FOOTINGS, FOUNDATIONS, & CONCRETE STAKING OUT HOUSE LOCATION Stake out dimensions for the house are provided on the plot plan Required instruments: - measuring tape (100’) - contractor’s level - transit - plumb bob - batter boards and stakes - string Staking steps: 1. Locate house corners from plot plan by driving stakes at these spots (use the 9-12-15 triangle to square the corners or a transit) 2. Place batter boards 4 feet off of the footing line (outside of footing) 3. Stretch string between batter boards directly above corner stakes. A line can be painted on the ground below the string to outline the house. A plumb bob is used to accurately locate the string over the corner stakes. EXCAVATION Removing the top soil is the first step in excavation. This material can be used for final grading purposes after the footings and foundations are completed. Excavations for footing should extend either, - at least 6” into undisturbed soil- no backfilling - at least 6” below the average maximum frost penetration - at least 6” into existing rock if the footing is sitting on bedrock *Soil bearing conditions will determine the excavation depth and width Excavation must be wide enough to allow for work space to construct the forms FOOTING SHAPES AND SPECIFICATIONS Footings increase the support capacity of the foundation wall by spreading the loads over a wider area Footing design (width and thickness) depend on the building loads and the soil bearing capacity General footing proportions W W 2W It is important to take into consideration any varying soil types within the footing area. Different soils (clay, sand, organic, rock, etc.) respond differently to loading. You don’t want differential settlement of varying degree occurring during construction. Longitudinal reinforcement with 2- #6 bars In hilly terrain, stepped footings can be used as long as the height of the step is not more than three-fourths (3/4) the horizontal distance between the steps. To “tie” the footing together, 2- #4 bars should be used. FOUNDATION WALLS Connects the footing to the slab or floor joists or may be a basement wall Materials: poured concrete, concrete block, stone or brick, pressure-treated wood Four basic types (Fig. 23.8 & 9) - T- foundation - Slab foundation - Pier or post foundation - Permanent wood foundation Basement walls must have adequate drainage at the footing to relieve hydrostatic pressure T-Foundation (Fig. 23.14) Most common type. Footings and foundation wall are formed to be cast as a single unit Slab foundation (Fig. 23.25) It is an extension of a slab floor. The slab and foundation are cast as one unit Advantages - requires less time - less expensive - less labor time to construct Slab foundation construction is used for interior bearing wall partitions Pier and post foundations (Figs. 23.30 & 33) Commonly used in combination with T-foundations as support for interior beams and floor joists. Also used with columns in a basement where the distance is too great to span with floor joists. A “lally” column is used in this situation Pier vs. column - The difference between and pier and a column is the length. - Pier are usually shorter and located under the house - Columns are made of two pieces, a footing and a post (Fig. 9-17) Wood foundations (Figs. 23.39 & 40) AKA permanent wood foundation (PWF), all-weather wood foundation (AWWF), and treated wood foundation A wood foundation is a below grade, pressure-treated plywood-sheathed stud wall. Common in climates where typical foundation work (concrete and masonry) stops in freezing or wet weather For crawl space - depth of excavation allows for 2” of sand or 6” of crushed rock that is placed below the maximum average frost depth - footing trench should be at least 12” deep and 10” to 12” wide (check local code) - level sand and rock is essential to ensure a level floor For basement walls - bottom of footing excavation is covered with 6” to 8” of porous gravel and leveled - footing plates are either 2x8, 2x10, or 2x12 pressure-treated material All fasteners (nails, brackets, etc.) must be made of silicon bronze, copper, or hot- dipped galvanized zinc coated steel Special caulking compounds are used to seal joints A polyethylene film is placed on the gravel base to prevent moisture and a concrete slab is placed on top of that Attention must be given to the connection of the floor joists to the basement walls to correctly transfer inward soil forces to the floor structure *Backfilling should not begin until the basement floor has cured and the first floor is installed CONCRETE AND MASONRY BASEMENT WALLS Wall thickness depends on lateral earth