Seismic and wind resistance

The Old House Web

Seismic Resistance

Editor's note: This story is adapted from the U.S. Department of Housing and Urban Development's Residential Rehabilitation Inspection Guide, 2000.Click here for other stories in this series.

Parts of this story: Introduction ~~ Seismic and wind resistance ~~Cracking and deterioration of masonry, general issues ~~Masonry foundations & piers~~Above ground masonry walls ~~Chimneys ~~Wood structural components ~~Iron and steel structural components ~~Concrete structural components

earthquake damage

This home was destroyed when an earthquake struck Northridge, California in January 1994. Approximately 114,000 residential and commercial structures were damaged and 72 deaths were attributed to the earthquake. Damage costs were estimated at $25 billion. FEMA News Photo

If the building is in seismic zones 2B, 3, and 4 (California, Idaho, Nevada, Oregon, Washington, and portions of Alaska, Arizona, Arkansas, Hawaii, Missouri, Montana, New Mexico, Utah, and Wyoming), have a structural engineer check the following conditions for structural vulnerability: (Note that wood frame buildings with brick or stone veneer are still considered wood frame).

  • Wood frame buildings that are not physically anchored to their foundations. Such buildings may be vulnerable to shifting or sliding.
  • Wood frame buildings and wood-framed portions (porches, for example) or other buildings when they are supported above ground on either short wood studs (cripple walls) or on piers of stone, masonry, or concrete. Such buildings may be vulnerable to tilting or falling over.
  • Non-reinforced and inadequately reinforced masonry buildings. Such buildings may be vulnerable to total or partial collapse due to inadequate reinforcement or to inadequate anchorage of roofs and walls to the floors. Use as a reference Seismic Strengthening Provisions for Non-reinforced Masonry Bearing Wall Buildings, Appendix Chapter 1 of the Uniform Code for Building Conservation.
  • Buildings of any type that have irregular shapes. Such buildings may be vulnerable to partial collapse.
  • Wood frame and masonry buildings with more than one story above grade where the story at grade is a large unobstructed open space, such as a garage. Such buildings may be vulnerable to collapse of the story at grade.
  • Wood frame and masonry buildings with more than one story above grade that are constructed on sloping hill-sides, and buildings of any type of construction and height that are constructed on steep slopes of 20 F (-7 C) or more. Such buildings may be vulnerable to sliding.

If the building is in seismic zone 2A (Connecticut, Massachusetts, Rhode Island, South Carolina, and portions of Georgia, Illinois, Indiana, Kansas, Kentucky, New Hampshire, New Jersey, New York, North Carolina, Oklahoma, Pennsylvania, Tennessee, Vermont, and Virginia) and has more than two stories above grade, consider having a structural engineer check for the last two conditions (large unobstructed open space at grade and sloped sites).

Buildings not of wood frame or masonry construction, such as stone, adobe, log, and post and beam structures, as well as buildings with more than one type of construction in any seismic zone, should be investigated by a structural engineer to determine their seismic vulnerability.

Masonry bearing wall buildings in seismic zones 2B, 3, and 4 should be investigated by a structural engineer for the presence of reinforcing steel. Use as a reference Seismic Strengthening Provisions for Unreinforced Masonry Bearing Wall Buildings, Appendix Chapter 1 of the Uniform Code for Building Conservation.

Have a structural engineer check the anchorage of wood framed structures to their foundations and investigate all such structures supported on cripple walls or piers in seismic zones 2, 3, and 4.

In all seismic zones, a structural engineer should investigate buildings with more than one story above grade where the story at grade is a large unobstructed open space or the building is on a sloping hillside and in seismic zones 2B, 3, and 4, buildings with an irregular shape.

Wind resistance

hurricane damage

This second floor of this home shows devastation of high winds from Hurricane Andrew in Florida in August 1992. The storm was one of the most destructive hurricanes ever recorded in America. One million people were evacuated and 54 died in this hurricane. FEMA News Photo

The coasts of the Gulf of Mexico, the south- and mid-Atlantic coast, the coastal areas of Puerto Rico, the U.S. Virgin Islands and Hawaii as well as the U.S. territories of American Samoa and Guam are vulnerable to hurricanes in the late summer and early fall.

Hurricanes are large, slow moving, damaging storms characterized by gusting winds from different directions, rain, flooding, high waves, and storm surges. Winter storms along the mid- and north-Atlantic coast can be more damaging than hurricanes because of their greater frequency, longer duration, and high erosion impacts on the coastline. Even in states not normally considered susceptible to extreme wind storms, there are areas that experience dangerously high winds. These areas are typically near mountain ranges and include the Pacific Northwest coast. Other extreme wind areas include the plains states, which are especially subject to tornadoes.

In addition to the direct effects of high winds and winter on buildings, hurricanes and other severe storms generate airborne debris that can damage buildings. Debris, such as small stones, tree branches, roof shingles and tiles, building parts and other objects, is picked up by the wind and moved with enough force to damage and even penetrate windows, doors, walls, and roofs. When a buildings exterior envelope is breached by debris, the building can become pressurized, subjecting its walls and roof to much higher damaging wind pressures. In general, the stronger the wind, the larger and heavier the debris it can carry and the greater the risk of severe damage.

If the building is in a hurricane or high-wind region, have a structural engineer check its structural system for continuity of load path, including resistance to uplift forces. If there is an accessible attic, check for improper attachment of the roof sheathing to the roof framing members by looking for unengaged or partially engaged nails. Check for the presence of hurricane hold-down clips for joists, rafters, and trusses at the exterior wall. Examine the gable end walls and the roof trusses for lateral bracing. Check to see whether the exterior wall and other load-bearing walls are securely attached to the foundation.

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