Design Improvements To Decrease Earthquake Damage

The following suggestions are based on field observations of damage resulting from the Northridge earthquake. They present an engineer's perspective on retail store construction and offer simple improvements which could be made to future designs to reduce typical damage.


While we cannot as yet predict when an earthquake will occur, a great deal of effort has gone into determining where earthquakes are likely to occur. Estimates are also made as to the magnitude of earthquake which might be expected from individual fault lines. This information and some assumptions regarding earthquake frequency can be used to estimate the Maximum Credible Earthquake for a given location and to arrive at a Site-Specific Seismic Coefficient. This process is sometimes called a Seismic Risk Analysis and is performed by a specialist in this area or by a professional engineer. The intent is to limit the total damage of a structure to levels which can easily be repaired (about 10% of total construction cost) following an earthquake. Risk reports typically require increases in member sizes, elimination of short term stress increases and set limits on the deflection of critical structural systems. If your store is moving into an existing mall, ask the management if they can provide you with a copy of this analysis for your reference.

While tenants generally cannot have an influence over the type of building they occupy, this does not mean that risk analysis lacks value in store design. We can preserve the intent by resorting to a more empirical approach; combining portions of risk analysis with our past experience to improve the earthquake resistance of our designs. Part of this process is understanding what we can and cannot control. To that end, the following lists are provided.

External Influences

A. Magnitude and Proximity of Earthquake

Two contributing factors over which you have no control. While the State of California and the Department of the Interior have made maps indicating where known fault lines exist and have historical records of earthquake magnitudes, this data does not as yet offer reliable guidance as to what forces might affect a particular location for a specific period of occupancy. Assumptions are required. One might combine the maximum horizontal acceleration of the largest recorded seismic event on each fault line with the perpendicular distance from the fault to generate an assumed maximum horizontal acceleration of the ground at a particular location. Given a number of fault lines, distances and spotty recording data this process becomes tedious and the results are questionable. A simpler approach is to assign maximum horizontal accelerations to large areas based on the data collected to date. The Uniform Building Code (UBC) selected this second option. The result is the Seismic Zone Map in chapter 23 (1991) or chapter 16 (1994) and a maximum acceleration of 0.4g.

Experience has shown that accelerations can and do exceed Code maximums. Your future designs should make some allowance for this fact. This does not necessarily mean a drastic revision to your store design. The strength of your systems can often be increased by using a slightly heavier gage of material or nominal increase in thickness.

B. Building Type & Rigidity

The ideal tenant space would be one in which the landlord's walls were rigid and the roof and floor were rigid and flat. Your interior walls would be designed with uniform connections top and bottom. Storefronts could be braced in two directions and would not move. Deflections (based on a calculated seismic coefficient) would be predictable and could be limited to reduce damage to adjacent components.

Unfortunately, structures do bend, deflect, flex, rack, etc. in response to an earthquake. The simple system described above is complicated by an additional degree of freedom: movement (translation) parallel with the ground surface. The amount of translation is dependent on :

1. The materials used in the Mall/building construction.

Whether from field surveys or from copies of the original design drawings, you will know the type of building you are occupying. Each material type (concrete, masonry, steel, wood) has certain properties which relate to deflection under load. The more flexible the material, the greater the effect on your interior work.

2. Your location in the larger structure relative to structural lateral-resisting elements.

If your tenant space is located in the middle of a large open floor area, your finishes will experience more damage than if you were adjacent to a wall or braced steel frame line. This is a result of the flexing of the floor or roof diaphragm above and the connection of your walls to that deflecting element.

3. Your plan orientation relative to the direction of the future earthquake.

This cannot be predicted. The Mall/building sits where it does and the future earthquake epicenter location will remain an unknown. By bracing all elements in two directions, we attempt to provide adequate resistance for forces impacting the interiors in any direction. The degree of damage sustained is directly related to the strength of the connections made.

Your future designs should make allowances for the deflection of the Mall/building structure wherever possible. The amount of anticipated deflection for most buildings is based on the floor to floor height in inches times a coefficient of 0.005 (UBC 2334, (h)2). While concrete structures tend to deflect less than this amount, you may still want to adjust your standard details for the higher result.

C. Mall Operations

One of the constraints which limits the useability of a tenant space is whether the Mall or larger building structure was damaged as a result of the earthquake. An improved store design will not be of any use if the Mall/building is closed for repairs. Consider the following scenario:

D. Mall Responsibilities

Deserving of a separate category are those items in your tenant space which belong to and are the maintenance responsibility of the landlord. This includes the hot/cold water runs, soil lines, gas lines, sprinkler pipes, smoke exhaust venting, rainwater leaders and overflows, main electrical feeds and HVAC ducting. A great deal of damage to stores in Mall structures was caused by the rupture of Mall sprinkler pipes. No matter how you modify your designs, you will still be subject to the unique weaknesses of the Mall/building design.

