Historic Buildings and Earthquake Codes

What are some of the principal risk factors of earthquakes for historic buildings? And how can we prepare for them?

Earthquakes result from sudden movements of the geological plates that form the earth’s crust, generally along cracks or fractures known as “faults.” When buildings are not designed and constructed to withstand these unpredictable and often violent ground motions, major structural damage, or outright collapse, can result, with grave risk to human life. Historic buildings are especially vulnerable to seismic events, particularly those built before seismic codes were adopted. Also, more and more communities continue to adopt higher standards for seismic retrofit of existing buildings. And, despite popular misconceptions, the risks of earthquakes are not limited to the West Coast, but exist across much of the United States.

Although historic and other older buildings can be retrofitted to survive earthquakes, the process of doing so may damage or destroy the very features that make such buildings significant. While life-safety issues remain foremost concerns, fortunately, there are various approaches which can help protect historic buildings from both the devastation caused by earthquakes and from the damage inflicted by well-intentioned, but insensitive, retrofit procedures. Building owners, managers, consultants, and communities need to be actively involved in planning for and readying irreplaceable historic resources from these threats.

The Federal Emergency Management Agency (FEMA) defines seismic risk as a function of earthquake hazard and vulnerability. Assessing the seismic risk of a historic property is the first step to avoid the potential loss of life and injuries, damage and loss of property, or disruption of services. Seismic evaluations of historic buildings within areas of earthquake hazard should be conducted if they have not been previously performed. This evaluation should identify both the potential structural deficiencies of the building (any structural component such as columns, beams, floors, etc., required to resist seismic forces), as well as the potential vulnerabilities of the nonstructural components of the building (all components that are not part of the structural system, which include exterior cladding, glazing, chimneys, interior partitions, ceilings, and other architectural features, as well as building systems, and equipment).

Nonstructural failures generally account for the majority of earthquake damage repair costs during earthquakes. Thus, it is critical to consider the risk and consequences of potential nonstructural failures. This is particularly important for historic buildings located in areas of low or moderate earthquake hazard, where the danger of collapse may be relatively small, but nonstructural elements such as unanchored stone veneers, cornices, parapets, chimneys, and gable ends may dislodge and fall to the ground during a moderate earthquake and pose severe life-safety hazards (Figure 6).

Other important nonstructural hazards to consider are the possibility that gas and water lines may rupture during an earthquake, which can cause fire and water damage. Many of these vulnerabilities can be mitigated by understanding how the forces unleashed in an earthquake affect a building, and then planning and implementing appropriate remedial treatments.

Putting a Team Together

A team that is experienced with both seismic retrofit requirements and historic preservation, and can adopt an inter-disciplinary approach, is important for achieving a seismic rehabilitation that is sensitive to the building’s historic character, features, and materials. Team members should be selected for their experience with similar projects, and may include architects, engineers, code specialists, contractors, and preservation consultants. Because the typical seismic codes are written for new construction, it is important that both the architect and structural engineer be knowledgeable about historic buildings and about meeting building code equivalencies and finding other options.

Local and state building officials can identify regulatory requirements, alternative approaches to meeting these requirements, and a historic preservation or building conservation code if one has been adopted by the jurisdiction. Even on small projects that cannot support a full professional team, consultants should be familiar with historic preservation goals. The State Historic Preservation Office (SHPO) and the local historic preservation office or commission may be able to identify consultants with experience in seismic rehabilitation of historic buildings, or be able to provide initial technical assistance on how to approach a seismic retrofit.

United States Geological Survey from 2014A simplified 2014 United States Geological Survey (USGS) seismic hazard map. Owners of certain classes of high-risk buildings in regions of high seismic activity are advised, and often required by local ordinances, to take immediate action in undertaking a comprehensive vulnerability assessment and make any necessary seismic rehabilitation measures. Owners of buildings in moderate seismic zones are advised to do further investigation of their building’s exposure to earthquake risk, identify seismic rehabilitation needs, and consider mitigation of risks primarily due to nonstructural hazards. Owners of buildings in low seismic areas are advised to consider low-cost rehabilitation measures that protect against casualties and property loss, if such measures are found to be necessary, even though the potential occurrence of an earthquake might be low.

