GBP Calculator / Field guide
The field guide.
Understand the inputs, inspect the model and read the result. A practical reference for working in GBP Calculator.
Supported model
This calculation uses a rectangular timber section with one centered load and uniform pressure over the effective bearing area. It does not model steel, aluminum, composite mats, eccentric loads, multiple loads or layered systems.
Explore the workspace
Support lab has four stages: LOAD, SUPPORT, TIMBER and RESOLVE. The first three collect the inputs; RESOLVE lets you review the case, confirm the model assumptions and calculate. You can move between input stages using the navigation rail.
Drag the interactive 3D model in Perspective to rotate it. Choose Section to read the side view or Layers to separate the parts. These are schematic views, not a simulation of soil movement or a detailed construction model. You can also use the step-by-step calculator, with the same inputs and engineering checks.
Load & ground
C is the pad contact dimension along the mat length, in ft. Enter the load on this mat in lb and the allowable ground pressure in psf. Ground capacity and load data must be established for the actual site and lifting arrangement.
Mat dimensions
Enter L, W and T in feet. C must not exceed L. W is the width that actually shares the load: use the full mat width only when load distribution across it has been established. Convert thickness from inches to feet before entering it.
Timber properties
Enter density in lb/ft³, adjusted allowable bending and shear stresses in psi, and Young’s modulus in ksi. Ultimate strength values are not allowable stresses. These inputs must account for the timber species, grade, condition and service conditions.
Before you calculate
All numeric values must be greater than zero. In RESOLVE, review the values and select the confirmation that the model assumptions apply to your case. Then select Calculate support. Demonstration values illustrate the workflow; they are not project recommendations.
Read and revisit results
The model accompanies the results in RESOLVE. Read all four checks: ground bearing, timber bending, timber shear and relative edge deflection. Each compares the calculated demand with its limit. Select a check to inspect its explanation and model view.
Select Edit inputs to return to the controls. Changing an input invalidates the previous result, so run the analysis again after a change. Inspect the inputs used and select Print report to print the results. Starting a new analysis clears the current case after confirmation.
Calculation method
The implementation follows David Duerr’s Effective Bearing Length of Crane Mats (2010, revised 2012). It evaluates bending, timber shear and model-deflection length limits, caps the effective length at the physical mat length, and checks the operating load against soil and timber limits. Decisions use unrounded values.
Understanding the result
PASS means the four implemented checks meet their limits for the entered data. It is not approval of a lifting operation or a site. The deflection value is a simplified mat calculation, not a prediction of soil settlement. Local crushing, load-spreader and connection design, timber condition, ground stability and other project requirements need separate assessment.
Engineering release — content pendingNeeds content
The product owner still needs the responsible engineer’s review and release, the final project-use manual and the approved limits of application. Reproducing published calculations and passing software tests do not replace that review.