Design Strength of Materials and Design Loads(Refer Clause 36, IS Code)
Design Strength of Materials : Partial Safety Factors for Materials (r_{m}), and Design Loads : Values of Partial Safety Factor (r_{f}) for Loads(Refer Clause 36, IS Code)
DESIGN VALUES (REFER CLAUSE 36, IS CODE)
Design Strength of Materials
The strength of any material obtained in a structure is always less than the characteristic strength of the material. It is because of the workmanship or quality control in the manufacture of materials.
The reduced value of strength which is obtained by applying partial safety factors to the characteristic strength is called as design strength of the material.
The design strength of the material f_{d} is given by
\[f_{d}=\frac{f}{r_{m}}\]
where f = Characteristic strength of the material
r_{m} = Partial safety factors appropriate to material and limit state being considered.
Partial Safety Factors for Materials (r_{m})
The values of partial safety factors for limit state of collapse, should be taken as :
r_{mc} = 1.5
r_{ms} = 1.15
where r_{mc} = Partial safety factor for concrete
r_{ms} = Partial safety factor for steel.
A higher value of partial safety factor for concrete i.e., 1.5 has been adopted because there are greater chances of variation of strength of concrete due to improper compaction, inadequate curing and variation in the properties of ingredients. The chances of variation in the properties of steel are small, as it is fabricated in the factories where good workmanship and better quality control can be an achieved. Hence, a lower value of 1.15 has been adopted.
Design Loads
The design loads are obtained by multiplying characteristic loads and the appropriate partial safety factors. The design load F_{d} is given by
\[F_{d} = F.r_{f}\]
where f = characteristic load
r_{f} = partial safety factors appropriate to the nature of loading and limit state being considered.
Design loads are used for calculating design bending moment, shear force and thrust etc.
The values of partial safety factors (r_{f}) for loads are given in the following Table
Values of Partial Safety Factor (r_{f}) for Loads
TABLE 3.1. Values of Partial Safety Factor (r_{f}) for Loads
Load combination | Limit State of Collapse | Limit State of Serviceability | ||||
DL | LL | WL | DL | LL | WL | |
DL+LL | 1.5 | 1.5 | – | 1.0 | 1.0 | – |
DL+WL | 1.5 or 0.9* | – | 1.5 | 1.0 | – | 1.0 |
DL+LL+WL | 1.2 | 1.2 | 1.2 | 1.0 | 0.8 | 0.8 |
Note : While considering earthquake effects substitute EL for WL.
*This value should be considered when stability against overturning or stress reversal is critical.
DL = Dead load.
LL = Live load,
WL = Wind load.
EL = Earthquake load,
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