Your future designs should not contribute to the existing problems. Where pipes pass through your tenant walls, provide sufficient clearance all around to permit them to move independently of the wall. Use sheet metal closures if required at fire walls. Ducts which pass through walls should be provided with flex sections (or isolated as above) each side of the wall to permit the wall to flex out of plane. Hard piping such as sprinkler lines have special problems because of their pressurized state. Your mechanical consultant should be contacted for his/her recommendations if you are planning on installing new lines. For existing pipes, please ask your real estate department to investigate how the owner will reimburse you for damage if it occurs then expect it to occur.

Internal Influences/Components

A. Tenant Space Walls

The landlord's walls are intended to divide larger spaces into smaller spaces. They are designed to UBC minimum requirements for strength but are rarely checked for their deflection at/near ceiling level or for their capacity to support large amounts of merchandise and resist seismic loads. By utilizing these walls as-is in your design, you include their weaknesses. The typical result : damaged ceiling tiles and bent tee sections along the perimeter of your store and merchandise on the floor. Think about the money you will spend to make repairs and the time you will lose. Consider the following statement : Cosmetic damage is no different from actual structural damage in the mind of the post-earthquake public.

As part of your future designs, these walls should be made to comply with the seismic coefficient you are using for the remainder of your store. They should also be checked for their deflection at the elevation of your ceiling unless you elect to decouple the ceiling grid (see details). Stud bracing from just above the ceiling level to the structure may offer a compromise solution.

B. Your Walls

We assume the original building designer has made a provision in his/her calculations for the weight of your interior walls as well as the lateral load they contribute to the roof and floors. All that remains is to make proper and adequate connections top and bottom for strength and with consideration for the flexibility of the main structure. Walls should not be directly connected to the floor/roof framing above as this will lead to the rupturing of the finishes. Slip tracks should be required everywhere. The juncture between the storefront and the tenant wall creates a problem as these two elements respond differently to the earthquake force. Please consider uncoupling these elements. A mastic joint or trim piece can preserve the design intent and will minimize the kind of damage seen in the recent earthquake. The use of a boxed column section also provides a transition between these elements as long as it is not connected to the tenant wall.

Two criteria are important here : Strength & Deflection. Once you have checked your design against the yield strength of the Mall/building, the deflection at the level of the ceiling must be compared against the expected flexibility of that system. Your calculations should include the height of the wall, wall weight and the seismic contribution of any merchandise which is shelved on that wall. If you cannot uncouple the ceiling from the wall, I suggest you hold the wall deflection below a maximum of (Wall Height In Inches)/480 at the ceiling level. This may result in an increase in stud gage but rarely requires an increase in stud depth. By spending a little extra on the walls, you reduce the cosmetic damage to ceilings and other elements which make a post-earthquake store look worse than it actually is. Construction schedules are not affected by increasing the gage of a few metal studs. The added cost of materials will be returned in savings on post-earthquake repairs and losses due to downtime.

C. Ceilings

In the early 1900's, ceiling were decorative items. They did little more than hide the floor/roof framing from the public. Many were constructed with stripping on the bottoms of the framing or with black iron and wires. All were held as high as possible. Lighting was provided by pendant fixtures or by perimeter uplighting. Blowers or radiators provided heating for retail spaces.

When ceilings were adapted to conceal mechanical ducts and support small light fixtures, they moved away from the structure by some distance. This was accomplished with either solid framing or again with wires. Most ceilings were constructed with either plaster or with gypsum board. The results were quite rigid. In fact, the above ceiling space often acted as a return plenum for the new forced air ducting. Gaps around the columns provided access to this plenum and eliminated cracking of the ceilings at these locations. The joint between the ceiling and the wall was sometimes highlighted with the use of an ornate cove section - this concealed any cracking which would be expected at this intersection of elements.

T-bar ceilings were introduced to permit easy access to the ever increasing ductwork, electrical runs, etc. above, to reduce floor to floor heights and to reduce the load on the structure. They were quickly modified and offered a variety of textures which were too costly to reproduce in plaster. One disadvantage which occurred after the first earthquake test was that the ceiling grid did not hold together very well; tiles fell out, lights dropped, grids bent. As a result, newer building codes required safety wires on all light fixtures and special seismic bracing wires with compression struts at regular intervals. The problems were supposed to go away. They did not.

Field observations have lead me to believe that ceilings with bracing wires and compression posts are actually quite rigid despite the materials used. They will actually try to resist the out of plane movement of the wall. The disadvantage is they will only do so once. Chipped tiles and bent tees are the result.

