Location of Base for Seismic Design

S tructural P erformanceS T R U C T U R E performance issues relative to extreme events Figure 2:Base atground floorlevel. is motion ground horizontal general, In base. the locating for importance mary pri of is motion ground Horizontal list. Some ofthefactorsare: location. its affect factors many as broad, seismic design. for base the locate to how to as engineers to guidance on provides article building This a piles. for or site sloping a on located buildings for example for ward, for straight not are conditions site when building a for is base the where question often Engineers engineers. by terpreted misin or misunderstood sometimes is “base” of definition This structure”. the motions are considered to to be imparted ground seismic horizontal the which at level as “the defined is design seismic for American Society of the Civil Engineers, base of the Institute by Engineering Structural published standard, reference ingsand Other Structures By Dominic J.Kelly, P.E. Location ofBaseforSeismicDesign Note that piles are not included in this in included not are piles that Note intentionally is “base” of definition The In the F G Ba • • • • • • • • • l r oo ound Slope ofgrade. Proximity toadjacentbuildings;and are supported; Manner inwhichbasementwalls Depth ofbasement; separations; seismic of extent and Location resisting system; elements oftheseismic-force- Location andstiffnessofvertical Openings inbasementwalls; the building; Soil conditionsadjacentto to floorlevels; Location ofthegraderelative s e Factors Affecting the r a Minimum Design Loads for Build t Location ofBase F i gu (ASCE/SEI 7-05) r e 2

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a t - - - - g r ound Copyright perimeter foundation walls, the base base the walls, foundation perimeter of top onsupported are system seismic-force- resisting the of elements vertical the at Where slab-on-grade. the else of top or the elements these of top the at established commonly is base pile the or caps, footings interior are on system supported seismic-force-resisting the ( lished near the ground-level estab slab elevation generally is base the basement, without a building a For generally grade. to is close base the soils, competent on LevelSites Buildings withoutBasements are toabuilding. imparted which horizontal seismic ground motions and have little to no impact on the level at erally with the horizontal ground motions lat move generally battered, and vertical and both Piles, slabs elements. foundation shallow of underside the on friction through and elements, foundation and and friction on the sides of basement walls impartedtothestructure through bearing Figure 1:Base foralevel site. Figure 1 For typical buildings on level sites with sites level on buildings typical For f Buildings onLevelSites l oo r ). Where the vertical elements of l e v e 8 l December 2009 F i gu r Ba e 1 s e – i Ba s a

s t e t h f e o r t o a - - p l e o v f o oae h bs.Te ae a usu can base The base. the locate to judgment her or his use must engineer an occurs, this Where walls. foundation perimeter or caps, pile footings, of top are supportedon the elevations at various verticalOftenwalls.elements foundation the of top the at established typically is above, grade. For the base to be located at feet few a as much as or to, adjacent level basement. the of bottom the at established be should base the basement, the of depth the over are present soils If these ered earthquake. consid in a maximum or collapse failure to susceptible are that cemented weakly are that soils or clays, sensitive highly or not include soils that are liquefiable, quick file over the depth of the basement should the level locatedclosesttograde,thepro to grade. If the base is to be established at close level a at force seismic the transfer to enough stiff are soils the whether to as determination is a making record for of responsible engineer The grade. near forces seismic transmit to earthquake an during laterally much too compress may soils are present near the surface, soils soft where or buildings heavy or walls. tall For basement the of height the over building the from and to transmitted be will forces seismic because basement the of depth the over required are soils stiff ( locate the base at the floor closest to grade to appropriate often is it site, level a on on LevelSites Buildings withBasements system. seismic-force-resisting the of elements vertical the supporting elements of tops the of elevation lowest the as established conservatively be ally e f Figure 2 Figure Occasionally, the base may be at the floor located basement a with building a For o l o s i t te i ng s ). For the base to be near grade, near be to base the For ).

