Strut and tie method – example 1

Example #1

Consider the 2 meter deep beam described in Figure 1.1 below. Use the strut-and-tie model to determine the required amount of reinforcement.
Additional details:
fc’ = 25 MPa, fy = 420 MPa, PDL = 800 kN dan PLL = 400 kN

Figure 1

Step 1: Evaluate the Total Factored Load, Pu
Pu = 1,2 PDL + 1,6 PLL
= 1,2 (800) + 1,6(400) = 1600 kN

Step 2: Check Bearing Capacity at Loading and Support Locations
Bearing strength at points of loading = φ0,85fc’βnAc
= 0.75(0.85)(25)(1.0)(450)(500) N
= 3586 kN > 1600 kN OK
Bearing strength at supports = φ0,85fc’βnAc
=0.75(0.85)(25)(0.80)(450)(500) N
= 2868 kN > 1600 kN OK

Step 3: Select the Strut-and-Tie Model to Use in Design

Figure 2

Step 4: Isolate Disturbed Region and Estimate Member Forces and Dimensions
The entire deep beam is a disturbed region, but it is only necessary to consider the left third of the structure to complete the design. The horizontal position of nodes A and B are easy to define, but the vertical position of these nodes must somehow be estimated or determined. What we do know is that the design strength of strut BC must be greater than or equal to the factored load in strut BC.

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