STRUT & TIE MODELS (S-T-M) Module 2

Topics Introduction Development 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 Define and Isolate D-regions # Compute the resultant forces on each D-region boundary # Select a truss model to transfer the forces across a D-region # Select dimensions for nodal zones # Verify the capacity of node and strut; for struts at mid-length and nodal interface # Design the ties and tie anchorage # Prepare design details and minimum reinforcement requirements # HOME

Strut and Tie design Methodology Strength and serviceability Strength criteria ACI A.2.6 Strength reduction factor 0.75 Serviceability checks Spacing of reinforcement within ties

Strut and Tie design Methodology Steps in design D-regions (ACI A) Region extending on both sides of a discontinuity by a distance ‘h’ HOME

Strut and Tie design Methodology Steps in design 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 HOME

Strut and Tie design Methodology Steps in design 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 Alternative truss models for a deep beam HOME

Strut and Tie design Methodology Alternative truss models for a deep beam Single tension tie direct load path Greater number of transfer points & ties More flexible truss Complex layout Upper tension tie Lower tension tie HOME

Strut and Tie design Methodology Steps in design Selecting dimensions for Struts and Nodal zones Width on magnitude of forces & dimensions of adjoining elements External effects bearing plate area on Nodal zone dimensions Angle between struts and ties at a node>25◦ Design of nodal zones Principal stresses within the intersecting struts and ties are parallel to the axes of these truss members Width of struts and ties α forces in the elements Width of strut by Geometry of bearing plate / tension tie – non-hydrostatic HOME

Strut and Tie design Methodology Steps in design 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 HOME

Strut and Tie design Methodology Steps in design 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 HOME

Strut and Tie design Methodology Steps in design 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 HOME

Strut and Tie design Methodology Length available for anchorage of ties la Extended nodal zone Extend beyond or hooks for full development HOME

Strut and Tie design Methodology Steps in design Design details and minimum reinforcement requirements Complete design demands the verification Tie reinforcement can be placed in the section Nodal zones confined by compressive forces or ties Minimum reinforcement requirements Tie details – development length, mechanical anchorage Shear reinforcement – permissible shear force(code), controlled longitudinal cracking of bottle shaped struts, minimum reinforcement (code) HOME

ACI Code Provisions Strength of struts Strength of nodal zones Strength of ties Shear reinforcement requirements (Deep beams) HOME

ACI Code Provisions Strength of struts Nominal compressive strength of a strut Where fce is the effective compressive strength of concrete in a strut or nodal zone Acs is the cross sectional area at one end of the strut = strut thickness x strut width fce=0.85 βs f΄c HOME

ACI Code Provisions Strength of struts HOME

ACI Code Provisions Strength of struts When compression steel is provided, strength is increased to depends on the strain in concrete at peak stress ACI A.3.5 Transverse reinforcement for bottle shaped struts Code says ‘it shall be permitted to assume the compressive force in the strut spreads at a slope of 2 longitudinal to 1 transverse to the axis of the strut’(cl.A.3.3) HOME

ACI Code Provisions Strength of struts For ≤ 6000 psi, A.3.3 transverse reinforcement - axis of the strut being crossed by layers of reinforcement satisfying HOME

ACI Code Provisions Strength of struts Rectangular or bottle-shaped strut? Horizontal struts as rectangular, inclined as bottle-shaped &page number HOME

ACI Code Provisions Strength of Nodal zones Nominal compressive strength of a nodal zone ,effective strength of concrete in nodal zone , is the smaller of (a) and (b) HOME

ACI Code Provisions Strength of Nodal zones Unless confining reinforcement is provided in the nodal zone ,maximum (A.5.2) , compressive strength of concrete in nodal zone βn , factor for degree of disruption –incompatibility between strains in struts and ties HOME

ACI Code Provisions Strength of Ties (cl. A.4) Nominal strength of a tie HOME

ACI Code Provisions Strength of Ties (cl. A.4) Effective width of a tie, wt Distribution of tie reinforcement If placed in single layer, wt = diameter of the largest bars in the tie + 2*the cover to surface of bars Or width of anchor plates HOME

ACI Code Provisions Strength of Ties (cl. A.4) The axis of reinforcement in a tie shall coincide with the axis of the tie in STM Anchor the reinforcement as required by mechanical devices, post-tensioning anchorage devices, standard hooks etc. Ties must be anchored before they leave the extended nodal zone at a point defined by the centroid of the bars in the tie and the extensions of either the strut or the bearing area. HOME

Extended nodal zone showing the distribution of force

ACI Code Provisions ACI Shear Requirements for Deep Beams Deep beams Beams with clear span less than or equal to 4 times the total member depth or with concentrated loads placed within twice the member depth of the support Design either by Non-linear analysis or by Strut and Tie method Nominal shear ≤ (11.7.3) Minimum reinforcement perpendicular to the span Minimum reinforcement parallel to the span HOME

ACI Code Provisions s and s2 may not exceed d/5 or 12 inches For STM, bw is thickness of element b

Applications Deep beams Beam-column joints Corbel

Deep beams One of the principal application Alternative solution –nonlinear analysis Question: A transfer girder is to carry two 24in. Square columns, each with factored loads of 1200kips located at third points of its 36 ft span, as in the fig, The beam has a thickness of 2ft and a total height of 12ft. Design the beam for the given loads, ignoring the self weight. Use fc’=5000psi & fy=60000psi

