They have more surface exposure per unit volume, the flange tips tend to cool faster than flange-to-web junctures.
- Tension members response to load is much similar to that of tensile-test coupon .
- Member behavior may differ from coupon behavior because of:
- Slip in bolted & riveted connections
- Non-linear behavior of connections
- Residual stresses in member
- Residual stresses result principally from non-uniform cooling of hot rolled or welded shapes and from cold straightening of bent members.
- They have more surface exposure per unit volume, the flange tips tend to cool faster than flange-to-web junctures.
- Similarly, Central portion of web tends to cool faster than junctures
- As a result, metal at junctures continues to contract as it cools after flange tips and web interior have cooled to temperature of surroundings.
- This contraction is partially restrained by cooler metal.
- Developing tensile stresses in regions of junctures & compressive stresses in remainder.
- These are called residual Stresses
- Magnitude and distribution of thermal residual stresses are influenced to considerable degree by geometry of x-section
- 20 W's were investigated
- An investigation on W's Revealed that flange tip stress varied from 4.1 to 18.7 Ksi
- Residual stresses in web varied from 41Ksi comp; to 18.2Ksi tension.
- Showing some W's develop residual tension over entire web, instead the pattern shown.
- Only one out of 20 sections was thicker than 1 in.
- Residual stresses tend to increase in magnitude with increase in thickness
- Because of high concentration of heat, tensile residual stresses at the weld in welded members usually equal the yield strength of the weld metal itself.
- Residual stresses in welded shapes are determined by section geometry and method of preparation of components.
- Example H may be fabricated from Universal-mill plates or from plates flame-cut to width.
- Because they are quenched and tempered, A514 rolled steel shapes are partially stress-relieved, so residual stresses are small
- Quenching is the act of rapidly cooling the hot steel to harden the steel.
- Quenched steel is hard and brittle. Often it is just too brittle and must be made more malleable, This is achieved by a process known as tempering.
- The quenched steel is heated again but this time to a temperature between 200 °C and 300 °C.
- When the metal reaches the tempering temperature, it is quenched again in cold water or oil. The result is a steel that is still hard but is more malleable and ductile
- Fabricating operations such as cambering and straightening by cold bending also induce residual stresses.
- These stresses are superimposed on the thermal residual stresses.
- If member is straightened by rotorizing, which is a continuous procedure, the residual stress distribution will be changed along entire length of member.
Effect on Behavior of tension Members