Understanding ULS vs SLS in Foundation Design

When designing foundations, engineers must balance two critical checks:

• Ultimate Limit State (ULS): ensures strength and structural safety.

• Serviceability Limit State (SLS): ensures functionality and limits deformations under service loads.

As the footing width increases, the behavior of both states changes:

• The ultimate bearing capacity (ULS) generally increases with larger footing widths.

• The settlement control (SLS) tends to become more challenging as the footing size grows.

The attached graph shows the interaction between these two limits.

•   At smaller widths, ULS governs designs

• At larger widths, SLS often becomes the controlling factor.

Targeting the right balance between strength and serviceability is a core part of practical geotechnical engineering.

It is often recommended to start sizing a footing based on SLS—especially in soft or loose soils—because once serviceability criteria are met, ULS requirements are usually satisfied automatically.