Helical anchor allowable bearing pressure
Helical anchor allowable bearing pressure
(OP)
I am designing helical anchors using the individual bearing capacity method. I typically limit the bearing pressure of the helices to less than the allowable soil bearing pressure given in the project geotech report. For instance, I recently was asked to design helicals to replace concrete-shaft drilled piers that had an allowable end bearing of 15000 psf, so I limited the helical bearing to less than 15000 psf. Is that necessary? I have not found such a design check in any of the literature I have or by an internet search, but it seems reasonable to me. So, is that check necessary?





RE: Helical anchor allowable bearing pressure
You also need to check the structural strength of the screwpile.
I would specify that they check the torque value also and base the acceptance of the load capacity on that.
RE: Helical anchor allowable bearing pressure
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RE: Helical anchor allowable bearing pressure
RE: Helical anchor allowable bearing pressure
As noted above, the geotechnical recommendation could be uniformly conservative, since the geotech may have a high factor of safety on their tip resistance. Low allowable bearing might be presented in part because if stronger and weaker portions of the deposit are available (variable non-homogeneous strength) one can't rely on the tip bearing on the strongest layer (either because one can't know exactly which depths the highest strength is, or one can't rely on the contractor not to overdrill, whereas you can advance the helical until torque is reached), or in particular they may have assumed cleanout of the shaft tip may not be perfect, particularly for small-diameter shafts.
RE: Helical anchor allowable bearing pressure
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RE: Helical anchor allowable bearing pressure
FOUNDATIONS (SCREW PILES)
THE FOUNDATION DESIGN IS BASED ON USING CONTINUOUS SCREW PILES
PILES HAVE BEEN DESIGNED ASSUMING A SKIN FRICTION VALUE OF 375 PSF AND AN END BEARING CAPACITY OF 4177 PSF
FOR EXTERIOR PILES, THE TOP 6.6 FT BELOW FINISHED GRADE HAS BEEN NEGLECTED FOR SKIN FRICTION
FOR INTERIOR PILES AND BASEMENT WALL PILES, THE TOP 3 FT HAS BEEN NEGLECTED FOR SKIN FRICTION
SKIN FRICTION HAS BEEN ASSUMED FROM THE ABOVE DATUM TO ONE HELIX DIA ABOVE THE FIRST HELIX
BEARING AREA HAS BEEN ASSUMED TO BE THE HORIZ PROJECTED AREA OF THE HELIX LESS THE DIA OF THE PILE FOR THE UPPER HELICES AND THE FULL PROJECTED AREA FOR THE LOWEST HELIX
PILE WALL THICKNESS AND SIZE SHOWN IS MINIMUM AND SHALL BE INCREASED AS NECESSARY TO ACCOMMODATE INSTALLATION EQUIPMENT
SCREW PILES SHALL BE PLACED NOT CLOSER THAN 2-1/2 HELIX DIA FROM ADJACENT PILE U/N
PITCH OF HELICES SHALL BE 6" UNLESS APPROVED BY THE GEOTECHNICAL CONSULTANT
SPACING OF HELICES ALONG THE PILE SHAFT SHALL NOT BE LESS THAN 3 TIMES THE HELIX DIA
SPACING OF HELICES SHALL BE A WHOLE NUMBER MULTIPLE OF THE HELIX PITCH
PROVIDE A 1" THICK MIN STEEL PILE HEAD WITH SUITABLE MEANS OF CONNECTING TO THE INSTALLATION EQUIPMENT
PROVIDE A ZINC RICH EPOXY PRIMER FOR THE TOP 10 FT OF ALL PILES AND THE PILE HEAD
Dik
RE: Helical anchor allowable bearing pressure
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RE: Helical anchor allowable bearing pressure
RE: Helical anchor allowable bearing pressure
The piles have a capacity of from roughly 50K to 75K with the first helix located approx 16' from grade. Clay is approx 60' deep.
Dik
RE: Helical anchor allowable bearing pressure
Dik
RE: Helical anchor allowable bearing pressure
RE: Helical anchor allowable bearing pressure
Dik
RE: Helical anchor allowable bearing pressure
www.PeirceEngineering.com
RE: Helical anchor allowable bearing pressure
Bks
RE: Helical anchor allowable bearing pressure
RE: Helical anchor allowable bearing pressure
There's no need to do any type of finite element modeling, unless you were trying to analyze the plate bending capacity. I normally do a yield line analysis to determine plate thickness. The design of the shaft itself for axial and lateral loading is no different than any other steel pile. The biggest difference is you will need to analyze the shaft and connections for torsional loading. The torsional load itself will be dependent on the soil/pile capacity. There are well established torque to bearing capacity correlations for "normal" shaft sizes; i.e. for a 1.5" square shaft, a Kt of 10 is accepted. This means you can multiply the torque (in ft_lb) by 10 to obtain the ultimate capacity of the pile (in lbs). The Kt drops as the hub (pile) diameter and helix plate thickness increase. For 2.875" pile the Kt is 8 to 9, for 3.5" pile its 7-8. At 7" it will be down to 4.5. To structurally design the pile, you first have to determine the ultimate capacity of the system. If you are designing the pile for a known load, this would be the required allowable capacity times a factor of safety (2 normally). If you are checking a particular blade configuration in a soil profile, you would calculate the ultimate bearing capacity using either the individual bearing capacity method or the cylindrical shear method. Once you know the ultimate capacity, you have to select a pile size (diameter and thickness) capable of handling the required torque, axial, and lateral loads. It can be somewhat trial and error, especially if you are working with a large array of pile sizes and working on non-typical stuff.
Is that what you were looking for?
bks
RE: Helical anchor allowable bearing pressure