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, - -i:= 67 XI. BASEMENT CONSTRUCTION 11.1 Introduction The design and construction of basements can be a ;complicated procedure with many alter- natives. As with most design work, certain assumptio~s are made and only commonly-used construction is considered. This reasonably simplifies ithe design and construction tasks. For further explanation of methods and application. the rf,ader is referred to the "Manual for the Construction of Residential Basements in Non-Coastal Flood Environs" NAH B Research Foun- dation 1977 (Reference No. 24). The purpose of this portion of the manual is not to ~Iesign every basement for every application. Rather, the standard calculations and conjstruction will be discussed, including the approximate extreme loading conditions that diff<rrent basements can accommodate. If loading conditions are expected to exceed those leVfjls, a special design should be done by a qualified engineer experienced in that type 01 work: Maximum loading conditions are often governed by buoyancy which is discussed following strength design for walls and floor slabs. Based on the following calculations and examples, if'should be evident that the allowable water loadings on basements is quite limited. Walls ahd floor slabs may be specially designed for higher water elevations, but their design is logicall~ limited because buoyancy considera- tion governs when water depth reaches five feet abo~e the floor slab. The allowable water loading is the lowest elevation at which one of the fo'ilowing will occur: wall fails, floor slab fails, house becomes buoyant or floats. 11.2 Types of Basements Modern basements are usually constructed of struct0ral plain concrete. reinforced concrete, unreinforced masonry block, or grouted reinforced m4sonry block. A plain concrete wall is typically eight inches thick ~ithout temperature or shrinkage reinfor- cement. The wall's ability to withstand lateral loads isilimited by its tensile strength which varies approximately linearly with compressive strengt:h. Concrete compressive strengths are usually designated as 2,500 psi or 3,000 psi for basenilent walls. Reinforced cast-in-place concrete walls are typically ~'ight inches thick for residental con- struction and contain both vertical sleel bars lor load iresistance and horizontal steel bars for temperature and shrinkage control. This type of construction is the best method when large lateral loads are anticipated. Commercial and industri*1 construction typically uses ten-inch wall thickness or greater. For the example in this secti:on, we have used an eight-inch and ten-inch wall thickness. For other wall thicknesses onG should go through the procedures outlined lor design. ' Unreinforced masonry block walls are eight or ten inf:hes thick and simply consist of hlock units set in mortar with no reinforcement. This type or wall should only be used for minimal loading; it is not suitable for large lateral loads. . Masonry block walls may be reinforced by adding vejrtical reinforcing bars which are grouted into the block cavities. Horizontal wire reinforcement imay also be added at regular intervals between SOme of the block courses. This kind of wall ican provide the necessary strength for large lateral loads. . Other wall types that have been used include Cllt slcine, rubble stone, and cribbing and planking. These are no longer commonplace, and theri,fore design guidelines are not presen- ted for them.