Thoughts on Foundation Stability Code Requirements
In this blog entry, we will provide a review of the structural stability requirements for foundations of buildings and other structures as stipulated in US design standards. We start by reviewing US code content relevant to overall structural stability and, if found, foundation stability. We welcome feedback from practitioners and code experts!
AI illustration - Not an actual ASCE book.
ASCE 7-16
ASCE 7 is the de facto US standard that defines minimum design loads and associated criteria for buildings and other structures. Here, we will review ASCE 7-16, the version we have in hand.
Stability as Basic Code Requirement
Structural stability is mentioned as a basic requirement in Section 1.3.1 Strength and Stiffness, which says:
“Buildings and other structures, and all parts thereof, shall be designed and constructed with adequate strength and stiffness to provide structural stability, protect nonstructural components and systems, and meet the serviceability requirements of Section 1.3.2.”We note that this section specifies how acceptable strength can be demonstrated (via the strength procedure of Section 1.3.1.1 which references the LRFD load combination of Section 2.3, or via the allowable stress procedures of Section 1.3.1.2 which references the ASD load combinations of Section 2.4). However, this section does not specify how adequate stiffness or structural stability can be demonstrated. In fact, the LRFD or ASD load combinations are not listed as procedures to verify stability or stiffness. Rather, they are listed as procedures that are focused on verifying strength.
Consideration of General Stability During Analysis
In Section 1.3.5 Analysis, we find:
“Load effects on individual structural members shall be determined by methods of structural analysis that take into account equilibrium, general stability, geometric compatibility, and both short- and long-term material properties.”To understand this quote a little better, we should consult ASCE 7’s definition of load effects. In Section 1.2 Definitions, load effects are defined as:
“LOAD EFFECTS: Forces and deformations produced in structural members by the applied loads.”The term “applied loads” is not defined anywhere in ASCE 7-16. Thus, it could mean nominal loads, factored loads, or a combination of nominal or factored loads. In all cases, the internal forces in members and the deformations resulting from these applied loads would be the load effects. Section 1.3.5 is saying that the load effects on elements of a structure shall be obtained through analysis methods that take into consideration the overall (general) stability of the structure. Section 1.3.5 is concerned with general stability of the structure, not specific member stability.
Counteracting Structural Actions
In Section 1.3.6 Counteracting Structural Actions, which is part of the basic requirements section, we find:
“All structural members and systems, and all components and cladding in a building or other structure, shall be designed to resist forces caused by earthquake and wind, with consideration of overturning, sliding, and uplift, and continuous load paths shall be provided for transmitting these forces to the foundation. Where sliding is used to isolate the elements, the effects of friction between sliding elements shall be included as a force. Where all or a portion of the resistance to these forces is provided by dead load, the dead load shall be taken as the minimum dead load likely to be in place during the event causing the considered forces. Consideration shall be given to the effects of vertical and horizontal deflections resulting from such forces.”This section reemphasizes the necessity to verify stability of all structural members, systems, components and cladding. A foundation would be considered a component of the structural system. This section also says that when dead load is providing resistance, the minimum dead load value present during the event is to be used, without specifying a reduction factor on this minimum dead load.
Stability Load Combinations
Whether using the LRFD or ASD approach, ASCE 7 has load combinations that are aimed specifically at situations where the structural actions due to lateral loads and gravity loads counteract each other, i.e., situations where stability needs to be verified for the whole structure and individual elements of the structure. These would be the governing load combinations for stability considerations. They are:
For LRFD (Section 2.3):
Load Combination #5: 0.9D+1.0W
Load Combination #7: 0.9D-Ev+Emh
For ASD (Section 2.4):
Load Combination #7: 0.6D+0.6W
Load Combination #10: 0.6D-0.7Ev+0.7Eh
These stability verifications must be carried out at the structure and component levels. Overall stability would be apparent from the analysis of the whole structure, as stipulated in Section 1.3.5 Analysis. However, foundation stability would NOT be apparent from that analysis and here is an example why:
For a frame structure, one must assume some boundary conditions at the supports. The analysis, whether by hand or using a computer program, will consider the overall stability of the structure based on the assumed support conditions (simple or pinned supports, fixed supports, roller supports, spring supports in any degree of freedom direction, etc.) and will verify element stability within the structure (or excessive movement) based on member stiffnesses and connections. The analysis will result in support reactions based on the assumed support conditions. The stability of a foundation that is subjected to those support reactions must still be verified because the real support conditions for the foundation are not the same as the idealized support conditions used in the analysis. For example, if a pinned support condition is assumed at a foundation, that support will have vertical and horizontal reactions. We would need to check bearing or uplift capacity for the vertical reaction and sliding capacity for the horizontal reactions. In this process, we may also add foundation self-weight and backfill on top of the foundation as additional dead load, combined with the reactions in the same fashion as that adopted for the load combination associated with those reactions. Note that bearing capacity for the foundation would be governed by load combinations other than the ones listed above.