® - - - STRUCTURE magazine Base at level closest to grade ® Base below grade Figure 3: Base at level closest to grade elevation. Figure 4: Base below substantial openings in basement wall. Figure 3 – Base at level closest to grade elevation Figure 4 – Base below substantial openings in basement a floor level above grade, stiff foundation walls condition where the basement walls maywall not Other Conditions Requiring the on or near all sides of the building should be stiff relative to the structure above grade Base to Be Below Grade extend to the underside of the elevated level is where the vertical elements of the seismic that is to be considered the base. The validity force-resisting system are long concrete shear Other conditions may also necessitate estab- of having the base above grade is based on the walls extending over the full height and length lishing the base below grade for a building same principles used to justify the two-stage of the building (Figure 5, page 10). For this with a basement that is located on a level site. equivalent lateral force procedure permitted case, theCopyright appropriate location for the base is Such conditions include: where seismic sepa- in Section 12.2.3.1 of ASCE/SEI 7-05. This likely at the foundation level of the basement rations extend through the floor structures procedure allows one to apply the equivalent walls, unless the soils against the basement located close to and above grade; where the lateral force procedure to a flexible upper walls are very stiff. floor diaphragms close to and below grade are portion of a building as if it were founded at Where the base is established below grade, not tied to the foundation wall; where the floor the ground level instead of on top of a lower the weight of the portion of the story that is diaphragms, including the diaphragm for the portion of the building that is 10 times as partially above and below grade must be included floor close to grade, are flexible; and, where stiff as the upper portion. For a floor level as part of the effective seismic weight. If the other buildings are located nearby. Knowledge above grade to be considered the base, grade equivalent lateral force procedure is used, this of dynamic response of buildings and engi- should generally not be lower than one-half can result in disproportionately high forces in neering judgment are often critical in defining the height of the basement story below the upper levels due to a large mass at this lowest the base of these structures. level considered base (Figure 3). The concept level above the base. The magnitude of these of establishing the base at a level above grade forces can often be mitigated by using the Effect of Seismic Separations is also applicable to small structures. Typical two-stage equivalent lateral force procedure For a building with seismic separations ex- examples include light-framed structures that where allowed, or by using a dynamic analysis to tending through the height of the building rest on a stiff basement or crawl space walls determine force distribution over the height including levels close to and below grade, the of concrete or masonry. The two-stage analysis of the building. If a dynamic analysis is used, separate structures will not be supported by procedure can be used to design the basementmagazine it may be necessary to include many modes the soil against a basement wall on all sides in or crawl space walls with the weight of the first to capture the required mass participation unless all directions. If there is only one joint through level Sincluded, but theT design of theR upper soilU springs are incorporatedC into theT model. theU building, assigning R the base to theE level stories would not be affected by the mass of Incorporation of soil springs into the model will close to grade may still be appropriate if the the concrete or masonry walls or the level of also minimize seismic forces in the upper levels. soils over the depth of the basement walls are frame supported on the top of these walls. If the base is located at the level closest to grade, the lateral stiffness of the basement walls should be substantially greater than the stiffness of the vertical elements of the seismic

force-resisting system. However, a condition visit ADVERTISEMENT Information, - For Advertiser where the basement walls extend to slightly above grade on a level site and may not provide adequate stiffness is where the basement walls have many openings for items such as light-wells, areaways, windows, and doors (Fig- ure 4). Where the stiffness of the basement walls is not much greater than the vertical elements of the seismic-force-resisting system

above grade, consideration should be given to www.

establishing the base at the basement slab or STRUCTUREmag basement foundation elements. If all of the vertical elements of the seismic-force-resisting

system are located on top of basement walls .org and there are many openings in the basement walls, it may be appropriate to establish the base at the bottom of the openings. Another

December 2009 STRUCTURE magazine 9 December 2009 Sloping grade

Base is at the top of footings

Figure 5: Base at foundation level where concrete shear walls extend over Figure 6: Building withF igutie-backre 6 B uoril dcantilevereding with tie-ba retainingck or can walltileve thatred is separate the full heightFigure and5 – lengthBase a oft f otheunda building.tion level where there is full from the building. retaining wall that is separate from the building length exterior shear walls stiff and the diaphragm is rigid. Stiff soils are a structure, the slabs may intentionally not be buildings with basements are located adjacent required so that the seismic forces can be trans- tied to the wall to allow relative movement to one or more sides of a building, it may be ferred between the soils and basement walls in between Copyrightthem. In other cases, a soft joint may appropriate to locate the base at the bottom of both bearing and side friction. The structural be provided. If shear forces cannot be trans- the basement. The interaction of the structure, engineer generally consults with a geotechnical ferred between the wall and a ground-level or the surrounding soils, and adjacent buildings, engineer to determine if the soils are stiff basement floor, the location of the base will and analyses that account for such interaction, enough. If the soils are not stiff, adequate side depend on whether forces can be transferred are complex. Well established rules of thumb friction may not develop for movement in through bearing between the floor diaphragm about when to consider interaction of adjacent the direction perpendicular to the joint and, and the basement wall, and between the structures through the soils are not available consequently, the base should be located at the basement wall and the surrounding soils. Floor for commercial buildings. Although not well level of the basement slab or foundations. diaphragms bearing against the basement established within commercial building design, For large footprint buildings, seismic separation walls must resist the compressive stress from a rule of thumb for when interaction occurs is joints may extend through the building in earthquake forces without buckling. If a seismic when the buildings are closer to one another two directions and there may be multiple paral- or expansion joint is provided in one of these by an amount that is less than two times the lel joints in a given direction. For individual buildings, the base will almost certainly need depth of the basements; however, this rule structures within these buildings, substantial to be located at the foundation level or a level is not always applicable and other factors differences in the location of the center of ri- below grade where the joint no longer exists. such as the mass and plan dimensions of gidity for the levels below grade relative to levels the structures can be equally as important. above grade can lead to torsional response. For Flexible Diaphragms near Grade In general, the closer buildings are to one such buildings, the base should usually be at If the diaphragm at grade is flexible and does another, the more likely it is that the base the foundation elements below the basementmagazine not have substantial compressive strength, the should be below grade. or, in the case where the joints are not present base of the building may need to be located in theS lower basement T levels, the baseR may be belowU grade. This conditionC is moreT com- UBuildings onR Sloping SitesE at a basement slab level where the separations mon with existing buildings. Newer buildings For buildings on sites with sloping grade, are not provided. with flexible diaphragms should be designed many of the same considerations for a level for compression within elements of the site are applicable. For example, on steeply Floors Separated from Basement Walls diaphragm to avoid the damage that would sloped sites, the earth may be retained by a Where basement floor levels are not tied to occur otherwise. tie-back wall so that the seismic-force-resisting foundation walls, the base may need to be system of the building does not have to resist located well below grade at the foundation Nearby Buildings the lateral soil pressures. For such a case, the level. An example is a building with tie-back The proximity to other buildings can also building will be independent of the wall, so walls and post-tensioned floor slabs. For such impact the location of the base. If other the base should be located at a level close

