturning needs to be seen)
Under Wind/EQ condition
Factor of safety against Overturning =1.5
Factor of safety against Sliding =1.25
Above criteria required for all structures
B] For Design of various members a structure
The load combinations are :
Under Normal condition
1.5*DL + 1.5*LL
Under Wind/EQ condition
1.2*DL + 1.2*WL + 1.2*EL
1.5*DL + 1.5*EL
0.9*DL + 1.5*EL
This comb of (0.9DL +1.5EL) is to ensure that
a structure which has overall stability FOS =1.5 , shall not fail
as result of any individual member's deficiency and it must
possess its strength to match with overall stability.Moreover
Actual EQ force would many times the design EQ.
pl. note that the permissible stresses in materials as well as allowable pressures in
soils need to be increased when earthquake forces are considered(ref. respective
codes).
The FOS mentioned, have any codal support or general values in the usage?
Also pl. confirm the load combination 1.2*DL + 1.2*WL + 1.2*EL , as wind and EQ
forces should not be considered simultaneously.
For anchor bolts - maximum tension has to be considered. Though the load
combination which you have mentioned is primarily to check for global stability.
For anchr bolts generally, there are different load combinations than the structure
load combination. You can take a look at appendix D of ACI 318 for the same.
Thanks for the correction and regret for the typo error in my earlier posting.
The comb is ; 1.2*DL + 1.2*LL + 1.2*EL(or WL)
FOS for stability of structure
Min FOS(OT) =2.0
Min FOS(SLD) =1.5
These values are generally followed and were as per earlier structural
design practice
However as per latest codes
As per IS 456:Clause 20
Overturning
Stability of structure as whole against OT shall be ensured
Restoring moment shall not be less than
1.2*(max OT mo due to DL) + 1.4*(max OT mo due to IL)
Sliding: FOS shall be not less than 1.4
Where DL provides restoring, only 0.9*DL to be considered
In order to compare with earlier practice FOS as per IS 456
against OT or sliding is =1.4/0.9=1.55
Overhanging members
Anchorages and counter weights shall be provided to ensure
static equilibrium even if overturning Mo is doubled
As per IS IRC2000:Clause 706.3.4 on open foundations
FOS (non seismic case)
Against OT = 2.0
Against sliding = 1.5
Against deep seated failure = 1.25
FOS ( seismic case)
Against OT = 1.5
Against sliding = 1.25
Against deep seated failure = 1.15
Other codes on retaining walls IS 14458 in hill areas also require
probably same FOS of 2.0(OT) and 1.5(SLD)
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