Ceilings are non-structural elements. We must treat them as such. I suggest we borrow from the past by separating the edges of all T-bar ceilings from the perimeter walls and interior columns. Various ways of accomplishing this exist. Two proposed details are included for your reference. Decoupling the ceiling from the walls also makes your wall design easier as you are not strictly constrained by deflection.

For gypboard ceilings, in addition to all the duct penetrations and light locations it would be desirable to include an occasional access panel for visual inspection. This can be an aid to both the inspecting engineer and to the contractor looking for broken pipes, etc.

D. Fixtures

The interior walls should be designed to resist the seismic force/impact from all of your store fixtures. Each fixture should be firmly attached to the wall to prevent it from falling over. This has long been a recommendation of earthquake-preparedness advocates. It requires your designers to work more closely with your fixture suppliers to determine various weights and critical dimensions. This information can become part of your standard CAD details. Some initial commitment is required, but a uniform interior wall type should be possible even given a variety of fixtures.

Specialty light fixtures require additional attention. Fragile fixtures should not only be supported for vertical loads, but should be contained against vertical acceleration and cushioned against impact with rigid support points.

E. Mechanical Equipment

Closer attention needs to be paid to the connection of mechanical equipment to the structure. If you are moving into an existing space, inspect the space's package units for proper connections (vertical, lateral) to the roof. If you are installing new units, the mechanical engineer's drawings should include connection details for these forces. The general contractor should be given the responsibility of verifying the completeness of the installation prior to the departure of the mechanical subcontractor.

F. Storefront Glazing

Tempered glass properties and characteristics have been determined by manufacturer's tests for various installation conditions. Tables are provided to assist your consultants in selecting the appropriate thickness of glass based on a theoretical failure rate. My field observations have prompted some additional criteria :

1. Deflection under wind loading and under the 5 pound per square foot UBC loading (for interior walls and partitions) should be a consideration when determining glass thickness for a given vertical span. In no instance should glass be permitted to deflect more than (Span in inches)/240.

2. Right angle joints should be caulked to prevent the panes from beating against each other. Caulked joints will also provide an additional supporting edge which, as can be seen from manufacturer's data, increases the strength of the installation.

3. Isolated clips to join adjacent glass panes should be joined with a through-fastener rather than glued in place. Glued clips failed when adjacent panes moved in opposite directions due to small differences in their widths and/or orientations.

G. Stockroom Lighting

Strip fixtures with exposed fluorescent tubes are used in stockrooms and service corridors because they are inexpensive and low maintenance, but field observations suggest they are all too easily damaged during an earthquake. Failures occurred when suspended fixtures impacted against stock shelving and/or interior walls. Some support chains pulled loose, leading to bulb breakage and obstructing passage through exit corridors. More thought needs to go into the type of lighting provided. My suggestions are :

1. Always provide plexiglass covers on fluorescent fixtures. If the fixture falls, the bulb debris will remain contained inside the fixture. If an employee carrying a ladder inadvertantly hits a fixture, the bulbs are less likely to break and fall into his/her eyes. Consider the lifetime of the bulbs and the small increase in time required to remove and replace such covers versus the chance of injury.

2. If strip fixtures are a necessity, provide bracing wires to the stock room walls. The small areas of stockrooms may permit you to run three or four horizontal wires from wall to wall, intersecting fixtures at each end. Diagonal bracing wires would be more involved and would require a compression post similar to a typical suspended ceiling installation.

3. Fluorescent bulbs in individual, cord-supported and shielded fixtures would not be as fragile as strip fixtures and could provide equal lighting. The fixture would protect the bulb from impact against walls and/or shelving and the long cord could sway without the need for bracing.

Items Related To Use / Safety

A. Stock/Fixture Storage

Adequate areas and wall surface must be provided for stock and fixture storage. If areas are not provided, employees will create them. Bootlegged storage areas only lead to inefficiency in stock management and can create real structural problems if placed in the wrong location. High-density rolling shelving is one alternative assuming the structure can support the concentrated load. Your engineering consultant can tell you if this is an option. Excess fixtures should be removed from the site on the same truck that delivers new merchandise. If this is not possible, then future designs must include locations or rooms inside new stores specifically for fixtures.

B. Emergency Exit Access

The lack of defined and/or adequate storage areas for excess fixtures tempts employees to store them in any available space. Unfortunately, this usually means in the hallways and fire exit corridors. The earthquake merely completes the problem by shifting fixtures into the path of the exiting public. Panicked shoppers will not stop to move your fixtures on their way out. More than likely, people will be injured falling over these elements and you will pay through the nose. Your liability insurance may not cover you for this risk as the blockage of an exit is a building code violation. Investing ahead of time in an onsite fixture room or removing excess fixtures (see above) makes more sense.

