Description

The AASHTO specifications give an approximate method for anchorage zone design, which takes the bursting force as roughly 25% of the jacking force — a result that aligns with a strut-and-tie model at a strut angle of about 30 degrees. This session applies that method to the anchor block of a 35 m PCI girder with four 17-strand tendons, a 1.2 load factor on the jacking force and a tendon inclination of 4.15 degrees, arriving at a dense layout of horizontal and vertical bursting reinforcement. AASHTO itself notes that the method is conservative, that the actual bursting force concentrates closer to the anchor plate, and that 3D effects should be investigated.

The same anchor block is then modeled in MIDAS FEA NX as a 3D cracked model: solid elements for the concrete, embedded rebar for the bursting, spalling, edge tension and spiral reinforcement, nonlinear stress-strain curves for both materials, and the jacking force applied as a uniform pressure of 40 MPa on the anchor plates. When the tensile stress in the concrete exceeds its capacity, cracking occurs and internal forces are redistributed to the reinforcement, so rebar stresses and crack widths are read directly from the model.

In the FEA model, the horizontal bursting rebar stress is 72 MPa against 241 MPa by the AASHTO method, and the vertical bursting rebar carries only about 25 MPa because the vertical bursting force spreads through a larger volume of concrete. The session attributes the gap to the strut-and-tie assumption of a freely elongating tie, where the 3D model keeps the bar bonded and restrained, and to part of the force being shared by the vertical reinforcement. The local zone compressive stress, by contrast, matches the AASHTO value of 18.32 MPa closely. Cracks remain only partially open, with widths within 27 microns against the BS 5400 limit of 0.1 mm, and the crack pattern is read against the AASHTO bursting rebar length for the governing horizontal direction.