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Structural PracticeS practical knowledge beyond the textbook ® Figure 1: Modeling options for base condition. ne of the least understood aspects For a typical building with one or more below of modeling building structures is grade levels, the perimeter basement walls create dealing with at- and below-grade a very largeCopyright and laterally components. This includes soil- stiff box. The ground floor Ostructure interaction, but also the question of diaphragm engages this which below-grade structural elements should box and integrates it into Backstay Effect be included in a lateral model and what is an the lateral system. Sticking accurate representation of the base conditions. with the beam analogy, The focus of this article is what is most commonly the result is an effectively larger beam section Basement Modeling in referred to as the backstay effect. Traditionally, lat- below grade. This results in shedding of lateral Tall Buildings eral systems have been viewed as simple cantilever load from the main lateral force resisting system beams fixed at the base. While this analogy is (LFRS) to the basement walls. Overturning and By Nat Tocci, P.E. and Sanya Levi reasonable for the above-grade structure, a more shear are shared between the perimeter walls and accurate analogy would also include the effects magazinecore rather than isolated beneath the building of the below-grade structure,S which behavesT likeR core. ConceptuallyU C this is fairlyT straightforward. U R E a backspan to the cantilever. In this analogy, the The complexity arises in properly modeling the lateral system is viewed as a beam overhanging change in section, and capturing an accurate dis- one support, where that support is created by the tribution of internal forces and external reactions. at-grade diaphragm and foundation walls. The degree to which lateral loads are transferred The backstay effect is not limited to restraint at into the foundation perimeter is dependent on many the grade level. Backstay effects are also seen at set- variables, many of which there is limited certainty Nat Tocci, P.E. is the owner of Tocci backs with changes to the lateral system, the most about, as they are not specified or controlled in Engineering PLLC in New York common example being lower level podiums. They a typical project. It is therefore fair to ask if it is City and can be reached at are often very large in plan and introduce new lat- more conservative to simply ignore any backstay [email protected]. eral elements, and are therefore significantly stiffer effects and model the building core as an isolated Sanya Levi is an engineer at Arup than the set-back structure above. Backstay effects element. However, it can be shown that in many New York and can be reached at are also impacted by multiple basement levels. For cases the backstay effect will create higher demands [email protected]. simplicity of explanation, this article will focus on in some structural elements, in particular shear in the most common example which is the effect of the main LFRS below grade as well as the backstay the ground floor diaphragm in contributing to diaphragms, and therefore cannot be ignored. backstay effects. The concepts can be extended to Figure 1 is a stick diagram presenting some of all conditions where backstay effects occur. the possible options for modeling the base condi- tions of a core wall building. The building is of Backstay Effect height H with a basement of height B. The most traditional model, a simple cantilever, is shown Backstay effects are most noticeable in buildings in Figure 1a. It is clear that the maximum shear with discrete lateral systems, such as shear walls, is V = F. The extreme case of the backstay effect as opposed to distributed lateral systems. Building is shown in Figure 1b. In Figure 1b the ground height is also a major factor in the magnitude of floor diaphragm and perimeter foundation are the backstay effects. For the purposes of illustra- very stiff and are therefore modeled as a pin. tion, this article focuses on a high rise shear wall Statics shows that the maximum shear in the core building with a single basement. now occurs below grade with V = 3H/2B F. The STRUCTURE magazine 23 overall base shear has not changed, but the should also be considered. This component is backstay effect may create conditions with typically small relative to the other elements much higher demands than anticipated in and may possibly be neglected in many cases. certain elements. It can also be shown that In addition, this force is present only in the the base overturning moment in the core compression cycle of loading and should be has been reduced and redistributed to the modeled as such. perimeter foundation walls. Clearly there are many parameters to consider. Although Figure 1b shows dramatic increases In most cases, the best that can be done is to in shear, this is overly conservative for most model all contributing elements and make an