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Chapter 5
FLOOD LOADS
5.1 GENERAL
The provisions of this section apply to buildings and other structures
located in areas prone to flooding as defined on a flood
hazard map.
5.2 DEFINITIONS
The following definitions apply to the provisions of this chapter:
APPROVED: Acceptable to the authority having jurisdiction.
BASE FLOOD: The flood having a 1 percent chance of being
equaled or exceeded in any given year.
BASE FLOOD ELEVATION (BFE): The elevation of flooding,
including wave height, having a 1 percent chance of being
equaled or exceeded in any given year.
BREAKAWAY WALL: Any type of wall subject to flooding
that is not required to provide structural support to a building or
other structure, and that is designed and constructed such that,
under base flood or lesser flood conditions, it will collapse in
such a way that: (I) it allows the free passage of floodwaters,
and (2) it does not damage the structure or supporting foundation
system.
COASTAL A-ZONE: An area within a special flood hazard
area, landward of a V-Zone or landward of an open coast without
mapped V-Zones. To be classified as a Coastal A-Zone, the principal
source of flooding must be astronomical tides, storm surges,
seiches, or tsunamis, not riverine flooding, and the potential for
breaking wave heights greater than or equal to 1.5 ft (0.46 m)
must exist during the base flood.
COASTAL HIGH HAZARD AREA (V-ZONE): An area
within a Special Flood Hazard Area, extending from offshore
to the inland limit of a primary frontal dune along an open
coast, and any other area that is subject to high-velocity wave
action from storms or seismic sources. This area is designated
on Flood Insurance Rate Maps (FIRMS) as V, VE, VO, or
Vl-30.
DESIGN FLOOD: The greater of the following two flood
events: (I) the Base Flood, affecting those areas identified as
Special Flood Hazard Areas on the community's FIRM; or
(2) the flood corresponding to the area designated as a Flood
Hazard Area on a community's Flood Hazard Map or otherwise
legally designated.
DESIGN FLOOD ELEVATION (DFE): The elevation of the
design flood, including wave height, relative to the datum specified
on a community's flood hazard map.
FLOOD HAZARD AREA: The area subject to flooding during
the design flood.
FLOOD HAZARD MAP: The map delineating Flood Hazard
Areas adopted by the authority having jurisdiction.
FLOOD INSURANCE RATE MAP (FIRM): An official
map of a community on which the Federal Insurance and
Mitigation Administration has delineated both special flood hazard
areas and the risk premium zones applicable to the community.
SPECIAL FLOOD HAZARD AREA (AREA OF SPECIAL
FLOOD HAZARD): The land in the floodplain subject to
a 1 percent or greater chance of flooding in any given year. These
areas are delineated on a community's FIRM as A-Zones (A, AE,
Al-30, A99, AR, AO, or AH) or V-Zones (V, VE, VO, or Vl-30).
5.3 DESIGN REQUIREMENTS
5.3.1 Design Loads. Structural systems of buildings or other
structures shall be designed, constructed, connected, and anchored
to resist flotation, collapse, and permanent lateral displacement
due to action of flood loads associated with the design flood
(see Section 5.3.3) and other loads in accordance with the load
combinations of Chapter 2.
5.3.2 Erosion and Scour. The effects of erosion and scour shall
be included in the calculation of loads on buildings and other
structures in flood hazard areas.
5.3.3 Loads on Breakaway Walls. Walls and partitions required
by ASCEISEI 24, to break away, including their connections to
the structure, shall be designed for the largest of the following
loads acting perpendicular to the plane of the wall:
I. The wind load specified in Chapter 6.
2. The earthquake load specified in Chapter 9.
3. 10 psf (0.48 k~/m').
The loading at which breakaway walls are intended to collapse
shall not exceed 20 psf (0.96 kN/m2) unless the design meets the
following conditions:
I. Breakaway wall collapse is designed to result from a flood
load less than that which occurs during the base flood.
2. The supporting foundation and the elevated portion of the
building shall be designed against collapse, permanent lateral
displacement, and other structural damage due to the
effects of flood loads in combination with other loads as
specified in Chapter 2.
5.4 LOADS DURING FLOODING
5.4.1 Load Basis. In flood hazard areas, the structural design
shall be based on the design flood.
