§ 21-16.1. Residential structures (coastal high-hazard areas)., Article V. Construction Standards, Chapter 21. Floodplain Management Regulations, Code of Ordinances, North Hempstead

Elevation. New construction and substantial improvements shall be elevated on pilings, columns or shear walls such that the bottom of the lowest horizontal structural member supporting the lowest elevated floor (excluding columns, piles, diagonal bracing attached to the piles or columns, grade beams, pile caps and other members designed to either withstand storm action or break away without imparting damaging loads to the structure) is elevated to or above two feet above base flood elevation so as not to impede the flow of water.

Determination of loading forces. Structural design shall consider the effects of wind and water loads acting simultaneously during the base flood on all building components.

(1)

The structural design shall be adequate to resist water forces that would occur during the base flood. Horizontal water loads considered shall include inertial and drag forces of waves, current drag forces and impact forces from waterborne storm debris. Dynamic uplift loads shall also be considered if bulkheads, walls or other natural or man-made flow obstructions could cause wave runup beyond the elevation of the base flood.

(2)

Buildings shall be designed and constructed to resist the forces due to wind pressure. Wind forces on the superstructure include windward and leeward forces on vertical walls, uplift on the roof, internal forces when openings allow wind to enter the house, and upward force on the underside of the house when it is exposed. In the design, the wind should be assumed to blow potentially from any lateral direction relative to the house.

(3)

Wind-loading values used shall be those required by the building code.

Foundation standards.

(1)

The pilings or column foundation and structure attached thereto shall be adequately anchored to resist flotation, collapse or lateral movement due to the effects of wind and water pressures acting simultaneously on all building components. Foundations must be designed to transfer safely to the underlying soil all loads due to wind, water, dead load, live load and other loads (including uplift due to wind and water).

(2)

Spread footings and fill material shall not be used for structural support of a new building or substantial improvement of an existing structure.

Pile foundation design.

(1)

The design ratio of pile spacing to pile diameter shall not be less than 8:1 for individual piles. (This shall not apply to pile clusters located below the design grade.) The maximum center-to-center spacing of wood piles shall not be more than 12 feet on center under load-bearing sills, beams or girders.

(2)

Pilings shall have adequate soil penetration (bearing capacity) to resist the combined wave and wind loads (lateral and uplift) associated with the base flood acting simultaneously with typical structure (live and dead) loads and shall include consideration of decreased resistance capacity caused by erosion of soil strata surrounding the piles. The minimum penetration for foundation piles is to an elevation of five feet below mean sea level (msl) datum if the BFE is +10 msl or less or at least 10 feet below msl if the BFE is greater than +10 msl.

(3)

Pile foundation analysis shall also include consideration of piles in column action from the bottom of the structure to the stable soil elevation of the site. Pilings may be horizontally or diagonally braced to withstand wind and water forces.

(4)

The minimum acceptable sizes for timber piles are a tip diameter of eight inches for round timber piles and eight inches by eight inches for square timber piles. All wood piles must be treated in accordance with requirements of EPEE-C3 to minimize decay and damage from fungus.

(5)

Reinforced concrete piles shall be cast of concrete having a twenty-eight-day ultimate compressive strength of not less than 5,000 pounds per square inch and shall be reinforced with a minimum of four longitudinal steel bars having a combined area of not less than 1% nor more than 4% of the gross concrete area. Reinforcing for precast piles shall have a concrete cover of not less than 1 1/4 inches for No. 5 bars and smaller and not less than 1 1/2 inches for Nos. 6 through 11 bars. Reinforcement for piles cast in the field shall have a concrete cover of not less than two inches.

(6)

Piles shall be driven by means of a pile driver or drop hammer, or jetted or augered into place.

(7)

Additional support for piles in the form of bracing may include lateral or diagonal bracing between piles.

(8)

When necessary, piles shall be braced at the ground line in both directions by a wood timber grade beam or a reinforced concrete grade beam. These at-grade supports should be securely attached to the piles to provide support even if scoured from beneath.

(9)

Diagonal bracing between piles, consisting of two-inch by eight-inch (minimum) members bolted to the piles, shall be limited in location to below the lowest supporting structural member and above the stable soil elevation and aligned in the vertical plane along pile rows perpendicular to the shoreline. Galvanized steel rods (minimum diameter of 1/2 inch) or cable-type bracing is permitted in any plane.

(10)

Knee braces, which stiffen both the upper portion of a pile and the beam-to-pile connection, may be used along pile rows perpendicular and parallel to the shoreline. Knee braces shall be two-by-eight lumber bolted to the sides of the pile/beam or four-by-four, or larger, braces framed into the pile/beam. Bolting shall consist of two five-eighths-inch galvanized steel bolts (each end) for two-by-eight members or one five-eighths-inch lag bolt (each end) for square members. Knee braces shall not extend more than three feet below the elevation of the base flood.

Column foundation design. Masonry piers or poured-in-place concrete piers shall be internally reinforced to resist vertical and lateral loads and connected with a movement-resisting connection to a pile cap or pile shaft.

Connectors and fasteners. Galvanized metal connectors, wood connectors or bolts of size and number adequate for the calculated loads must be used to connect adjoining components of a structure. Toe nailing as a principal method of connection is not permitted. All metal connectors and fasteners used in exposed locations shall be steel, hot-dipped galvanized after fabrication. Connectors in protected interior locations shall be fabricated from galvanized sheet.