conditions. The true restraint at the ground educated estimate of the element stiffnesses. floor is far from rigid and may range from The number of possibilities is too numerous Typical concrete core building configuration. very stiff to almost non-existent. A more real- for a prescriptive approach that will work for istic model is one in which the ground floor all buildings, which is perhaps why there is interface slip, and other unknowns, the stiffness restraint is modeled as a spring, producing little literature on the subject. Most building of the slab should be reduced for both shear results somewhere between Figures 1a and codes provide requirements for loading and (GAv) and flexural (EI) stiffness. Similar mod- 1b. Figure 1c shows this option. design of structural elements, but rarely provide eling guidelines ®and stiffness reductions should The complexity of an accurate model lies in detailed guidance on modeling procedures. A also be applied to basement wall elements. the fact that the spring in Figure 1c represents very good resource for an in depth discussion Soil stiffness should also be bracketed, typi- the cumulative stiffness of numerous elements of the backstay effect and recommendations for cally starting with recommendations provided in the building structure and supporting soil. A modeling is Modeling and Acceptance Criteria by the project geotechnical engineer. The sup- partial list of elements represented by the ground for Seismic Design and Analysis of Tall Buildings, porting stiffness under all elements should be floor spring would include: diaphragm to core PEER/ATC 72-1,Copyright which is available as a free taken at an upper and lower bound, and passive connection, diaphragm stiffness, diaphragm download from the PEER (Pacific Earthquake resistance provided against the perpendicular to basement wall connection, basement wall Engineering Institute) website. wall should also be bracketed if it is modeled. stiffness, foundation stiffness, and passive soil The backstay concept is more familiar to PEER/ATC 72-1 Table A-2 and Table A-3 resistance against the basement wall. engineers working in high seismic regions provide recommended upper and lower Ground floor diaphragms are often thick and has had less attention in other regions. bounds for bracketing the stiffness of the concrete plates with high relative stiffness. The concepts, however, are applicable for both above elements. PEER/ATC 72-1 also recom- However, this stiffness may be reduced by wind and seismic loading. mends that elements outside of the backstay cracking, bond slip, and discontinuities such influence (primarily tower elements) need not as large openings or slab elevation changes. Modeling be bracketed and should be modeled with In addition to the stiffness of the diaphragm the same assumptions used for their design. itself, the connections at each end must be A reasonablemagazine first step may be to assess whether Since these recommendations are intended for considered for their abilityS to transfer T the Rthe backstay U effect isC a consideration T forU the buildings R in highE seismic regions, it may be backstay shears. The same can be said for the building under investigation. A quick study of appropriate to adjust the recommendations basement walls which will have varying stiff- the parameters that create the backstay effect for wind controlled design to account for ness dependent on the same factors. may quickly rule out the need for a more in primarily elastic behavior. The overall stiffness of the diaphragm and depth analysis. The building system or con- Due to the complexity of capturing backstay basement wall system is also affected by the figuration may also determine the potential effects in the analysis, it may be desired to elimi- supporting foundation elements. Differences for backstay effects. nate the phenomenon in the actual building. in relative stiffness between core and perim- For buildings where backstay effects need This can be accomplished by isolating the LFRS eter wall soil support conditions may magnify to be considered, it will most likely be nec- from the foundation elements by providing lat- or lessen backstay effects. essary to consider multiple scenarios. Both eral joint at the backstay diaphragms. Typically The passive resistance provided by the soil on an overestimation and underestimation of this is done by providing a corbel or similar the basement wall face in the direction of force backstay effects can produce underestimates detail at the diaphragm to shear wall interface. of demand. For example, overestimating backstay restraint may underestimate the Conclusion overturning demand at the base of the main LFRS. The common approach is Ignoring the contribution of at- and below- to consider reasonable extremes for both grade structural elements in lateral models may The easiest to use software for calculating conditions and design each element for underestimate demands in key elements.

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