Guy Anchor Holding Capacity in Soil

Nothing says whether the ground will hold it, and an anchor that pulls is the failure mode that takes the pole with it. An anchor in tension fails by pulling a cone or cylinder of soil up with it, and for an anchor deep relative to its bearing area the capacity is a bearing-capacity problem turned upside down: the helix or plate area times the strength the soil can mobilize at that depth. In clay that strength is dominated by cohesion and the depth term is small; in sand there is no cohesion at all and the whole capacity comes from overburden times a bearing factor that climbs steeply with friction angle. A 12 in helix at 7 ft in a stiff clay at 1,000 psf cohesion holds 7,673 lb ultimate and 3,837 allowable at a factor of safety of 2; the same anchor at the same depth in a loose sand with no cohesion holds 6,048 ultimate and 3,024 allowable. Same hardware, same depth, and the soil is the whole variable. For power-installed screw anchors there is a second and better number: installing torque. The torque the machine reads at final depth correlates with capacity through an empirical factor set by shaft size, and it has the enormous advantage of measuring the soil that is actually there rather than the soil someone guessed at from a boring two hundred feet away. Both are reported, and where they disagree by a wide margin THAT DISAGREEMENT IS THE FINDING -- it means the assumed soil is not the soil the anchor went into, and the torque reading is the one to believe. This is a single-helix bearing estimate from soil properties the user supplies, and it is only as good as they are. It does not sum multiple helices, apply the spacing rules that stop them from acting as one block, or check whether the anchor is deep enough for the deep-failure assumption to hold -- a shallow anchor fails by lifting a soil cone to the surface, which is a different and weaker mechanism this does not model. It does not evaluate the shaft, the eye, the guy hardware, or the rod's own tensile capacity, which can govern instead of the soil; it does not address group effects, cyclic or sustained loading, frost heave, corrosion, or installation in fill, rock, or saturated soil. The torque correlation factor is empirical and manufacturer-specific. The anchor manufacturer's data, a geotechnical evaluation of the actual site, the utility's construction standards, and the applicable NESC edition govern.

cohesive Q_u = A (9 c + gamma D); granular Q_u = A gamma D N_q; allowable = ultimate / factor of safety; screw-anchor torque correlation Q_u = K_t x T.

The deep-anchor uplift bearing relations as standard practice, by name, with the empirical screw-anchor torque correlation reported alongside. Single helix, deep-failure assumption. The anchor manufacturer's data, a geotechnical evaluation of the actual site, the utility's construction standards, and the applicable NESC edition govern.

A bearing calculation on soil properties the user supplies; no manufacturer capacity table is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: helix_diameter_in, installed_depth_ft, cohesion_psf, friction_bearing_factor, soil_unit_weight_pcf, factor_of_safety, installing_torque_ftlb, torque_factor_per_ft, guy_tension_lb, helix_area_ft2, ultimate_capacity_lb, allowable_capacity_lb, torque_capacity_lb, margin_lb

• Deep-failure assumption a shallow anchor fails by a weaker cone mechanism not modelled hereanchor design practice
• Single helix multiple helices and their spacing rules are not summedthe anchor manufacturer's data
• The torque factor is manufacturer-specific it is set by shaft size and is empiricalthe anchor manufacturer's data