5.4.2 Hydrostatic Loads. Hydrostatic loads caused by a depth
of water to the level of the DFE shall be applied over all surfaces
involved, both above and below ground level, except that for surfaces
exposed to free water, the design depth shall be increased
by 1 ft (0.30 m).
Reduced uplift and lateral loads on surfaces of enclosed spaces
below the DFE shall apply only if provision is made for entry and
exit of floodwater.
Minimum Design Loads for Buildings and Other Structures
5.4.3 Hydrodynamic Loads. Dynamic effects of moving water
shall be determined by a detailed analysis utilizing basic concepts
of fluid mechanics.
EXCEPTION: Where water velocities do not exceed 10 ft/s (3.05 mls),
dynamic effects of moving water shall be permitted to be converted into
equivalent hydrostatic loads by increasing the DFE for design purposes
by an equivalent surcharge depth, dh, on the headwater side and above the
ground level only, equal to
where
V = average velocity of water in ft/s (mls)
g = acceleration due to gravity, 32.2 ft/s (9.81 m/s2)
a = coefficient of drag or shape factor (not less than 1.25)
The equivalent surcharge depth shall be added to the DFE design
depth and the resultant hydrostatic pressures applied to, and
uniformly distributed across, the vertical projected area of the
building or structure that is perpendicular to the flow. Surfaces
parallel to the flow or surfaces wetted by the tail water shall be
subject to the hydrostatic pressures for depths to the DFE only.
5.4.4 Wave Loads. Wave loads shall be determined by one of the
following three methods: (I) by using the analytical procedures
outlined in this section, (2) by more advanced numerical modeling
procedures, or (3) by laboratory test procedures (physical
modeling).
Wave loads are those loads that result from water waves uroua- 1 1
gating over the water surface and striking a building or other
structure. Design and construction of buildings and other structures
subject to wave loads shall account for the following loads:
waves breaking on any portion of the building or structure; uplift
forces caused by shoaling waves beneath a building or structure,
or portion thereof; wave runup striking any portion of the
building or structure; wave-induced drag and inertia forces; and
wave-induced scour at the base of a building or structure, or its
foundation. Wave loads shall be included for both V-Zones and
A-Zones. In V-Zones, waves are 3 ft (0.91 m) high, or higher; in
coastal floodplains landward of the V-Zone, waves are less than
3 ft high (0.91 m).
Nonbreaking and broken wave loads shall be calculated using
the procedures described in Sections 5.4.2 and 5.4.3 that show
how to calculate hydrostatic and hydrodynamic loads.
Breaking wave loads shall be calculated using the procedures
described in Sections 5.4.4.1 through 5.4.4.4. Breaking wave
heights used in the procedures described in Sections 5.4.4.1
through 5.4.4.4 shall be calculated for V-Zones and Coastal AZones
using Eqs. 5-2 and 5-3.
where
HI, = breaking wave height in ft (m)
d, = local still water depth in ft (m)
The local still water depth shall be calculated using Eq. 5-3,
unless more advanced procedures or laboratory tests permitted by
this section are used.
where
BFE = BFE in ft (m)
G = ground elevation in ft (m)
5.4.4.1 Breaking Wave Loads on Vertical Pilings and
Columns. The net force resulting from a breaking wave acting
on a rigid vertical pile or column shall be assumed to act at the
still water elevation and shall be calculated by the following:
where
FD = net wave force, in lb (kN)
y,, = unit weight of water, in lb per cubic ft (kN/m3), = 62.4 pcf
(9.80 kN/m3) for fresh water and 64.0 pcf (10.05 Wm3)
for salt water
CD = coefficient of drag for breaking waves, = 1.75 for round
piles or columns, and = 2.25 for square piles or columns
D = pile or column diameter, in ft (m) for circular sections, or
for a square pile or column, 1.4 times the width of the pile
or column in ft (m)
HI, = breaking wave height, in ft (m)
5.4.4.2 Breaking Wave Loads on Vertical Walls. Maximum
pressures and net forces resulting from a normally incident breaking
wave (depth-limited in size, with HI, = 0.78d,) acting on a
rigid vertical wall shall be calculated by the following:
and
where
Pmax =maximum combined dynamic (C[,y,,d,) and static
(1.2y,,d,) wave pressures, also referred to as shockpressures
in lb/ft2 (kN/m2)
Ft = net breaking wave force per unit length of structure, also
referred to as shock, impulse, or wave impact force in
lb/ft (kN/m), acting near the still water elevation
CI, = dynamic pressure coefficient (1.6 < CI, < 3.5) (see
Table 5-1)
y,, = unit weight of water, in lb per cubic ft (kN/m3), = 62.4 pcf
(9.80 k ~ / mf~or )fr esh water and 64.0 pcf (10.05 Wm 3 )
for salt water
d, = still water depth in ft (m) at base of building or other
structure where the wave breaks
This procedure assumes the vertical wall causes a reflected or
standing wave against the waterward side of the wall with the
crest of the wave at a height of 1.2d, above the still water level.
