Design philosophy.pdf

CL501 Design of Structures –I

B. Tech. Civil Semester V

Design Philosophy Design Philosophy  There are three three design philosophy 1. Working stress Method or Elastic Analysis 2. Plastic Method or Ultimate Load analysis or Collapse Load Method 3. Limit State Method

Working stress Method (IS 800:1984)    

Design criteria = initial yielding Working stress < permissible stress. , where permissible stress is fraction of  yield stress of material. Advantage: serviceability criteria is always satisfied, as stresses will be less than yield stress. Limitation:  Reserve strength beyond elastic limit is not utilized. Failure mode is not visualize.   Load are not factored, load vary with time and this uncertainty of load is not Considered.

Plastic Design Method or Ultimate load method.    

Design criteria = ultimate strength Working stress < collapse load, where collapse load is working load × load factor. Advantage: Plastic behaviour is considered, reserved strength is used after  yielding. Limitation:  Doesn’t consider serviceability criteria. Factor of safety (FOS) is not applied on material.  No scientific definition of load factor. 

Limit state method (IS 800: 2007)   



Design criteria = limit state of o f strength and limit state of serviceability. serviceability. Design action ≤ Design strength. Design strength. Advantage: all the limitation of working stress method and plastic design method is overcome. overcome. Probabilistic and statistical statistical approach is used to determine limit states. Limitation:  100% probabilistic approach is not possible, so Factor of safety (F.O.S.) is used based on this probabilistic approach.

Characteristic Characterist ic Loads: 

Characteristic Characteristic loads means the value of the load which has a 95% probability not being exceeded during the lifetime of the structure.

Prof. Utsav Koshti

Department of Civil Engineering, IT, NU

CL501 Design of Structures –I

 

 

B. Tech. Civil Semester V

Characteristic Characteristic dead load is the weight of the structure itself. Characteristic live load and wind load are taken as per IS 875 and characteristic characteristic seismic loads are taken as per IS 1893. Design load = Characteristic load × Partial safety factor for load. Partial safety factor for loads γf  for  for limit states is given in IS 800:2007 table 4, page 29(Clause no. 3.5.1 and 5.3.3)

Characteristic Strength: 





Characteristic strength of a material is that value of the strength of the material below which not more than 5% of the test result are expected to lie. Design strength is determined from ultimate strength (which is derived by characteristic characteristic strength). strength).  ℎ  ℎ ℎ =    Partial safety factor for material is given in table 5, page.30, IS 800: 2007.

Limit State of Serviceability: Serviceability limit state is related to criteria governing normal normal use. Serviceability limit state is a limit state beyond which the service criteria are no longer met. Service criteria are as followings: 1. Deflection Limit (Refer Table 6, page. 31, IS 800: 2007) 2. Vibration Limit (Depend on machine supplier criteria) 3. Durability Limit (Refer section 15, page. 103, IS 800:2007) 4. Fire resistance (Refer section 16, page. 105, IS 800:2007)

Prof. Utsav Koshti

Department of Civil Engineering, IT, NU