Design Method

Following correction of the obvious inconsistencies, we may progress to the larger issue of the appropriate design strength of your interiors for future earthquakes.

Building Type vs. Strength

All materials used in present-day construction possess specific, measurable properties which allow designers to predict how they will react to imposed loads. Experimentation has demonstrated that each material also has a limited range of stresses within which it may function repeatedly. Stresses beyond this upper limit will induce either permanent deformations (yielding) or failure (rupture) of the material. We wish to determine the (Yield)/(Allowable Load) capacity for each common material as part of formulating your store's design seismic coefficient (also see Mall Operations above). The four materials which concern us are :

  1. Steel - Allowable stress = 20 k.s.i. The UBC permits a one third increase on this allowable stress when caused by wind/seismic forces. Yield stress = 36 k.s.i. for the majority of steel used in retail structures. Factor = (36)/(20 x 1.33) = 1.36.
  2. Concrete - Based on ultimate strength design and serviceability considerations, Yield occurs at 1.30 x 1.1 x (1/0.90) over normal loading. Factor = 1.59.
  3. Wood - Original tests of plywood diaphragms to failure demonstrated significant reserve capacity in such assemblies. Factors in excess of 4.0 were recorded. Experience though says that wood buildings do fail before the diaphragm capacity is reached. This may be a result of deflection of diaphragms under load and/or orthogonal effects. The connections are critical to determining the ultimate strength of the building. The UBC permits a 33 percent increase on all connections subjected to seismic loads. Designers almost never use the minimum number of fasteners and the variety of connections makes selecting a uniform factor difficult. For purposes of discussion, let us assume that each connection has a 50 percent reserve capacity. Factor = 1.50 x 1.33 = 2.0.
  4. Masonry - Allowable design stresses in masonry walls are based on fractions of the ultimate compressive strength of the block material. For bending, this factor is 0.33. The general quality of masonry construction is such that I would anticipate partial failure at one-half of the assumed design strength, so regardless of theoretical coefficients, the Factor = 1.66.

Building Type & Code Forces

Based on the equation for seismic force on building structures, V = (Z)(I)(C)(W)/(Rw).

The seismic zone (Z) is 0.40 for Zone 4, 0.30 for Zone 3, etc. The importance factor (I) is 1.0 unless very special circumstances occur. The soil type/structure period modification coefficient (C) is 2.75. The factor relating to building type (Rw) varies depending on construction. For example, a two story mall has a wood roof and metal deck floor with concrete fill. The lateral force is taken into exterior masonry walls. Referencing at Table 23-O of the UBI (B,3,b), Rw is listed as 8. The final version of the equation for a Zone 4 store is : V = (0.40)(1.0)(2.75)(W)/(8), V = 0.1375(W).

Increase From Seismic Risk Analysis

If the building was designed for forces higher than Code minimums, this factor should be included in your interior component design. In our experience, we have never been asked to design for more than 1.5 times UBC minimums.

Modifications to Standard UBC Design Requirements

Given the components in the above example (wood, masonry, concrete) we would use the smallest Factor related to materials (1.59) multiplied by the seismic coefficient (0.1375) and then by the seismic risk analysis factor (1.5) to determine the store's strength design coefficient (SSDC). SSDC = (1.59)(0.1375)(1.50) = 0.328. Comparing this with the minimum requirements of the UBC, we find that the Code requires a force coefficient of 0.30 to be applied to interior walls and partitions. The larger value should be used. While this may seem like an insignificant increase, consider a one story masonry wall building with a wood roof; SSDC = (1.66)(0.183)(1.50) = 0.46 versus 0.30.

The SSDC is then used to calculate the deflection of the interior walls including tributary weights of merchandise and/or fixtures. By comparing these with permissible amounts and/or empirical limits, uniform stud depths and gages are selected and connections are designed. The result is a tenant space which actually out-performs the larger structure and requires minimum post-earthquake repairs. Should earthquake forces exceed those which are safely resisted by the larger structure, you should be able to repair the minor damage in your space before your landlord makes his/her repairs, so you have not lost productive sales hours.


Whether you elect to use the modifications proposed above or not, you should still make the changes necessary to accommodate the dissimilar deflections of your wall and ceiling elements in future designs. Storefront glazing thicknesses should be appropriate for both the unsupported span and edge condition(s). Lighting fixtures should be selected for both efficiency and safety. Sufficient space should be provided for stock and fixture storage so exit corridors remain clear. Some commitment of time and money is required to modify your existing designs and to overcome the initial objections to change within your company. You know how much you spent during the last earthquake, can you afford not to change?

Back to McVicker Associates, Inc.