Factors that influence how and why historic buildings are damaged in an earthquake:

  1. Depth of the earthquake and subsequent strength of earthquake waves reaching the surface
  2. Duration of the earthquake, including aftershock tremors
  3. Proximity of the building to the earthquake epicenter, although distance is not necessarily a direct relationship
  4. Building construction type, including structural systems and materials
  5. Building design, including plan and elevation configuration, overall massing, arrangement of interior spaces, and detailing of nonstructural elements
  6. Existing building condition, including maintenance level
  7. Site and soil conditions

In the process of assessing the potential seismic risk, these are crucial factors that should be considered:

  1. Type of construction and condition of the building
  2. Site seismic hazards
  3. Occupancy and use

Type of Building Construction. To a great extent, a historic building’s construction and materials determine its behavior during an earthquake. Some buildings, such as a broad class of wood-frame structures, are able to absorb substantial movements with little risk of collapse. Others, such as unreinforced masonry or adobe buildings, tend to be more susceptible to damage from shaking. If an earthquake is strong, or continues for a long time, building elements that are poorly attached or unreinforced may collapse or dislodge. Buildings of more rigid or stronger construction methods such as reinforced concrete or steel-frame buildings may also have seismic deficiencies depending on when they were constructed and whether or not they have been well-maintained over time.

A thorough assessment of the building’s existing conditions is the basis for any seismic rehabilitation. This begins by gathering any available information about the building’s original construction. Many historic buildings in earthquake zones have survived episodes of ground shaking and may even have undergone previous seismic reinforcement work. Compiling any available documentation that quantifies their proven seismic resistance or describes seismic reinforcement work or any other changes that have occurred over time is extremely useful. Some of these records may have been already compiled in previous documentation assembled to nominate the structure to the National Register of Historic Places or for a Historic Structure Report. (If not previously done, for many buildings preparing a Historic Structure Report is highly recommended; see Preservation Brief 43: The Preparation and Use of Historic Structure Reports). Early real estate or insurance maps, such as Sanborn Maps, and assessor’s records may also note building changes over time.

Original construction documents, plans and specifications, when available, and engineering drawings, in particular, which include structural layout and connection details are especially useful. When drawings documenting improvements or alterations over time are not available, building permits can also provide useful information. Historic photographs of the building under construction or before and after previous earthquakes are also invaluable. The compiled information, along with a thorough evaluation of the condition and strength of the existing building materials, will provide a sound basis for calculating the potential seismic hazards of the building and preparing a seismic retrofit plan.

Structural deformation is when several parts of a structure are pulled, pushed, or twisted in ways that harm structural stability.This image shows structural deformation due to stress concentration in structures with re-entrant corners, the inside corner where the two perpendicular exterior walls meet.

Building Configuration. The geometry and shape of a building also play a role in how a building behaves during an earthquake. Buildings with regular plans, whether they are round, square, or rectangular, have a greater resistance to damage during an earthquake because their geometry allows for equal resistance of lateral forces in all directions.

Buildings with complex and irregular plans, however, may be more prone to damage during an earthquake because of uneven strength and stiffness. For example, structures with an L,T, H, or other plan configurations with inward-facing, or re-entrant corners, have unequal resistance to stress concentrated at those corners and intersections. This is of particular concern if the buildings have flexible structural systems and/or have an irregular layout of shear walls, which may cause portions of the building to pull apart.

Similarly, the more complex and irregular buildings are in elevation, the more susceptible they are to damage, especially tall structures. Other building features such as large ground-level storefront or garage openings, or floors with columns and walls running in only one direction, are commonly known as “soft” or “weak” stories, which increase the seismic vulnerability of historic buildings.

Open first floors do not have as much lower secondary support in the event of buckling or seismic events.Double-height floors may face structural damage moreso than other floors.Top: open first floor. Bottom: double height second floor. These renderings show examples of “weak” or ”soft” story irregularities.

