Building Design and Construction Handbook - Merritt - Ventech!

PROTECTION AGAINST HAZARDS 3.13
TABLE 3.1 Modified Mercalli Intensity Scale (Abridged)
Intensity Definition
I Detected only by sensitive instruments.
II Felt by a few persons at rest, especially on upper floors. Delicate suspended
objects may swing.
III Felt noticeably indoors; not always recognized as an earthquake. Standing
automobiles rock slightly. Vibration similar to that caused by a passing
truck.
IV Felt indoors by many, outdoors by few; at night some awaken. Windows,
dishes, doors rattle. Standing automobiles rock noticeably.
V Felt by nearly everyone. Some breakage of plaster, windows, and dishes.
Tall objects disturbed.
VI Felt by all; many frightened and run outdoors. Falling plaster and damaged
chimneys.
VII Everyone runs outdoors. Damage of buildings negligible to slight, depending
on quality of construction. Noticeable to drivers of automobiles.
VIII Damage slight to considerable in substantial buildings, great in poorly
constructed structures. Walls thrown out of frames; walls, chimneys,
monuments fall; sand and mud ejected.
IX Considerable damage to well-designed structures; structures shifted off
foundations; buildings thrown out of plumb; underground pipes damaged.
Ground cracked conspicuously.
X Many masonry and frame structures destroyed; rails bent; water splashed
over banks; landslides; ground cracked.
XI Bridges destroyed; rails bent greatly; most masonry structures destroyed;
underground service pipes out of commission; landslides; broad fissures in
ground.
XII Total damage. Waves seen in surface level; lines of sight and level distorted;
objects thrown into air.
3.3.3 Effects of Ground Conditions
The amplitude of ground motion at a specific location during an earthquake depends
not only on distance from the epicenter but also on the types of soil at the location.
(Some soils suffer a loss of strength in an earthquake and allow large, uneven
foundation settlements, which cause severe property damage.) Ground motion usually
is much larger in alluvial soils (sands or clays deposited by flowing water) than
in rocky areas or diluvial soils (material deposited by glaciers). Reclaimed land or
earth fills generally undergo even greater displacements than alluvial soils. Consequently,
in selection of sites for structures in zones where severe earthquakes are
highly probable during the life of the structures, preference should be given to sites
with hard ground or rock to considerable depth, with sand and gravel as a less
desirable alternative and clay as a poor choice.
3.3.4 Seismic Forces
During an earthquake, the ground may move horizontally in any direction and up
and down, shifting the building foundations correspondingly. Inertial forces, or seis-