pressure (soil and hydostatic pressure) and vertical load (8” minimum) Factors influencing strength and stability - height and thickness - mortar bond (3/8”) - vertical load - support from cross walls, pilasters or wall stiffeners and support from the first floor framing Strong earth pressures require stiffening through, - pilasters - vertical and horizontal bar reinforcement through hollow core block with concrete Basement walls extend a minimum of 8” above finished grade (using wood framing) Basement walls are slightly shorter than first and secondary story walls Load-bearing cross walls are tied to exterior walls through metal tie bars (1/4” thick, ¼” wide, and 28” long) Floor loads are distributed uniformly along the basement wall top course of block using - 4” solid block - solid top block - reinforced concrete masonry bond beam - cores in the top course filled with concrete or mortar Damp proofing is essential- use a parge coat (2-1/4” mortar coats and hot tar or equivalent material Wall drains and sump pumps BEAMS AND GIRDERS A beam or girder is used to support the floor joists and prevent sagging. Placed equidistant from exterior walls and under bearing walls (walls designed to support part of the load) Beams are either wood or steel (wood is typical for residential construction) Wood beams are either built-up or solid Steel beams are commonly S-beams or W-beams (wide flanged) Fig. 26.10 Calculations to determine the beam size depends on the load the beam will support Weights are combinations of Live or Dead loads Live load (LL)- fixed or moving weights not a structural part of the house - furniture, people, snow, wind, etc. Dead load (DL)- static or fixed weights of the structure - weight of roof, walls, floors, siding, joists, foundation walls, etc. Simplified loads First floor- LL + DL= 50 pounds per square foot (psf) Second floor- LL + DL= 50 psf Ceiling- LL + DL= 30 psf Walls- LL + DL= 10 psf Roof- LL + DL= Typically none since most load from the roof transfer directly to the exterior walls Weight Calculations See Appendix B for Mathematical Calculations for loads Tributary Area- Area supported by a beam, girder or column and is equal to half the span length in any given direction Loads are based on kips (One kip is equal to 1000 pounds) Tables for maximum allowable loads are: Fig. B.38, American Standard I-beams with lateral support (S-beams) Fig. B-42, Allowable concentric loads for standard steel pipe columns If a span is too great, additional columns must be provided to reduce the span length Example calc for beam Structural component Tributary Area Loads Weight (sf) (psf) (kips) 2nd floor 560 50 28.0 1st floor 560 50 28.0 1st ceiling 560 30 16.8 2nd bearing wall (bw) 320 10 3.2 1st bw 320 10 3.2 Total weight on beam 79.2 Lintels Lintel- A horizontal structural member that supports the load over an opening such as a door or window Constructed of: - precast concrete - cast-in-place concrete - lintel blocks - steel angle iron CONCRETE AND MASONRY Concrete is ordered by the yard (27 cubic feet = 1 yard) Minimum compressive strength of structural concrete is 3000 pounds per square inch (psi) Curing time and temperature affect the curing of concrete Concrete reaches its maximum compressive strength at about 28 days It must be kept moist for several days The process for placing concrete - Pour and vibrate - Screed - Float - Trowel - Contraction joints Pour and Vibrate- The vibrating or tamping reduces the air pockets which reduce strength Screed- Using a long straightedge, the concrete is worked back and forth to smooth the surface, bring excess water to the surface and settle aggregate Float- A short board with a handle and flat sides. Floating embeds large aggregate, removes slight imperfections, lumps and voids and consolidates mortar at the surface Trowel- Rectangular and used in circular motion. Troweling hardens the surface and develops a very smooth surface Contraction joints- Control the cracking of concrete from expansion and contraction due to temperature. They are placed in line with interior columns, at changes in slab width, or at maximum spacing of 20 ft. Concrete slab floors should not be bonded to footings or columns. A 1” cushion of sand should be provided where the slab rests on the footing. Building paper (a sleeve of 3 thicknesses) can be used to separate the column from the slab. Concrete Blocks (Fig. 25.8) Used to form exterior and sometimes interior walls. Typical block size is an 8”x8”x16”, but the actual dimensions are 7 5/8” x 7 5/8” x 15 5/8”. These dimensions allow for 3/8” mortar joints.
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