Building Condition. Damaged and deteriorated building materials increase the risk of serious damage during an earthquake. This condition can be the result of poor quality workmanship and materials from when the building was built, or lack of proper maintenance. Material damage and degradation due to moisture, erosion, mold, or insect infestation are typical problems resulting from poor maintenance. Well-maintained buildings, even without added reinforcement, survive better than similar buildings that have not been maintained. In unreinforced masonry buildings, deteriorated mortar joints can weaken entire walls. Regular cyclical maintenance is therefore essential.

The capacity of the structural system to resist earthquakes may also be severely reduced if previous alterations or earthquakes have weakened structural connections. Unrepaired cracks or damage from previous earthquakes can progressively weaken a building, increasing the potential for greater damage during the next earthquake. Cumulative earthquake damage can be significant; therefore, it is important to analyze the structural capacity of the building.

Over time, structural members can become loose and pose a major liability. Unreinforced masonry buildings typically have a friction-fit connection between horizontal and vertical structural members, and the shaking caused by an earthquake pulls them apart. Insufficient bearing surfaces for beams, joists, and rafters against the load-bearing walls or support columns is another important factor to consider. The resulting structural inadequacy can cause a partial or complete building collapse, depending on the severity of the earthquake and the internal wall configuration.

Evaluation of the general physical condition of the building’s interior and exterior, and identification of areas vulnerable to seismic damage, often requires testing and analysis to determine the durability and strength of materials and structure. This should be performed by a qualified engineer who is knowledgeable of historic materials and construction methods. In order to evaluate the actual strength and condition of the historic materials, selective destructive testing may be required.

Site Seismic Hazards. In addition to the shaking motion of the ground during an earthquake, there is risk of damage due to site-specific hazards, such as fault rupture; liquefaction and other soil failures; landslides; hazards from adjacent buildings, including pounding; or potential inundation from nearby dam failure or a tsunami. If such hazards exist, they should be addressed along with any needed seismic rehabilitation of the building.

Occupancy and Use. A building’s occupancy and use have a direct relationship to its seismic risk, as well as the social, economic, or environmental consequences that an earthquake may pose. From a life-safety perspective, warehouses, barns, and certain industrial buildings and structures with low human occupancy may present a lower risk compared to high-rise office buildings, theaters, and other high-occupancy buildings. Specific uses such as medical facilities, housing for persons of limited mobility, or buildings that support vital community services or utilities fall within use categories where the risk of damage or collapse during an earthquake requires special consideration. Owners of historic buildings that are being repurposed for a new use should be aware that, depending on the change, the new use may pose a higher risk to life safety and may require significant seismic reinforcement to mitigate its seismic risk. Inversely, if the change in use lowers the risk to life safety, the need for extensive seismic retrofit work may not be necessary.

Basic Maintenance and Earthquake Preparedness

Regular maintenance ensures that existing historic materials remain in good condition and are not weakened by rot, rust, decay, or other moisture problems. Without exception, historic buildings should be well maintained. An evacuation plan should also be developed. With the knowledge that an earthquake may occur at any time in the future, building owners should have emergency information and supplies on hand.

  • Check roofs, gutters, and foundations for moisture problems, and check for corrosion of metal ties at parapets and chimneys. Make repairs and keep metal painted and in good condition.
  • Inspect and keep termite and wood-boring insects away from wooden structural members.
  • Check exit steps and porches to ensure that they are tightly connected and will not collapse during an emergency exit.
  • Check masonry for deteriorating mortar, and never defer repairs. Repoint, matching the historic mortar in composition and detailing.
  • Contact utility companies for information on flexible connectors for gas and water lines and earthquake-activated gas shut-off valves. Strap oil tanks down and anchor water heaters to wall framing.
  • Collect local emergency material for reference and implement simple household or office mitigation measures, such as installing latches to keep cabinets from flying open or braces to attach tall bookcases to walls. Keep drinking water, tarpaulins, and other emergency supplies on hand.

Photo by daveynin