Design of Compression Members Based on IS 800-2007 and IS 800-1984-Comparsion

JOURNAL OF INFORMATION, KNOWLEDGE AND RESEARCH IN

CIVIL ENGINEERING

Design of Compression Members Based on IS 800-2007 and IS 800-1984- COMPARISON

M. KRISHNAMOORTHY, D.TENSING

M.Tech (Structures) Student, PRIST University, Thanjavur

ISSN: 0975 –6744| NOV 11 TO OCT 12 | Volume 2, Issue 1 Page 73

JOURNAL OF INFORMATION, KNOWLEDGE AND RESEARCH IN

CIVIL ENGINEERING

Principal, ASL Pauls College of Engineering and Technology, Coimbatore

ABSTRACT: The design methodologies for the steel structures namely, working stress design method and limit state design methods are briefly explained. The importance of limit state design method is highlighted. Columns form the main component of a structure which serves the basic purpose of supporting and transmitting the entire loads both vertical and horizontal for which the overall structure is intended to the foundation system. Beams are generally subjected only to flexure about the horizontal axis whereas columns are subjected to axial load along with bending moment about the major axis. The minor axis moment in columns are generally nil or very nominal since in standard structural system, the columns are so oriented that the frames along the major axis of the columns are moment resistant frames, and column bracings are provided in the frames along the other perpendicular direction. This paper focuses entirely to the procedure involved in design of compression members. Typical problem have been worked out using allowable stress design methods and limit state method and comparative studies is made.

ISSN: 0975 –6744| NOV 11 TO OCT 12 | Volume 2, Issue 1 Page 73

JOURNAL OF INFORMATION, KNOWLEDGE AND RESEARCH IN

CIVIL ENGINEERING

ALLOWABLE STRESS DESIGN

With the development of linear elastic theories in the 19th century the stress-strain behavior of new materials like wrought iron & mild steel could be accurately represented. The first attainment of yield stress of steel was generally taken to be the inset of failure. The limitations due to non-linearity and buckling were neglected. The allowable stress is defined in terms of a “factor of safety” which represented a margin for overload and other unknown factors which could be tolerated by the structure.

Allowable stress =

LIMIT STATE DESIGN

An improved design philosophy to make allowances for the shortcomings in the “allowable stress design” was developed in the late 1970’s and has been extensively corporated in design standards and codes formulated in all the developed countries. Although there are many variations between practices adopted in different countries the basic concept is broadly similar. The probability of operating conditions not reaching failure conditions forms the basis of “Limit States Design” adopted in all countries. Ultimate limit states are those catastrophic states, which require a larger reliability in order to reduce the probability of its occurrence to a very low level.” Serviceability limit state” refers to the limits on acceptable performance of the structure.

LOAD AND LOAD COMBINATIONS

To design a structure, it is analyzed first for its intended structural configuration and assumed sectional properties against various loads individually and in combination with each other in a way by which the structure may be subjected any time or at all time during the life of the structure for which is to be built. The various primary loads and other secondary effects required to be considered for Indian condition m while computing maximum stresses in a structure are mainly as follows

a) Dead load b) imposed load or live load c) wind load d) seismic load e) erection load f) Secondary effects due to contraction or expansion resulting from temperature changes, shrinkage, creep in compression members etc.

As a general approach, a structure is analyzed for all the probable primary load cases and their combinations are mentioned above. Only for special structures or under stringent conditions, the secondary effects are considered in the overall analysis and in the design of connections of the structural components. While designing a structure using the popular “Allowable stress design method”, the above load combinations are considered with an individual load factor of unity. As per IS: 800-1984, the permissible stress can be increased upto 33%, whenever wind or seismic load is taken in to consideration.

In the proposed Limit state method of design also the above load combinations are considered, but with variable load factors called the “partial safety factor for load as described in Table4. This variable load factors basically account the loading and thus enable to use steel efficiently and economically in different structural systems.

Similarly, to determine the strength of the member to be designed against the factored loads as described above, a reduction factor for strength called “partial safety factor for material” is taken into consideration, which accounts for uncertainty in material strength and quality as well as manufacture tolerance. Various material safety factors as have been adopted in IS: 800-2007 are given in the table –5

DESIGN PROCEDURES

The detail design procedure of compression member using allowable stress design method as per IS: 800-1984 and also limit state design method as per IS: 800-2007 have been discussed with the help of example and comparatives study as been done

ISSN: 0975 –6744| NOV 11 TO OCT 12 | Volume 2, Issue 1 Page 73