Beam-to-pile connections. The primary floor beams or girders shall span the supports in the direction parallel to the flow of potential floodwater and wave action and shall be fastened to the columns or pilings by bolting, with or without cover plates. Concrete members shall be connected by reinforcement, if cast in place, or (if precast) shall be securely connected by bolting and welding. If sills, beams or girders are attached to wood piling at a notch, a minimum of two five-eighths-inch galvanized steel bolts or two hot-dipped galvanized straps 3/16 inch by four inches by 18 inches, each bolted with two one-half-inch lag bolts per beam member, shall be used. Notching of pile tops shall be the minimum sufficient to provide ledge support for beam members without unduly weakening pile connections. Piling shall not be notched so that the cross section is reduced below 50%.

Floor and deck connections.

(1)

Wood two-inch-by-four-inch (minimum) connectors or metal joist anchors shall be used to tie floor joists to floor beams/girders. These should be installed on alternate floor joists, at a minimum. Cross-bridging of all floor joists shall be provided. Such cross bridging may be one-inch-by-three-inch members, placed eight feet on center maximum, or solid bridging of same depth as joist at same spacing.

(2)

Plywood should be used for subflooring and attic flooring to provide good torsional resistance in the horizontal plane of the structure. The plywood should not be less than a total thickness of 3/4 inch and should be exterior grade and fastened to beams or joists with 8d annular or spiral thread galvanized nails. Such fastening shall be supplemented by the application of waterproof industrial adhesive applied to all bearing surfaces.

Exterior wall connections. All bottom plates shall have any required breaks under a wall stud or an anchor bolt. Approved anchors will be used to secure rafters or joists and top and bottom plates to studs in exterior and bearing walls to form a continuous tie. Continuous plywood sheathing of 15/32 inch or thicker, overlapping the top wall plate and continuing down to the sill, beam or girder, may be used to provide the continuous tie. If the sheets of plywood are not vertically continuous, then two-by-four nailer blocking shall be provided at all horizontal joints. In lieu of the plywood, galvanized steel rods of one-half-inch diameter or galvanized steel straps not less than one inch wide by 1/16 inch thick may be used to connect from the top wall plate to the sill, beam or girder. Washers with a minimum diameter of three inches shall be used at each end of the one-half-inch round rods. These anchors shall be installed no more than two feet from each corner rod, no more than four feet on center.

Ceiling joist/rafter connections. All ceiling joists or rafters shall be installed in such a manner that the joists provide a continuous tie across the rafters. Ceiling joists and rafters shall be securely fastened at their intersections. A metal or wood connector shall be used at alternate ceiling joist/rafter connections to the wall top plate. Gable roofs shall be additionally stabilized by installing two-by-four blocking on two-foot centers between the rafters at each gable end. Blocking shall be installed a minimum of eight feet toward the house interior from each gable end.

Projecting members. All cantilevers and other projecting members must be adequately supported and braced to withstand wind and water uplift forces. Roof eave overhangs shall be limited to a maximum of two feet and joist overhangs to a maximum of one foot. Larger overhangs and porches will be permitted if designed or reviewed and certified by a registered professional engineer or architect.

Roof sheathing.

(1)

Plywood, or other wood material, when used as roof sheathing, shall not be less than 15/32 inch in thickness and shall be of exterior sheathing grade or equivalent. All attaching devices for sheathing and roof coverings shall be galvanized or be of other suitable corrosion-resistant material.

(2)

All corners, gable ends and roof overhangs exceeding six inches shall be reinforced by the application of waterproof industrial adhesive applied to all bearing surfaces of any plywood sheet used in the sheathing of such corner, gable end or roof overhang.

(3)

In addition, roofs should be sloped as steeply as practicable to reduce uplift pressures, and special care should be used in securing ridges, hips, valleys, eaves, vents, chimneys and other points of discontinuity in the roofing surface.

Protection of openings. All exterior glass panels, windows and doors shall be designed, detailed and constructed to withstand loads due to the design wind speed of 75 miles per hour. Connections for these elements must be designed to transfer safely the design loads to the supporting structure. Panel widths of multiple panel sliding glass doors shall not exceed three feet.

Breakaway wall design standards.

(1)

The breakaway wall shall have a design-safe loading resistance of not less than 10 and not more than 20 pounds per square foot, with the criterion that the safety of the overall structure at the point of wall failure be confirmed using established procedures. Grade beams shall be installed in both directions for all piles considered to carry the breakaway wall load. Knee braces are required for front row piles that support breakaway walls.

(2)

Use of breakaway wall strengths in excess of 20 pounds per square foot shall not be permitted unless a registered professional engineer or architect has developed or reviewed the structural design and specifications for the building foundation and breakaway wall components and certifies that the breakaway walls will fail under water loads less than those that would occur during the base flood; and the elevated portion of the building and supporting foundation system will not be subject to collapse, displacement or other structural damage due to the effects of wind and water loads acting simultaneously on all building components (structural and nonstructural). Water-loading values used shall be those associated with the base flood. Wind-loading values shall be those required by the building code.