Sample Design Calculations Page 18
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37C
APPENDIX
SAMPLE DESIGN CALCULATIONS
EXAMPLE C3. LOAD COMBINATIONS (continued)
When a structure is located in a flood zone, the following load combinations should be considered in
addition to the basic combinations on Section 2.4.1 of ASCE 7-10:
• In Zone V or A Coastal Zone – 1.5 F
should be added to load combination 5, 6, and 7 and E
a
should be set equal to zero in Nos. 5 and 6
• In non-Coastal A Zones – 0.75 F
should be added to load combination 5, 6, and 7 and E
a
should be set equal to zero in Nos. 5 and 6
Each possible building failure mode (sliding, overturning, uplift, buoyancy) must be investigated
using the most restrictive load combination.
• Controlling condition for sliding: The sum of the forces in the horizontal direction must
be LESS than the sliding resistance provided by the soil in order for building to not slide.
By inspection, load combination 7 is most restrictive sliding condition because dead load
is reduced and flood and wind loads are included for the direction parallel to the ridge. By
inspection, load combination 7 is also the most restrictive load combination equation for the
direction perpendicular to the ridge, even though no flood load is involved in this direction.
• Weight of the structure used in sliding and overturning calculations must incorporate the use
of flood vents. The buoyancy force for the slab and 5 ft of wall must be subtracted from the
total weight of the structure:
Parallel to ridge direction: The coefficient of soil friction is 0.3. Using load combination 7:
for the sliding force
for the sliding resistance providing by the soil
OK in parallel to ridge direction
Perpendicular to ridge direction: The coefficient of soil friction is 0.3. Using load combination 7:
where F
= 0 (flow and flood-borne debris directions are parallel to this
a
direction) and using the coefficient of soil friction 0.3
for the sliding force
for the sliding resistance providing by the soil
OK in perpendicular to ridge direction
Therefore, the structure can resist sliding.
Solution for #2: Check if the structure can resist overturning as follows:
• Controlling condition for overturning: The moments caused by the lateral forces must be less
than the counter-moment provided by the building weight.
C-18
ENGINEERING PRINCIPLES AND PRACTICES for Retrofitting Flood-Prone Residential Structures
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