A presentation on Strut and Tie Models (S-T-M)
•Introduction
•Development
Strut and Tie Model•Design Methodology
•IS and ACI provisions
•Applications
–Deep beams
–Corbels
–Beam-column joints
•Hydrostatic state of stress
–Nodal zone dimensions proportional to the applied compressive forces
–One dimension by the bearing area
–Other two, for a constant level of stress ‘p’
–Preselected strut dimensions , non hydrostatic
•Extended Nodal zone
–Inadequate length of hydrostatic zone for tie anchorage
–Intersection of the nodal zone and associated strut
–The portion of the overlap region between struts & ties, not already counted as part of a primary node
Strut and Tie design Methodology
•Steps in design Strut and Tie
1)Define and Isolate D-regions
2)Compute the resultant forces on each D-region boundary
3)Select a truss model to transfer the forces across a D-region
4)Select dimensions for nodal zones
5)Verify the capacity of node and strut; for struts at mid-length and nodal interface
6)Design the ties and tie anchorage
7)Prepare design details and minimum reinforcement requirements
•Strength and serviceability
•Strength criteria
–ACI A.2.6
–Strength reduction factor 0.75
•Serviceability checks
–Spacing of reinforcement within ties
•Steps in design
1)D-regions (ACI A)
Region extending on both sides of a discontinuity by a distance ‘h’
2)Resultant forces on D-region boundaries
–Helps in constructing the geometry of the truss model
–Subdividing the boundary into segments
–Distributed load
–Moments at faces of beam column joints
3)The Truss model
–Multiple solutions
–Axes of truss members to coincide with centroids of stress fields
–Struts must intersect only at nodal zones; ties may cross struts
–Effective model-minimum energy distribution through D-region
–Stiffest load path
–Minimum no. of ties
–Equilibrium ,structural stiffness
–Effectively mobilizes ties -cracking
–Points of maximum stresses
4)Selecting dimensions for Struts and Nodal zones
–Thickness of strut, tie and nodal zone typically equal to that of the member
–If thickness of bearing plate < thickness of member, reinforcement perpendicular to the plane of the member to be added – confinement, splitting
5)Capacity of Struts
–Based on, the strength of the strut & strength of nodal zone
–Insufficient capacity of strut – revising the design
•Add compression reinforcement
•Increase size of nodal zone
•Bearing area of plate and column
6)Design of Ties and Anchorage
At service loads, stress in reinforcement well below yield stress (crack control)
Geometry of tie – reinforcement fits within tie dimensions, full anchoring
Anchorage – nodal and extended nodal zones + available regions on far side
•Length available for anchorage of ties la
•Extended nodal zone
•Extend beyond or hooks for full development
7)Design details and minimum reinforcement requirements
oComplete design demands the verification
§Tie reinforcement can be placed in the section
§Nodal zones confined by compressive forces or ties
§Minimum reinforcement requirements
oTie details – development length, mechanical anchorage
oShear reinforcement – permissible shear force(code), controlled longitudinal cracking of bottle shaped struts, minimum reinforcement (code)
–ACI Code Provisions
•Strength of struts
•Strength of nodal zones
•Strength of ties
•Shear reinforcement requirements (Deep beams)
For more and detailed information on the Design of Strut and Tie Model, go through the PPT given below
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