Engineers
Compression Screwpile Design
Screwpile design is conducted using our proprietory software and we do not make it available to the public domain. It is based on common geotechnical and structural principals and calculations. In addition it takes the following into consideration:
- Intended structures minimum design life
- Site corrosion classifications
- Installation practicalities
- Interface between the sub-foundation and the foundation system
Whether acting in compression or in tension screwpiles must be designed within the appropriate Standards or Codes for the Region. In Australia this is primarily the Australian Piling Code AS2159-2010 which has just been revised.
To assist structural engineers in assessing screwpile suitability for their next project the following information is made available.
In addition to the design parameters indicated above and the following criteria needs to be addressed. This includes allowable eccentric loads, installation tolerance, corrosion allowance applicable to the site, how the loads are transferred from structure to piles, concrete crushing effects, helix deflections, effective lengths and pile buckeling, overburden soil pressures and water tables but to name a few. Explanations are as follows:
Allowable Eccentric Loads; Piling codes state as a minimum the amout of eccentricity that the designing engineer must use in this piling calculation, however due to issues such as the intended design of the structure, installation tolerance practicalities and allowance for the ground conditions any one of these may further influence necessary offsets of positioning and hense can have a dramatic impact on the short, but in particular the long term effect this has on the pile when corrosion factors are applied to the piling design calculations.
Installation Tolerances; In situations where the piles are to be installed next to existing structures or foundations what will be the actual and installation position of the screwpile. Will there be any influences caused by the existing walls or foundations on achievable machinery or pile positioning and can the installer be reasonably expected to install the screwpile within the engineered allowable eccentricities? In addition are there likely to be any obstructions underground which may cause pile deflection during installation such as existing underground structures or rocks etc?
Corrosion Allowances; Corrosion allowance has two main criteria. (1) What is the sites corrosion classification and therefore the allowable calculated sacrificial loss per year? (2) What is the specified minimum design life of the structure to be supported? The maximum allowable loss per year is multiplied by the minimum design life of the structure. This allowance is then combined with the allowable eccentric loads and installation tolerances and used in the design of the screwpile.
Load Transfer; Design needs to address how the loads are transferred from the structure to the piles. In the situation where the screwpiles are to be cast inside a concrete footing it is important to ensure that no crushing of the concrete occurs around the pile head and that there is sufficient head cover to ensure there is no punching effect occurs to the footing. The interface between the footing and the pile must be such that neither occurs and is done by various bar reinforcement and concrete configuration methods. Sometimes shear bars are specified whilst on others starter bars are specified. Filling the screwpiles shaft with a specified Mpa concrete may or may not be included in the specification.
Helix Deflection; Calculated in the overall screwpile design helix deflection is the amount of flex or bending that occurs at the hinge point of the connection between the helix and the pile shaft. This degree of bending or flex and point of structural failure will depend on:
- Helix form (helical shape and pitch)
- Number of helix plate thicknesss in accordance with the intended design load
- Quality and specification of welded connection
- Soils bearing capacity
- Installation method
It is an intrigal part of our screwpile design software and Screwpile Australia needs to be consulted.
Effective Length; Determines the pile length at which a design load is effectively transfered from a pile to the soil within the maximum allowable distance between the inflection points on a pile shaft prior to it bending.
Pile Buckeling; Developed by excessive axial load in accordance with the supported or unsupported shaft (column) and any existing or potential lateral, horizontal or bending forces applied to the shaft.
Overburden Soil Pressures; The pressure or stress exerted on the bearing surface by the soil strata above the bearing surface.
Water Table; The point where still or moving water exists below a predetermined ground level. This includes purched water tables as is the case above certain clays.
