(a) General. All buildings and structures to be constructed under this article shall be in accordance with ASCE 24 or this section.
(b) Stability.
(1) Overturning or sliding. All buildings and structures to be constructed under this article shall be designed and constructed to provide a minimum factor of safety of 1.50 against failure by sliding or overturning when subjected to combined loads as specified in subsection (d).
(2) Flotation. All buildings and structures to be constructed under this article shall be designed and constructed to resist flotation from floodwater at the base flood elevation with a safety factor of 1.33.
(c) Loads. The following loads shall be considered in the design and construction of buildings and structures subject to this article:
(1) Hydrostatic loads;
(2) Hydrodynamic loads;
(3) Impact loads. Assume concentrated load acting horizontally at the base flood elevation or at any point below it, equal to the impact force produced by a 1,000-pound mass traveling at the velocity of the flood water and acting on a 1-square-foot surface of the structure;
(4) Soil loads. Consideration shall be given to loads or pressures resulting from soils against or over the structure. Computation shall be in accordance with accepted engineering practice with proper consideration for effect of water on the soil. Special consideration shall be given in the design of structures when expansive soils are present; and
(5) Tsunami. Structural design of buildings and structures subject to tsunamis shall be in accordance with subsection (f).
(d) Combined loads. All loads stipulated in this chapter and all flood-related loads specified under subsection (c) shall be applied on the structure and on structural components, alone and in combination, in such manner that the combined effect will result in maximum loads and stresses on the structure and members. Application of these loads shall be as follows:
(1) Dead loads. Use at full intensity;
(2) Live loads. Use at reduced intensity as provided in this chapter for design of columns, piers, walls, foundation, trusses, beams, and flat slabs. Live loads on floors at or below the base flood elevation and particularly in basement slabs, shall not be used if their omission results in greater loading or stresses on such floors. Similarly, for storage tanks, pools, and other similar structures designed to contain and store materials, which may be full or empty when a flood occurs, both conditions shall be investigated in combination with flood-related loads of the containing structure being full or empty;
(3) Wind load. Use at full intensity as required in this chapter on areas of the building and structure above the base flood elevation; and
(4) Earthquake load. Combined earthquake and flood-related loads need not be considered.
(e) Allowable soil pressures. Under flood conditions, the bearing capacity of submerged soils is affected and reduced by the buoyancy effect of the water on the soil. For foundations of buildings and structures covered by this article, the bearing capacity of soils shall be evaluated by a recognized acceptable method. Expansive soils should be investigated with special care. Soils that lose all bearing capacity when saturated, or become “liquefied” shall not be used for supporting foundations.
(f) Coastal flood water design.*
(1) Buildings or structures shall be designed to resist the effects of coastal floodwaters due to tsunamis. The base flood elevation due to tsunamis is considered to result from a nonbore condition, except where a bore condition is shown on the flood insurance maps or in the flood study adopted for the county.
(2) Habitable space in building structures must be elevated above the base flood elevation by such means as posts, piles, piers, or shear walls parallel to the expected direction of flow of the tsunami wave. The forces and effects of floodwaters on the structure shall be fully considered in the design.
(3) Allowable stresses (or load factors in the case of ultimate strength or limit design) for the building materials used shall be the same as the building code provides for wind or earthquake loads combined with gravity loads, i.e., treat loads and stresses due to tsunamis in the same fashion as for earthquake loadings.
(4) The main building structure shall be adequately anchored and connected to the elevating substructure system to resist all lateral, uplift, and downward forces. In wood construction, toenailing is not allowed.
(5) Scour of soil from around individual piles and piers shall be provided for in the design in the coastal flood hazard area. Shallow foundation types are not permitted, unless the natural supporting soils are protected on all sides against scour by a shore protection structure, preferably a bulkhead. Shallow foundations may be permitted beyond 300 feet from the shoreline; provided that they are founded on natural soil and at least 2 feet below the anticipated depth of scour, and provided that not more than 3 feet of scour is expected at the structure. The table below gives estimated minimum depths of soil scour below existing grade as a percentage of the depth (h) of water at the location.
