Part 1 General Design Requirements Flashcards
What is the purpose of steel design
Design the members and connection so that they can resist and transfer the applied load safely and economically
How many theories exist regarding the design of steel structures and name them
There are three theories for steel design
- Working stress method or the elastic method
- Ultimate load method or the plastic method
- Limit state method
Explain the principles and the fundamentals of the working stress method or the elastic method
This method is based on the principle of Elastic theory. Hear the structures are analyse elastically for the worst possible combination of the working load and the members are designed such that the permissible load is never exceeded
The working stress is lesser than or equal to the permissible stress ( yield stress divided by the factor of safety)
How many drawbacks of the working stress method exist and explain them one by one
- In this case the linear elastic behaviour is assumed that is the Reserve strength of the member is not utilised thus in resulting in uneconomical design
- It assumes all load 2 act simultaneously with same degree of uncertainty
- The assumption that working stress shall be kept lesser than the permissible stress is not practically possible due to
Thermal stress residual stress stressed due to fabrication ( stress concentration) and sinking of supports
Explain the principles of the ultimate load method that is the plastic method
- This principle is based upon the ultimate strength of the material
- This method is based upon the failure condition rather than the working condition
- The ultimate load or the collapse load shall never be acceded by the design load
- Working load multiplied by the factor of safety is kept lesser than the collapse load
( this principle results in the development of smaller section )
Drawbacks of the ultimate load method or the plastic method
It does not takes into consideration the survisibility aspects
It consuders all the loads to be acting simultaneously and with the same factor of safety
Explain the limit state method and the philosophy of partial factor of safety
This method over comes the drawback of both working stress method and the plastic method.
The partial factor of safety for both loading and strength of the material are incorporated.
The partial factor of safety takes into accounts the overloading and the under development of strength in the material
Define the term factors strength and factor loading
Factor load equals to characteristic load multiplied by the partial factor of safety of load
Factors strength is equal to characteristics strength divided by the partial factor of safety for material
What do you mean by characteristics value
All the characteristic values are those which have which do not have the exceedence probability of 5%
What factors does the partial factor of safety for load accounts into it
There are four factors
- Possibility of the load exceeding the characteristic load
- Possibility of Innacurate computation of the load
- Uncertainty in the application of load.
- Uncertainty in the limit state assessment.
What factors does the partial factor of material strength accounts into it
There are four factors
- Possibility of the strength falling below the characteristic value.
- Reduction in member size due to inacurate ordering.
- Strength reduction due to poor fabrication .
- Uncertainty and approximation in the calculation of members strength
What do you mean by limit state explain the two types of limit States
Limit state is a stage beyond which the structure or the member fails to satisfy the performance requirement.
There are two types of limit States
A. Limit state of collapse or strength
B. Limit state of service ability
In lsm method the structure is designed for the limit state of collapse but checked also for the limit state of survisibility
What factors are taken care of in the limit state of service ability
Deflection vibration corrosion deformation due to excessive cracking Ponting of water fire resistance
When can the factor of safety for load be lesser than one , why cannot the earthquake load and the wind load be taken simultaneously
Sometimes the factor of safety for loading is 0.9 when the dead load provides stability against over turning or reduces the stresses due to other loads.
The earthquake load and wind load are not taken together as the probability of their occurrence to occur simultaneously is very little and hence of no practical significance
Important loads and the is codes corresponding to them
Dead load is 875 part 1
Live load is 875 part 1 and 2
Erection, accidental load is 875 part 2
Wind load is 875 part 3
Earthquake load is 1893
