FEMA P55 Coastal Construction Manual, Fourth Edition - Mad Cad

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Volume II DESIGNING THE FOUNDATION 10 EXAMPLE 10.4. PIER FOOTING UNDER UPLIFT LOAD (concluded) Submerged footing dead load = Footing volume required = For a 12-inch-thick footing, the footing area = 37 ft 2 The analysis shows that a square, 6 ft by 6 ft by 12 in., submerged concrete footing and a 5-ft tall, 16-in. square, partially submerged concrete column are required to resist 2,514 lb of uplift. Increasing the footing thickness to 2 ft would allow the footing dimensions to be reduced to 4 ft 6 in. At some value of lateral load or eccentricity, the compressive stresses on one side of the footing go to zero. Because there are no tensile connections between the footing and the supporting soils, the footing becomes unstable at that point and can fail by rotation. Failure can also occur when the bearing strength on the other side of the footing is exceeded. Equation 10.6 relates soil bearing pressure to axial load, lateral load, and footing dimension. For a given axial load, lateral load, and footing dimension, the equation can be used to solve for the maximum and minimum soil bearing pressures, q on each edge of the footing. The maximum can be compared to the allowable soil bearing pressure to determine whether the soils will be overstressed. The minimum stress determines whether instability occurs. Both maximum and minimum stresses are used to determine footing size. Alternatively, for a given allowable soil bearing pressure, axial load, and lateral load, the equation can be solved for the minimum footing size. EQUATION 10.6. DETERMINATION OF SOIL PRESSURE where: q = minimum and maximum soil bearing pressures at the edges of the footing (lb/ft2) Pt = total vertical load for the load combination being analyzed M = applied moment Pl (hcol + x) (ft lbs) where x and hcol are as defined previously and Pl is the lateral load applied at the top of the column When designing a pier and footing, P t and P l depend on the load combination being analyzed. COASTAL CONSTRUCTION MANUAL 10-43
10 DESIGNING THE FOUNDATION Volume II EXAMPLE 10.5. PIER FOOTING UNDER UPLIFT AND LATERAL LOADS Given: • • • • • • • • • • • • • Figure 10-20 Stillwater flood depth (d s ) = 2 ft Lateral load on pier (P l ) = 246 lb (from design example in Chapter 9: (205 plf)/6 ft times 5 piers assumed to be resisting this force) Uplift load on pier (P w ) = 2,514 lb (derived from 419 psf from Chapter 9 times 6 ft) Height of pier above grade (h col ) = 4 ft Distance from grade to bottom of footing (x) = 2 ft Column width (W col ) = 1.33 ft Column thickness (t col ) = 1.33 ft Unit weight of column and footing material (w c ) = 150 lb/ft 2 Soil bearing pressure (q ) = 2,000 psf Footing thickness (t foot ) = 1 ft Home is 24 ft x 30 ft consisting of a matrix of 30 16-in. square piers (see Example 10.3, Illustration A) Piers spaced 6 ft o.c. (see Illustration A) Find: The appropriate square footing size for the given uplift and lateral loads. Solution: The square footing size can be found using Equation 10.6: For simplicity, this example assumes the pier is partially submerged and exposed to uplift forces (as in Example 10.4) but that there are no loads from moving floodwaters or wave action. In an actual design, those forces would need to be considered. Also, if the vertical load is applied at an eccentricity “Δ”, the moment P tΔ must be combined with P l (H + x) (by vector addition) to determine the total moment applied to the footing. 2 The total induced moment at the footing can be modeled by considering an effective reaction R numerically equal to the total vertical load P t but applied at an eccentricity e from the centroid of the footing. The lateral load is modeled at the centroid of the footing where it contributes only to sliding. The equivalent eccentricity e is given by the following formula: 2 Unless the eccentricity from the lateral loads is collinear with the eccentricity from the vertical loads, the footing will be exposed to biaxial bending. For biaxial bending, soil stresses must be checked in both directions. 10-44 COASTAL CONSTRUCTION MANUAL
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Coastal Construction Manual Princip
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All illustrations in this document
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PREFACE Volume II Chapter 8 - Site-
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1 CHAPTER TITLE Contents COASTAL CO
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1 CHAPTER TITLE Acronyms COASTAL CO
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Volume II ACRONYMS H HMA Hazard Mit
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Volume II ACRONYMS T TMS The Masonr
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GLOSSARY Volume II Basement - Under
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GLOSSARY Volume II Conditions Great
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GLOSSARY Volume II F Federal Emerge
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GLOSSARY Volume II G Gabion - Rock-
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GLOSSARY Volume II Location - The l
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GLOSSARY Volume II O Open foundatio
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GLOSSARY Volume II Z Zone A - Under
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INDEX Volume II Building envelope,
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INDEX Volume II stillwater flood de
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INDEX Volume II I IBHS (see Insuran
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INDEX Volume II date of constructio
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INDEX Volume II Seawall, 7-7, 7-17,
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INDEX Volume II velocity pressure,
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FEMA P55 Coastal Construction Manual, Fourth Edition - Mad Cad

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