It is important here to remember that the reactions obtained from the structural analysis are not load effects, rather, they are loads to be used to design the foundations. Load effects from these reactions would be internal stresses within the foundation, soil reaction pressure under the foundation, and deformations of the foundation. Such load effects would depend, to a large extent, on soil-structure interactions between the foundation and the supporting and surrounding soils.
Mixing of LRFD and ASD Requirements
Per Section 2.1 General, buildings and other structures shall be designed using the LRFD or ASD provisions. However, where an element of a structure is designed by a particular material standard (such as concrete, steel or wood for example), it shall be designed exclusively using either LRFD or ASD provisions, i.e., without mixing and matching of provisions. Unless the designer knows exactly what he/she is doing, it is not recommended to mix and match LRFD and ASD provisions.
IBC 2018
In IBC 2018 Section 1605.1.1 Stability, we find:
“Regardless of which load combinations are used to design for strength, where overall structure stability (such as stability against overturning, sliding, or buoyancy) is being verified, use of the load combinations specified in Section 1605.2 or 1605.3 shall be permitted. Where the load combinations specified in Section 1605.2 are used, strength reduction factors applicable to soil resistance shall be provided by a registered design professional. The stability of retaining walls shall be verified in accordance with Section 1807.2.3.”Sections 1605.2 and 1605.3 are the LRFD (strength design) and ASD (allowable stress) load combinations, respectively. Here, the code talks about overall structural stability and says that either the LRFD or ASD approach can be used, which is in line with what ASCE 7 requires by default when the 2.3 or 2.4 load combinations are used for the overall structural stability.
Foundations Subjected to Eccentric Loading
Foundations for building structures are typically designed so that the resultant vertical force is within the kern, i.e., load eccentricity is less than B/6, where B is the foundation width. When foundations are sized this way for an eccentric load, there is no tension (or zero-pressure) area under the foundation. The pressure is typically assumed to have a trapezoidal or triangular distribution and is all compressive under the foundation. Foundations designed in this fashion are unlikely to overturn but they may still slide. Their overturning and sliding stability must be checked.
On the hand, foundations for tall, inverted pendulum-type structures, such as wind turbines, are subjected to highly eccentric loads. ASCE 7-16 Section 11.2 Definitions defines inverted pendulum-type structures as:
“INVERTED PENDULUM-TYPE STRUCTURES: Structures in which more than 50% of the structure’s mass is concentrated at the top of a slender, cantilevered structure and in which stability of the mass at the top of the structure relies on rotational restraint to the top of the cantilevered element.”Wind turbine support structures would be classified as inverted pendulum-type structures. Attempting to keep the resultant force within the foundation’s kern is nowhere close to being practical as the resulting foundation would be VERY large. Subject to industry-adopted factors of safety on overturning and sliding, industry practice has always been to allow part of the foundation area to be under zero pressure (or gapping). The industry question has been focused on how much gapping to allow. Current practice is to allow up to half the foundation width to be under zero pressure (gapping) for the extreme load case and not to allow any gapping under operational loads. These limits have been allowed to be exceeded if subgrade degradation can be assessed and can be shown to be tolerable.
Closing Thoughts
The LRFD and ASD load combinations given in ASCE 7 and IBC are focused on strength considerations. There are two load combinations in each design approach specifically intended to verify overall stability of the structure. They are also the likely combinations that will govern the stability verifications of the foundations, which must be carried out independently of the whole structure stability verifications (sliding, overturning and uplift).