Small wind / Buyer questions
Structure and civil works
Should a wind foundation compare net or gross bearing pressure with the geotechnical recommendation?
The short answer
Use the pressure definition specified by the geotechnical recommendation. Gross pressure includes the applicable total load at the bearing plane; net pressure represents an increase relative to the defined pre-excavation overburden condition. Comparing one with a recommendation expressed on the other basis can create an inconsistent bearing or settlement assessment.
Define both sides of the comparison
Ask whether the recommended value is net or gross and whether it addresses resistance, settlement or both. Also identify its assumed bearing elevation, groundwater and loading basis. The word allowable does not, by itself, answer those questions.
On the demand side, identify which weights are included: turbine and tower actions, foundation concrete and retained soil above the footing. Then document how excavated soil is accounted for under the adopted definition.
Use one accounting convention throughout
Hypothetically, a defined gross contact pressure of 120 kPa and an initial overburden pressure of 30 kPa correspond to a net increase of 90 kPa under a simple consistent convention. These figures illustrate pressure accounting only; none is a soil allowance or wind foundation design value.
The subtraction must use the same reference plane and appropriate water/stress convention. It should not be repeated elsewhere if the geotechnical recommendation or load calculation has already incorporated displaced-soil accounting.
Resolve ambiguity before resizing the footing
Request a reconciliation of the demand and recommendation, including pressure distribution under overturning. Net versus gross definitions do not eliminate the separate need to assess eccentricity, contact, sliding or rotation.
If correcting the convention changes the apparent margin, let the responsible designers evaluate the complete calculation before altering geometry. A larger concrete volume selected from mismatched pressure definitions can increase cost without resolving the underlying error. Keep the agreed basis with the site-specific foundation design rather than attaching an unlabeled bearing number to a turbine quotation.
Sources
Examples are illustrative. Confirm product-specific performance, compatibility and supply scope in the applicable documents.