Thus, the dynamic static and total pressure distributions against
the wall are as shown in Fig. 5-1.
This procedure also assumes the space behind the vertical wall
is dry, with no fluid balancing the static component of the wave
force on the outside of the wall. If free water exists behind the
wall, a portion of the hydrostatic component of the wave pressure
and force disappears (see Fig. 5-2) and the net force shall be
computed by Eq. 5-7 (the maximum combined wave pressure is
still computed with Eq. 5-5).
where
Ft = net breaking wave force per unit length of structure, also
referred to as shock, impulse, or wave impact force in lb/ft
(kN/m), acting near the still water elevation
CI, = dynamic pressure coefficient (1.6 < C1, < 3.5) (see
Table 5-1)
y,, = unit weight of water, in lb per cubic ft (kN/m3), = 62.4 pcf
(9.80 kN/m3) for fresh water and 64.0 pcf (10.05 kN/m3)
for salt water
ASCE 7-05
d, = still water depth in ft (m) at base of building or other struc- Ft = net breaking wave force (normally incident waves) acting
ture where the wave breaks on a vertical surface in lblft (kN1m)
5.4.4.3 Breaking Wave Loads on Nonvertical Walls. Breaking
wave forces given by Eqs. 5-6 and 5-7 shall be modified in
instances where the walls or surfaces upon which the breaking
waves act are nonvertical. The horizontal component of breaking
wave force shall be given by
FIX, = Ft sin2 a (5-8)
where
FIX, = horizontal component of breaking wave force in lblft
(kNlm)
a = horizontal angle between the direction of wave approach and
the vertical surface
5.4.5 Impact Loads. Impact loads are those that result from
debris, ice, and any object transported by floodwaters striking
against buildings and structures, or parts thereof. Impact loads
shall be determined using a rational approach as concentrated
loads acting horizontally at the most critical location at or below
the DFE.
Ft = net breaking wave force acting on a vertical surface in
lblft (kN1m) 5.5 CONSENSUS STANDARDS AND OTHER
a = vertical angle between nonvertical surface and the hori- REFERENCED DOCUMENTS
-
zontal This section lists the consensus standards and other documents
which are adopted by reference within this chapter:
5.4.4.4 Breaking Wave Loads from Obliquely Incident Waves.
Breaking wave forces given by Eqs. 5-6 and 5-7 shall be modified ASCEISEI
in instances where waves are obliquely incident. Breaking wave American Society of Civil Engineers
forces from non-normally incident waves shall be given by Structural Engineering Institute
1801 Alexander Bell Drive
F,,, = Ft sin2 a (5-9) Reston, VA 20191-4400
where ASCEISEI 24
F,,, = horizontal component of obliquely incident breaking wave Section 5.3.3
force in lblft (Wm) Flood Resistant Design and Construction, 1998
TABLE 5-1 VALUE OF DYNAMIC PRESSURE COEFFICIENT, Cp
Building Category
111 3.2
IV 3.5
Minimum Design Loads for Buildings and Other Structures
Vertical Wall
Crest of reflected wave
Dynamic pressure
1.2 d, I Crest of incident wave
0.55 d,
---------
Stillwater level
ds pressure
Ground elevation
FIGURE 5-1 NORMALLY INCIDENT BREAKING WAVE PRESSURES AGAINST A VERTICAL WALL (space behind vertical
wall is dry)
ASCE 7-05
Vertical Wall
Crest of reflected wave
Dynamic pressure
Crest of incident wave
Stillwater level
Net hydrostatic pressure
Ground elevation
FIGURE 5-2 NORMALLY INCIDENT BREAKING WAVE PRESSURES AGAINST A VERTICAL WALL (still water
level equal on both sides of wall)
Minimum Design Loads for Buildings and Other Structures