ASCE 7-16 Minimum Design Loads And Associated Criteria PDF

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Commentary

Load Combinations Including Flood Load.

The nominal flood load, Fa , is based on the 100-year flood (Section 5.1).

The recommended flood load factor of 2.0 in V-Zones and Coastal A-Zones is based on a statistical analysis of flood loads associated with hydrostatic pressures, pressures caused by steady overland flow, and hydrodynamic pressures caused by waves, as specified in Section 5.4.

The flood load criteria were derived from an analysis of hurricane-generated storm tides produced along the United States East and Gulf coasts (Mehta et al. 1998), where storm tide is defined as the water level above mean sea level resulting from wind-generated storm surge added to randomly phased astronomical tides.

Hurricane wind speeds and storm tides were simulated at 11 coastal sites based on historical storm climatology and on accepted wind speed and storm surge models.

The resulting wind speed and storm tide data were then used to define probability distributions of wind loads and flood loads using wind and flood load equations specified in Sections 5.3 and 5.4.

Load factors for these loads were then obtained using established reliability methods (Ellingwood et al. 1982; Galambos et al. 1982) and achieve approximately the same level of reliability as do combinations involving wind loads acting without floods.

The relatively high flood load factor stems from the high variability in floods relative to other environmental loads.

The presence of 2.0Fa in both combinations (4) and (6) in V-Zones and Coastal A-Zones is the result of high stochastic dependence between extreme wind and flood in hurricane-prone coastal zones.

The 2.0Fa also applies in coastal areas subject to northeasters, extratropical storms, or coastal storms other than hurricanes, where a high correlation exists between extreme wind and flood.
Flood loads are unique in that they are initiated only after the water level exceeds the local ground elevation.

As a result, the statistical characteristics of flood loads vary with ground elevation.

The load factor 2.0 is based on calculations (including hydrostatic, steady flow, and wave forces) with stillwater flood depths ranging from approximately 4 to 9 ft (1.2–2.7 m) (average stillwater flood depth of approximately 6 ft (1.8 m)) and applies to a wide variety of flood conditions.

For lesser flood depths, load factors exceed 2.0 because of the wide dispersion in flood loads relative to the nominal flood load.

As an example, load factors appropriate to water depths slightly less than 4 ft (1.2 m) equal 2.8 (Mehta et al. 1998). However, in such circumstances, the flood load generally is small.

Thus, the load factor 2.0 is based on the recognition that flood loads of most importance to structural design occur in situations where the depth of flooding is greatest.

Deterministic ( MCER ) Ground Motions.

Deterministic ground motions are to be based on characteristic earthquakes on all known active faults in a region.

The magnitude of a characteristic earthquake on a given fault should be a best estimate of the
maximum magnitude capable for that fault but not less than the largest magnitude that has occurred historically on the fault.

The maximum magnitude should be estimated considering all seismic-geologic evidence for the fault, including fault length and paleoseismic observations.

For faults characterized as having more than a single segment, the potential for rupture of multiple segments in a single earthquake should be considered in assessing the characteristic maximum magnitude for the fault.

For consistency, the same attenuation equations and ground motion variability used in the PSHA should be used in the deterministic seismic hazard analysis (DSHA).

Adjustments for directivity and/or directional effects should also be made, when appropriate.

In some cases, ground motion simulation methods may be appropriate for the estimation of long-period motions at sites in deep sedimentary basins or from great ( M ≥ 8) or giant ( M ≥ 9 ) earthquakes, for which recorded ground motion data are lacking.

Author
Language English
No. of Pages30
PDF Size5 MB
CategoryGeneral
Source/Creditssbcc.wa.gov

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