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EGB376 (completed)

Behaviour and Design of Tension Members – AS 4100 Section 7

QuestionAnswer
Tension members are used in... • Chords and Web members of trusses and lattice girders in buildings and bridges • Bracing members in buildings • Cables in bridges
Tension Members • Concentrically loaded pure tension members are the simplest and most efficient structural elements • Similar to the behaviour of steel coupons in standard tensile tests
What are the shortcomings of Tension Members? • End connections • Load reversal due to wind action
Tension Members are often subject to combined tension and bending actions • eccentricity of connections • frame action • transverse loads and • self-weight
If the tension member is predominantly loaded in axial tension, use Section 7 of AS 4100
Sections for Tension Members - Rigid members: UB, UC, CHS, RHS, SHS, Angles, Channels RHS, CHS, SHS - increasingly used (suitable for load reversals)
Sections for Tension Members - Flexible members: Rods, Bars, Cables. They are often used as cross-bracing members – often pre-tensioned (not suitable for load reversals)
Sections for Tension Members - Compound Members: double angles, double-channel and I-sections for longer lengths and larger forces
Effects of End Connections • Introduce Eccentricity • Bolted connections – bolt holes reduce available area and introduce stress concentrations; but steel has good ductility, so use net area
Effects of End Connections (2) • Welded connections – no such problems, but cannot be used often due to practical and economical reasons • Bolted/welded splices are sometimes needed in the middle of long tension members
Design Action Effects • Maximum N* or M* from appropriate load combinations • Use simple statics or computer programs
Design Action Effects (2) • If M* is considerable, design as beam-columns • Otherwise, design as tension members, but allow for eccentricity and other related factors through Reduction factors
What do you know about Tension Members? • Tensile stress = F/A • Tensile strain = Stress/E = F/EA • Extension = FL/EA • Do Not Fail By Buckling • They fail at the weakest point in the Cross-section
Section Capacity in Tension N_t depends on two failure modes ➢ Yielding of the gross section ➢ Fracture of the net section
Tension members may fail either in... Yielding of the gross section Fracture of the net section
Yielding of the Gross Section - Failure is ductile and associated with large deformations - Does not occur at connections
Yielding of the Gross Section - For sections with different flange and web yield stresses, the... lower yield stress is used for the entire cross-section
Fracture of the Net Section - Brittle failure and involves less deformation - Occurs at connections
Fracture of the Net Section Equation N_t = 0.85k_t*A_n*f_u
Fracture of the Net Section - f_u is used as yielding and stress redistribution at the holes allows strain hardening to occur
Factor 0.85 is used to allow for... the brittle failure
Correction factor k_t has to be used here unlike for yielding failure, which... occurs away from the connection where the stress will be uniform
Correction Factors k_t - If k_t is NOT included, you need to consider... combined axial tension and bending actions for design
Compound Members - AS4100 has minimum requirements for battens and lacing elements to enable the COMPOUND members... to act as a SINGLE member
Serviceability Design •Extension = FL/EA – Not critical •Sag is more important •Sag limit is L/100 to L/150 for tension members •Use pre-tensioning to reduce sag for long members (turnbuckles in cables and rods) •Vibration and fatigue, mainly in bridges
Summary - Concentrically loaded tension members: Simplest structural elements
Summary - Two Main Problems with Tension Members: End connections and Load reversal
Summary - Two modes of failure: Gross section yielding ϕN_t = ϕA_g *f_y Net section fracture ϕN_t = ϕ0.85 k_t*A_n*f_u
Summary - Tension members do not fail by... buckling
Summary - Strength limit state requirement: N* ≤ ϕN_t
Created by: Asher - S
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