Types of Tension Structural Members/Design for Axial Tension/Forces.

Regarding the Vertical Elements of a Car Frame, such as the one in Elephant & Castle Underground Station----I did an Engineering Calculation on the Frame and its Structural Upright, in my previous Article I mentioned my considered View on the Fastenings/their Grade of Steel which should have been--10 or better. Theoretically the tension Member between 2 Points on a Structure Cross-Head [Horizontal Structural Member], in some situations (such as cross-braced) structural connecting Members. In some circumstances when the Load reverses the Member acts as a Strut and can also Buckle when the Load is taken up by another Structural Member, such as when a Lift Accelerates in the Downward direction in EMERGENCIES--Free fall does not easily occur, but if the Safety Gear Blocks do not Grip correctly with the "T" Guide Rail, the efficiency of Braking could be 10% or less on one side & up to 40% on the opposite "T" Rail,thus a Twisting Moment can occurs with Major Cracks in the Steel Structures, at the Bottom end of the Lift Car Frame which supports the Platform and below and alongside the Vertical Members the Fixings/Fastenings of the Housing of the SFG could twist thus causing Shear Failures of Bolts & Abutments. This could lead to the SFG Housing sliding downwards.

In Compound Sections consisting of Double Angles or Channels at least one Axis of Symmetry is present hence the Eccentricity at the End connection can be Optimised.

Perforated Bars/Flats (enhance the lightness in the vertical plane, whereas as horizontal ones increase the Stiffness of the Member)-This can be added to existing horizontal structural members and welded in position, thus increasing the Flexural Strength of the Cross-Member.

When Designing for AXIAL Tension Rolled sections to British Standards, at --tensile stress f _1 = F/A. and Elongation "Delta_l" = FL/AE (in the linear elastic range)-------with a Load at Yield, P_y =p_y x A = Load Failure. The design of axially loaded tension Members is compliant with Standards given in B. S. 5950 : Part 1-----The factor K_g are found in BS 4360 and in Combined Bending and Tension the Loads arise from;

a) Eccentric Connections b) Lateral Loading c) Rigid Connections.

John Irwin C. Eng. M. I. Mech. E. of 22 Princess Court, Queensway, London W2 4RB