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What is the difference between working stress approach and limit state approach?

For working stress approach, service loads are used in the whole design and the strength of material is not utilized in the full extent. In this method of design, stresses acting on structural members are calculated based on elastic method and they are designed not to exceed certain allowable values. In fact, the whole structure during the lifespan may only experience loading stresses far below the ultimate state and that is the reason why this method is called working stress approach. Under such scenario, the most economical design can hardly be obtained by using working stress approach which is now commonly used in the design of temporary works.

For limit state approach, for each material and load, a partial safety factor is assigned individually depending on the material properties and load properties. Therefore, each element of load and material properties is accurately assessed resulting in a more refined and accurate analysis of the structure. In this connection, the material strength can be utilized to its maximum value during its lifespan and loads can be assessed with reasonable probability of occurrence. Limit state approach is commonly used for the majority of reinforced concrete design because it ensures the utilization of material strength with the lowest construction cost input.

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A Comparative Study of Retrofitting Of R.C. Building Using Steel Bracing And Infill Walls

By
Prof. Pravin B. Waghmare
Acharya Shrimannarayan Polytechnic Pipri (M)- Wardha-Maharashtra

Abstract
The objective of this study is to identify an efficient retrofitting method for existing open ground story reinforced concrete frame buildings. Failure of several soft-stored buildings in the past earthquakes underscores the need to retrofit existing soft-story buildings. A common cause for the collapse of multi-storied buildings is the occurrence of soft story in the ground floor due to the presence of infill walls in the upper story. During the Bhuj (Gujarat) earthquake of 6thJanuary 2001 several soft storied building failed there by confirming the vulnerability of such buildings to earthquake loading. This underscores the need to retrofit existing soft story buildings to prevent their total collapse. The existing building structures, which were designed and constructed according to early codal provisions, do not satisfy requirements of current seismic code and design practices. A two dimensional R.C. frame designed with linear elastic dynamic analysis using response spectrum method. The computer software package STAAD Pro–2005 is used for dynamics analysis technique is used to assess the performance of a (G + 4) reinforced concrete buildings, of which the ground storey is a parking facility the ground storey is 3.5m high while the upper stories giving a total height of 15.5 m. the building is located in Seismic Zone IV.

The RC frame is retrofitted by three methods namely,
1) Brick masonry infill in the ground story.

2) Steel braces in the ground story.

3) R.C. Structural wall in the ground story.

The study concludes that the building designed as per provisions of IS: 456:2000 using limit state method of design, and analyzed as per existing seismic code IS: 1893-2000 of all these three methods studied the use of structural wall in the ground story panel gave the maximum strength and ductility.

Keywords: Open ground storey, brick infill, RC wall Infill and Steel Bracing.
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Should high-yield steel or mild steel be designed as road reinforcement?

High yield steel is the preferred material for the reinforcement of concrete carriageway because of the following reasons:

(i) The principal function of steel reinforcement in concrete pavement is to control cracking. If mild steel is adopted for reinforcement, upon initiation of crack formation mild steel becomes overstressed and is
prone to yielding. High yield steel offers resistance to crack growth. The above situation is commonly encountered where there is abnormal traffic loads on concrete carriageway exceeding the design limit.

(ii) High-yield steel is less prone to deformation and bending during routine handling operation.

(iii) In the current market, steel mesh reinforcement is normally of high-yield steel type and the use of mild steel as road reinforcement requires the placing of special orders to the suppliers.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Analysis and Design Of 2-D Tubular Frame Using USFOS Modeling

Analysis and Design Of 2-D Tubular Frame Using USFOS Modeling by
SOURADEEP GUPTA
Department of Civil and Environmental Engineering
National University Of Singapore

Abstract:
USFOS is the analytical tool for predicting both the resistance of structures subject to accidental loads and the residual strength of damaged structures after such loads. It is based on finite element modeling. USFOS covers static collapse analysis, non-linear time series dynamic analysis as well as eigenvalue analysis of typically jackets, jack-ups, topsides and floaters. Primarily the purpose of this paper is to analyze two types of 2-D offshore frame and study the progressive collapse mechanism in these two frames due to different load combinations along X-axis and Y-axis. First the boundary conditions were fixed for the vertical members using USFOS modeling and they were tested for collapse under four different load combinations. Differences in behavior of two frames have been studied and different brace-chord sizes have been fixed. This type of analysis is useful to test if an offshore jacket with some specified size can stand the load coming on it from waves, wind or impact of ships. By utilizing the inherent redundancy found in most offshore structures the progressive collapse limit state can be used to design for accidental damage or extreme loads. Whereas in traditional elastic design redistribution of load is not normally considered. Collapse or plastic limit state design allows for local failure in yield or buckling and even partial collapse, provided the overall integrity of the structure is maintained. In short, plastic limit state design allows the designers to take advantage of any reserve capacity in the structure.
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Behavioural Studies On Hollow Double Skinned Steel Concrete Composite Columns

Research Paper by N.Balasubramanian,R.B.Karthika and Dr.R.Thenmozhi
Government College Of Technology, Coimbatore-641 013, India

ABSTRACT
This paper comprises of the experimental study of eight double skinned concrete filled steel tubular (DSCFT ) beam columns of concentrically placed circular sections filled with self compacting concrete. Tests on the specimens were made by applying eccentric loads. The main experimental parameters for beam-columns were slenderness ratio and load eccentricity. Testing of specimens investigates the behaviour on load deflection, confinement effect, and the strength of the columns. The experimental observations were shown by load-deflection curves. Various characteristics such as strength, stiffness, ductility and failure mode are discussed. The predicted load versus deformation relationships are in good agreement with beam-column test results. The DSCFT columns in-filled with SCC show good strength and ductility. Modified equations are suggested to find the ultimate compressive strength of DSCFT columns filled with SCC.

Keywords : Composite; Double skinned concrete filled steel tubular columns; D/t thickness, fabrication and casting, load deflection, ductility.

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