Shear Wall Moment Frames

Shear Wall Sloping grade Sloping grade

Base is at the top of footings Base is at the top of footings Figure 7: Building with vertical elements of the seismic force-resisting system Figure 8: Building with vertical elements of the seismic force-resisting system supporting lateral Fearthigure pressures.7 - Building with Vertical Elements of the supporting lateralF earthigure pressures.8 - Building with Vertical Elements of the Seismic Force-Resisting System Supporting Seismic Force-Resisting System Supporting Lateral Earth Pressures Lateral Earth Pressures STRUCTURE magazine 10 December 2009 STRUCTURE magazine to the elevation of grade on the side of the building where it is lowest (Figure 6). Where a building’s vertical elements of its seismic- Dominic Kelly, P.E. is an Associate Principal with Simpson Gumpertz & Heger Inc. STRUCTURE force-resisting system also resist lateral soil pressures (Figure 7), the base should also be located in Waltham, MA. Dominic’s twenty- magazine located at a level close to the elevation of three years of experience includes design, Electronic editions of this grade on the side of the building where grade rehabilitation, and investigation of building ® is low. For these buildings, the seismic-force- and nonbuilding structures. He has been a month’s issue and our resisting system below highest grade is often member ASCE/SEI 7 Task Committee on 2009 Trade Show in Print much stiffer than the system used above it Seismic Provisions since 2000 and a member (Figure 8), and the seismic weights for levels of ACI Faculty since 2004. Dominic can be Now at reached at [email protected]. close to and below highest grade are greater www.STRUCTUREmag.org than for levels above highest grade. Use of a two-stage equivalent lateral force procedure can be useful to minimize design forces in the upper levels of these buildings. If the equivalent lateral force procedure is used without Copyright doing the two-stage procedure, lateral forces in the upper more flexible portion of the building will be greater, as the mass of the heavy lower portion is included in the base ex⋅pert(s) Pronunciation [ek'spûrt']˘ shear that must be distributed over the height of the building. Where the site is moderately sloped so that it does not vary in height by more than a 1. A person or persons story, stiff walls often extend to the underside of the level close to the elevation of high with a high degree of skill grade, and the seismic-force-resisting system in or extensive knowledge above grade is much more flexible than it is below grade. If the stiff walls extend to the of a certain subject. underside of the level close to high grade on all sides of the building, locating the base at visit ADVERTISEMENT Information, - For Advertiser the level closest to high grade may be appropriate. If the stiff lower walls do not extendmagazine to the underside of the level located closest to (Some times it’s good to be called names!) high gradeS on or near Tall sides of theR building, U C T U R E the base should be assigned to the level closest to low grade. If there is doubt as to where to locate the base, it should conservatively be taken at the lower elevation.

Since 1912, CHANCE has been the international leader and www. industry expert in the world of earth anchoring STRUCTUREmag Summary and foundation solutions. The decision as to where to locate the

seismic base is not always straight forward. Whether it’s foundation underpinning, deep foundations, .org In general, the base of the structure should lighting foundations, tiebacks or soil nailing, CHANCE and be located where seismic forces enter and exit its network of over 400 certified installers and distributors the building. Designers generally locate the can provide the right solution that fits your needs. base at the level closest to the grade. If there is Contact your local CHANCE distributor today or check out doubt as to where to locate the base, it should what’s new @ WWW.ABCHANCE.COM generally be taken at the lower elevation. The Seismic Code Committee of ASCE 7 recently considered changes to its requirements that would establish the location of the seismic ANCHORING THE WORLD! base. Discussions of the committee led to the realizations that numerous parameters influence the location of seismic base, and that creating www.abchance.com comprehensive provisions adequately establish- email: [email protected] ing the seismic base was unrealistic. Language similar to that which appears in this article is being considered for incorporation into the 01-094 IHP ©Copyright 2009 Hubbell Incorporated commentary to SEI/ASCE 7-10.▪

December 2009 STRUCTURE magazine 11 December 2009