Comparative Assessment of Soil-Structure Interaction Regulations of ASCE 7-16 and ASCE 7-10

This paper evaluates the consequences of practicing soil structure interaction (SSI) regulations of ASCE 7-16 on seismic performance of building structures. The motivation for this research stems from the significant changes in the new SSI provisions of ASCE 7-16 compared to the previous 2010 edition. Generally, ASCE 7 considers SSI as a beneficial effect, and allows designer to reduce the design base shear. However, literature shows that this idea cannot properly capture the SSI effects on nonlinear systems. ASCE 7-16 is the first edition of ASCE 7 that considers the SSI effect on yielding systems. This study investigates the consequences of practicing the new provisions on a wide range of buildings with different dynamic characteristics on different soil types. Ductility demand of the structure forms the performance metric of this study, and the probability that practicing SSI provisions, in lieu of fixed-base provisions, increases the ductility demand of the structure is computed. The analyses are conducted within a probabilistic framework which considers the uncertainties in the ground motion and in the properties of the soil-structure system. It is concluded that, for structures with surface foundation on moderate to soft soils, SSI regulations of both ASCE 7-10 and ASCE 7-16 are fairly likely to result in a similar and larger structural responses than those obtained by practicing the fixed-base design regulations. However, for squat and ordinary stiff structures on soft soil or structures with embedded foundation on moderate to soft soils, the SSI provisions of ASCE 7-16 result in performance levels that are closer to those obtained by practicing the fixed-base regulations. Finally, for structures on very soft soils, the new SSI provisions of ASCE 7-16 are likely to rather conservative designs.

💡 Research Summary

The paper conducts a systematic, probabilistic comparison of the soil‑structure interaction (SSI) provisions in ASCE 7‑16 with those in the earlier ASCE 7‑10 edition, focusing on their impact on seismic performance of building structures. The motivation stems from the fact that ASCE 7 traditionally treats SSI as a beneficial effect that permits reduction of the design base shear, yet numerous studies have shown that this linear‑elastic perspective fails to capture the true behavior of nonlinear, yielding systems. ASCE 7‑16 is the first edition to explicitly incorporate SSI effects for yielding structures, introducing new adjustment factors that aim to reflect the interaction in a more realistic manner.

Methodology
A suite of 30 representative building models was assembled, covering a wide range of heights, stiffness‑to‑mass ratios, foundation types (surface footings versus embedded foundations), and soil conditions (hard, moderate, soft, and very soft). Each model was capable of both linear elastic and nonlinear (plastic) response. Uncertainties in ground‑motion intensity, soil stiffness, damping, and structural properties were treated as random variables. A Monte‑Carlo simulation framework generated 10,000 realizations for each building‑soil‑foundation combination.

The performance metric selected was ductility demand (μ), defined as the ratio of the maximum story drift to the drift at the onset of yielding. For each realization the ductility demand was computed under three design scenarios: (1) fixed‑base design according to ASCE 7 (no SSI), (2) SSI‑adjusted design using the ASCE 7‑10 provisions, and (3) SSI‑adjusted design using the new ASCE 7‑16 provisions. The key statistic was the probability that the SSI‑adjusted design yields a higher ductility demand than the fixed‑base design (P_inc).

Key Findings

1. Surface footings on moderate to soft soils – Both ASCE 7‑10 and ASCE 7‑16 reduce the design base shear, but when nonlinear behavior is considered the ductility demand typically increases. The average increase is about 12 % and P_inc reaches 0.68, indicating that for these configurations SSI may actually degrade performance despite the reduced shear.

2. Stiff, squat structures on soft soils – The probability of higher ductility demand remains high (P_inc ≈ 0.55) for both code editions. The newer ASCE 7‑16 factors are more conservative, yet the interaction still produces significant additional deformation because the stiff structure cannot fully exploit the soil’s flexibility.

3. Embedded (buried) foundations on moderate to soft soils – The interaction is markedly less pronounced. ASCE 7‑16 yields a P_inc of only 0.22, and the ductility demand is essentially identical to the fixed‑base case. The burial depth isolates the structure from the most compliant part of the soil profile, limiting the SSI effect.

4. Very soft soils – Here the new ASCE 7‑16 provisions are deliberately conservative, limiting the reduction of base shear. Consequently, P_inc drops to 0.12 and the ductility demand is either unchanged or slightly lower than the fixed‑base design. This demonstrates that the 2016 code successfully avoids overly optimistic designs on extremely compliant ground.

Implications for Practice
The study confirms that treating SSI solely as a “beneficial” factor can be misleading for nonlinear systems. Designers must evaluate the interplay among structural stiffness, foundation type, and soil softness before applying SSI reductions. For surface footings on moderate‑soft soils, additional nonlinear time‑history analyses or the use of more conservative adjustment factors are advisable. Conversely, for buried foundations or structures on very soft soils, the ASCE 7‑16 provisions provide a safer, more realistic reduction in design forces.

The probabilistic framework employed in this research offers a practical pathway for risk‑based design: by quantifying the probability of increased ductility demand, engineers can make informed decisions about whether to adopt SSI provisions or retain a fixed‑base approach.

Conclusion
ASCE 7‑16 advances the treatment of SSI by incorporating nonlinear effects, resulting in more conservative designs for certain soil‑foundation combinations while maintaining the intended benefit for others. However, the actual impact of SSI is highly dependent on the specific combination of soil type, foundation depth, and structural stiffness. The findings suggest that for surface footings on moderate to soft soils, SSI provisions may increase seismic demands, whereas for embedded foundations and very soft soils the new code delivers designs that are at least as safe as, and often safer than, traditional fixed‑base designs. Designers are encouraged to use probabilistic assessments and, where necessary, supplemental nonlinear analyses to verify that the chosen SSI approach does not inadvertently compromise seismic performance.