Installing and Testing Multi-Bar High Capacity Rock Tiedowns-Paper

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One Manhattan West, a 67-story office tower in Manhattan, required high-capacity rock tiedowns to resist uplift from substantial lateral loads. Site access, crane limits, excavation geometry and schedule restricted drilling to 16-inch-diameter sockets. Designers selected 83 permanent, double-corrosion-protected tiedowns, each with three 3-inch high-strength threaded bars and an allowable capacity of about 923 tons.<br /><br />Installation took place in strong Hartland Formation bedrock, but a large, previously undetected rock void disrupted drilling for tiedown B-17. Spoils could not be cleared reliably, and the hole repeatedly refilled. After conventional hammer and roller-bit methods failed, the contractor used a modified underreaming bit to advance threaded casing through the unstable zone. The completed anchor was successfully tested and locked off.<br /><br />Group testing required a 1,400-ton center-hole jack to load all three bars together. Test elongations met recommended limits, and permanent set was under three-quarters of an inch. However, multi-bar anchors introduced challenges not typical of single-bar systems: uneven load sharing, jack-to-anchor eccentricity, reaction-plate deformation, and visible movement of bar tails during testing. Even small alignment errors can create substantial bending forces at these high loads.<br /><br />The authors recommend accounting for construction tolerances in design, protecting factory-installed grout during transport and handling, and strengthening quality control during installation. Testing improvements could include aligning the jack with the bar group’s centroid or testing bars individually, and using strain gauges to monitor and balance load among bars. The case demonstrates that high-capacity tiedowns can make efficient use of limited rock-socket dimensions, but scaling up capacity requires greater attention to constructability, installation control and test monitoring.

Keywords

Manhattan West

office tower

rock tiedowns

high-capacity anchors

Hartland Formation

double-corrosion protection

underreaming bit

rock void

load testing

construction tolerances