The Working Stress Method is used in the Indian Standard Code of Practice for Steel Bridges (IS: 1915–1961). Most worldwide standards have adopted the Limit States Method of design because it is more rational and results in a more efficient, inexpensive design with consistent dependability. The Working Stress Method is still used in India as well as some other countries such as China, Vietnam, and Indonesia.
In the Working Stress Method, each component in the structure is assigned a tolerance limit that determines how much it can be loaded before failing. If any load on the bridge exceeds this value, then it will fail. For example, if a beam is declared to carry 10 tons, then it can be loaded up to 10 tons, but no more. If the beam is loaded 11 tons then it will fail.
A bridge is designed so that all its components will fail at different loads. For example, if a bridge has two girders, one may be able to support a load greater than the other. They are called "limit states" because they represent the maximum load that the girder can withstand without failure. Any load above this level will cause the girder to fail.
Now let's say that one of the girders fails when it is being loaded with 5 tons. This means that the remaining girder must be able to support at least 6 tons in order to fulfill its requirement.
The following is the Indian standard code for structural steel design for all sorts of constructions. IS: 816: 1969-Code of Practice for the Use of Metal Arc Welding in General Construction in Mild Steel IS: 1161: 1998-Specification for Structural Steel Tubes. For more information about these codes, visit Indian Buildings.
The American Iron and Steel Institute (AISI) has developed a set of guidelines to help users select suitable materials for their projects. The AISC 310: 2010 Guidelines for Designing with Steel - Primary Structure provides recommendations for choosing between various types of steel for different applications. The following are some general principles that apply to all types of steel:
The strength of steel is directly related to its mass. The greater the mass, the stronger the steel will be. However, there are limits beyond which increasing the weight would not improve the performance of the structure.
The type of steel affects the strength and other properties of the material. For example, hot-rolled steel is stronger than cold-drawn steel of equal thickness. Cold-formed steel is stronger than hot-formed steel of equal size. Strip steel is stronger when it is thicker. The yield point of steel is affected by its temperature. Lowering the temperature of hot metal reduces the risk of failure due to heat damage.
IS Codes List-Indian Civil Engineering Standards
|IS – 432||Specification for Mild steel and medium tensile bars and hard drawn steel wire.|
|IS – 1139||Specification for hot rolled mild steel, medium tensile steel and high yield strength steel deformed bars for concrete reinforcement.|
|BIS CODE : INDIAN STANDARD for Structural Engineering|
|IS: 800-1984||Code for practice for general construction in steel.|
|. IS:2062 -1992 (4th Rev)||Specification for steel for general structural purposes.|
|IS:3443 -1980(1st Rev)||Specification for crane rail section|
For building safe bridge constructions, the engineering design process involves the following steps: 1 gaining a thorough grasp of the situation, 2 identifying prospective bridge loads, 3 combining these loads to determine the greatest potential load, and 4 computing mathematical correlations to determine the highest potential load for which the structure will be able to support it.
Factors such as material strength, environmental conditions, cost, etc. are also taken into account during the design phase.
The goal is to provide reasonable protection against potential loads while minimizing the cost of construction. Cost may be reduced by using proven designs or by omitting some features (such as center piers) that add little protection but increase the expense of construction. Proven designs are often based on previous experience with similar structures. For example, a highway bridge might be designed using data from other similar bridges.
Some factors are difficult or impossible to control during construction. For example, wind can cause severe damage to bridge decks even if it is not strong enough to collapse the structure. It is therefore important to allow for some margin of safety in deck thicknesses.
Finally, cost cannot be the only factor considered when choosing a bridge design. If a cheaper design would suffice, then it should be used instead. For example, a single-span truss bridge costs less than two double-truss bridges of equal size and weight capacity.