Estimated Minimum Scour | ||
Distance from Shoreline | ||
Up to 300 Feet1 | Greater than 300 Feet2 | |
Estimated Minimum Scour | ||
Distance from Shoreline | ||
Up to 300 Feet1 | Greater than 300 Feet2 | |
Loose sand | 80% h | 60% h |
Dense sand | 50% h | 35% h |
Soft silt | 50% h | 25% h |
Stiff silt | 25% h | 15% h |
Soft clay | 25% h | 15% h |
Stiff clay | 10% h | 5% h |
1 Values may be reduced by 40% if a substantial dune or berm higher than the base flood elevation protects the building site. | ||
2 Values may be reduced 50% if the entire region is essentially flat. | ||
(6) Forces that must be considered in the design of structures elevated to resist floodwaters include:
(A) Buoyant forces—uplift caused by partial or total submergence of a structure;
(B) Surge forces—caused by the leading edge of a surge of water impinging on a structure;
(C) Drag forces—caused by velocity of flow around an object;
(D) Impact forces—caused by debris such as driftwood, small boats, portions of houses, etc., carried in the flood currents and colliding with a structure; and
(E) Hydrostatic forces—caused by an imbalance of pressure due to a differential water depth on opposite sides of a structure or structural member.
(7) Buoyant force. The buoyant force on a structure or structural member subject to partial or total submergence will act vertically through the center of mass of the displaced volume and is calculated from the following equation:
FB = pgV
where FB = buoyant force acting vertically
p = density of water (2.0 lb-s2/ft4 for salt water)
g = gravitational acceleration (32.2 ft/s2)
V = displaced volume of water (ft3)
(8) Surge force. The total force per unit width on a vertical wall subjected to a surge from the leading edge of a tsunami that approaches the structure as a bore or bore-like wave is calculated from the equation below. The resultant force acts at a distance approximately h above the base of the wall. (Note: This equation applies for walls with heights equal to or greater than 3h. Walls whose heights are less than 3h require surge forces to be calculated using the appropriate combination of hydrostatic and drag force equations for the given situation.)
FS = 4.5 pgh2
where FS = total force per unit width of wall
p = density of water (2.0 lb-s2
/ft4
for salt water)
g = gravitational acceleration (32.2 ft/s2
)
h = surge height (ft)
(9) Drag force.
FD
| = | pCD
Au2
|
2 |
where FD
= total drag force (lbs) acting in the direction of flow
p = density of water (2.0 lb-s2
/ft4
for salt water)
CD
= drag coefficient (nondimensional) (1.0 for circular piles, 2.0 for square piles, 1.5 for wall sections)
A = projected area of the body normal to the direction of flow (ft2
)
u = velocity of flow relative to body (ft/s) (estimated as equal in magnitude to depth in feet of water at the structure)
The flow is assumed to be uniform, so the resultant force will act at the centroid of the projected area immersed in the flow.
(10) Impact force.
where FI
= impact force (lb)
m = mass of the water displaced by the body impacting the structure (slugs)
Ub
= velocity of the body (ft/s) (estimated as equal in magnitude to depth in feet of water at the structure)
t = time (s)
dUb
= acceleration (deceleration) of the body at (ft/s2
)
dt
This single concentrated load acts horizontally at the base flood elevation or at any point below it and is equal to the impact force produced by a 1000-pound weight of debris traveling at the velocity of the flood water and acting on a 1 square-foot surface of the structural material where impact is postulated to occur. The impact force is to be applied to the structural material at a most critical or vulnerable location determined by the designer. It is assumed that the velocity of the body goes from Ub to zero over some small finite time interval (Δt) so the following approximation can be made:
For structural material of wood construction, assume t, the time interval over which impact occurs, is 1 second. For structural material of reinforced concrete construction, use Δt of 0.1 second and for structural material of steel construction, use t = 0.5 second.
(11) Hydrostatic force.
where FH
= hydrostatic force (lb/ft) on a wall, per unit width of wall
p = density of water (2.0 lb-s2
/ft4
for salt water)
g = gravitational acceleration (32.2 ft/s2
)
h = water depth (ft)
up
= component of velocity of flood flow perpendicular to the wall (ft/s) (total velocity, u, estimated as equal in magnitude to depth in feet of water at the structure)
The resultant force will act horizontally at a distance of
above the base of the wall.
(Sec. 16-7.5, R.O. 1978 (1983 Ed.); Sec. 16-5.5, R.O. 1978 (1987 Supp. to 1983 Ed.)) (1990 Code, Ch. 16, Art. 11, § 16-11.5) (Am. Ord. 90-57, 12-34)
Editor’s note:
* Reference is made to the January 31, 1980 report by Dames & Moore entitled “Design and Construction Standards for Residential Construction in Tsunami-Prone areas in Hawaii” prepared for the Federal Emergency Management Agency for a more detailed study and analysis of tsunami wave forces.