Deep beams Solution: Span/depth =3.0 deep beam Use strut and tie model Step 1 : Define D-region Entire structure as D-region Thickness of struts and ties = thickness of beam = 2ft=24in Assume effective depth =0.9h=0.9x12=10.8ft Maximum shear capacity of the beam , = 0.75x10x√5000x24x(10.8x12/1000) =1650kips > Vu=1200kips Thus design may continue Dividing by 1000, to convert to kips

Deep beams Step 2 : Force Resultants on D-region boundaries Reactions at supports = 1200kips (equilibrated by the column loads on the upper face of beam) Assume centre to centre distance between horizontal strut and tie = 0.8h = 9.6ft Angle between trial diagonal struts and horizontal tie =38.66 0 Analyze the truss to find the forces in struts and tie

Deep beams Step 3 : Truss model Based on the geometry and loading, a single truss as shown, is sufficient to carry the column loads The truss has a trapezoidal shape Nodes that are not true pins and instability within the plane of truss. Not a concern in Strut and Tie models. Hence this is an acceptable solution The truss geometry is established by the assumed intersection of the struts and ties used to determine θ

Deep beams Step 4 : Selecting dimensions for struts and nodal zone Two approaches 1) constant level of stress 2) minimum strut width CCC node βn =1.0 = 0.75 x 0.85 x 1.0 x 1.0 x 5000/1000 = 3.19 ksi >2.08 ksi, demand from the column, smaller sizes possible Width of the strut ac

Deep beams Step 4 : Selecting dimensions for struts and nodal zone wab=?, wtie=? new c/c distance between horz strut and tie Angle Revised forces iterate

Deep beams Step 5 : Capacity of struts Horizontal rectangular strut, inclined bottle shaped Strut ac Adequate Strut ab The capacity of strut ab established at node b The capacity of strut ac established at node b

Deep beams Step 5 : Capacity of struts Capacity at the end of the struts and at the nodes exceeds the factored loads Hence the struts are adequate

Deep beams Step 6 : Design of ties and anchorage Selection of area of steel Design of the anchorage Validation that tie fits within the available tie width Area of steel, Provide 22 No.s No.11 bars, Placing the bars in two layers of 5 bars each and three layers of 4 bars each, total tie width matching tie dimensions Note 2.5 in. clear cover, 4.5in. clear spacing b/w layers

Deep beams Step 6 : Design of ties and anchorage Anchorage length Ld, chapter 12 of ACI 318-11 For no. 11 bars, Length of nodal zone and extended nodal zone = 24 + 0.5 x 30.7 x cot 380 = 43.6 in. < Ld Provide 900 hooks / mechanical anchorage 1.5 in cover on both sides, side face reinforcement No.5 bars transverse & horizontal, 2db spacing between No.11 bars required total thickness Fit within the 24 in. beam thickness

Deep beams Step 7 : Design details and minimum reinforcement requirement Shear reinforcement requirement in deep beams- ACI

Deep beams Step 7 : Design details and minimum reinforcement requirement Av = 0.0025 x 24 x 12 =0.72 in2/ft Providing No. 5 (0.625in,0.31in2) bars, s = 12 x 0.31x2faces /0.72 = 10.33 in. At spacing 10 in, Av provided = 12 x 0.31 x 2 /10 = 0.74 in2/ft

Deep beams Step 7 : Design details and minimum reinforcement requirement Avh = 0.0015 x 24 x 12 =0.43 in2/ft Providing No. 4 (0.5 in,0.20in2) bars, s = 12 x 0.20x2faces /0.43 = 11.16 in. At spacing 10 in, Av provided = 12 x 0.20 x 2 /10 = 0.48 in2/ft

Deep beams Step 7 : Design details and minimum reinforcement requirement Read & understand RA 3.3 ACI 318-11 Two No. 5 bars Av =0.62 in2 ; Two No.4 bars Avh = 0.4 in2 This ensures sufficient reinforcement is present to control longitudinal splitting

Deep beams Step 7 : Design details and minimum reinforcement requirement Staggered hooks used for anchorage Horizontal U-shaped No.4 bars @ 4 in (3db) across the end of the beam to confine No. 11 hooks

Column brackets or Corbel

Column brackets & Corbels Brackets – in precast construction - to support precast beams at columns When brackets are projected from a wall, rather than from a column, they are properly called corbels Both terms may be used interchangeably Design - Vertical reaction Vu at the end of supported beam Horizontal force Nuc if adequate measures are not taken to avoid horizontal forces by shrinkage, creep, temperature change

Column brackets & Corbels Bearing plates or angles on the top surface of the bracket Elastomeric bearing pads – frictional forces – volumetric change Account for horizontal forces Strut and Tie model The steel required by STM , main bars anchorage

Corbel

Corbel

Corbel

Beam-Column Joints Inadequate attention to the detailing of reinforcement Mainly at the connection of main structural elements The basic requirement at joint – all of the forces existing at the ends of the members must be transmitted through the joint to the supporting members

References: Design of concrete structures, by A H Nilson