FEMA P55 Coastal Construction Manual, Fourth Edition - Mad Cad

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10 DESIGNING THE FOUNDATION Volume II A properly designed pile foundation must include a consideration of the effects of scour and erosion on the foundation system. Scour washes away soils around the piling, reducing pile embedment, and increases stresses within the pile when the pile is loaded. The reduced embedment can cause the foundation to fail at the pile/soil interface. The increased stresses can cause the pile itself to fracture and fail. Erosion is even more damaging. In addition to reducing pile embedment depths and increasing stresses on piles, erosion increases the flood forces the foundation must resist by increasing the stillwater depth at the foundation that the flood produces. Pile foundations that are adequate to resist flood and wind forces without being undermined by scour and erosion can fail when exposed to even minor amounts of scour and erosion. An example analysis of the effects of scour and erosion on a foundation is provided in Erosion, Scour, and Foundation Design (<strong>FEMA</strong> 2009a), published as part of Hurricane Ike Recovery Advisories and available at http://www.fema.gov/library/viewRecord.do?id=3539. The structure in the example is a two-story house with 10-foot story heights and a 32-foot by 32-foot foundation. The house is away from the shoreline and elevated 8 feet above grade on 25 square timber piles spaced 8 feet apart. Soils are medium dense sands. The house is subjected to a design wind event with a 130-mph (3-second gust) wind speed and a 4-foot stillwater depth above the uneroded grade, with storm surge and broken waves passing under the elevated building. Lateral wind and flood loads were calculated in accordance with ASCE 7-05. Although the wind loads in ASCE 7-10 vary from ASCE 7-05 somewhat, the results of the analyses do not change significantly. Piles were analyzed under lateral wind and flood loads only; dead, live, and wind uplift loads were neglected. If the neglected loads are included, deeper pile embedment and possibly larger piles than the results of the analysis indicated may be needed. Three timber pile sizes (8-inch square, 10-inch square, and 12-inch square) were evaluated using pre-storm embedment depths of 10 feet, 15 feet, and 20 feet and five erosion and scour conditions (erosion = 0 or 1 foot; scour = 2.0 times the pile diameter to 4.0 times the pile diameter). The results of the analysis are shown in Table 10-10. A shaded cell indicates that the combination of pile size, pre-storm embedment, and erosion/scour does not provide the bending resistance and/or embedment required to resist lateral loads. The reason for foundation failure is indicated in each shaded cell (“P” for failure due to bending and overstress within the pile and “E” for an embedment failure from the pile/soil interaction). “OK” indicates that the bending and foundation embedment criteria are both satisfied by the particular pile size/pile embedment/erosion-scour combination. The key points from the example analysis are as follows: Scour and erosion can cause pile foundations to fail and must be considered when designing pile foundations. Failures can result from either overloading the pile itself or from overloading at the pile/soil interface. Increasing a pile’s embedment depth does not offset a pile with a cross section that is too small or pile material that is too weak. Increasing a pile’s cross section (or its material strength) does not compensate for inadequate pile embedment. 10-22 COASTAL CONSTRUCTION MANUAL
Volume II DESIGNING THE FOUNDATION 10 Table 10‑10. Example Analysis of the Effects of Scour and Erosion on a Foundation Pile Embedment Before Erosion and Scour 10 feet 15 feet 20 feet COASTAL CONSTRUCTION MANUAL Erosion and Scour Conditions Pile Diameter (a) 8 inches 10 inches 12 inches Reason for Failure Erosion = 0, Scour = 0 P, E E OK Erosion = 1 foot, Scour = 2.0a P, E E E Erosion = 1 foot, Scour = 2.5a P, E E E Erosion = 1 foot, Scour = 3.0a P, E E E Erosion = 1 foot, Scour = 4.0a P, E P, E E Erosion = 0, Scour = 0 P OK OK Erosion = 1 foot, Scour = 2.0a P OK OK Erosion = 1 foot, Scour = 2.5a P OK OK Erosion = 1 foot, Scour = 3.0a P OK OK Erosion = 1 foot, Scour = 4.0a P, E P, E E Erosion = 0, Scour = 0 P OK OK Erosion = 1 foot, Scour = 2.0a P OK OK Erosion = 1 foot, Scour = 2.5a P OK OK Erosion = 1 foot, Scour = 3.0a P OK OK Erosion = 1 foot, Scour = 4.0a P P OK Two-story house supported on square timber piles and located away from the shoreline, storm surge and broken waves passing under the building, 130-mph wind zone, soil = medium dense sand. a = pile diameter E = foundation fails to meet embedment requirements OK = bending and foundation embedment criteria are both satisfied by the particular pile size/pile embedment/erosion-scour combination P = foundation fails to meet bending 10.5.6 Grade Beams for Pile Foundations Piles can be used with or without grade beams or pile caps. Grade beams create resistance to rotation (also called “fixity”) at the top of the piles and provide a method to accommodate misalignment in piling placement. When used with grade beams, the piles and foundation elements above the grade beams work together to elevate the structure, provide vertical and lateral support for the elevated home, and transfer loads imposed on the elevated home and the foundation to the ground below. Pile and grade beam foundations should be designed and constructed so that the grade beams act only to provide fixity to the foundation system and not to support the lowest elevated floor. If grade beams support the lowest elevated floor of the home, they become the lowest horizontal structural member and significantly higher flood insurance premiums would result. Grade beams must also be designed to span between adjacent piles, and the piles must be capable of resisting both the weight of the grade beams when undermined by erosion and scour and the loads imposed on them by forces acting on the structure. 10-23
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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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Volume II CONTENTS Chapter 9. Desig
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9 DESIGNING THE BUILDING Volume II
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12 INSTALLING MECHANICAL EQUIPMENT
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13 CONSTRUCTING THE BUILDING Volume
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14 MAINTAINING THE BUILDING Volume
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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 Risk tolerance -
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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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