Concrete Shear Walls

The problems that are most difficult to fix are those caused by the irregular configuration of a building (e.g., abrupt changes in stiffness, soft stories, large floor openings, and

Rigid Shear Wall
Figure 6.8. (Continued)

reentrant floor corners). These cases may require the addition of vertical or horizontal rigid structural elements, as well as strengthening of existing foundations or addition of new ones.

There are several approaches to the reinforcement of existing concrete shear walls, discussed in the following sections. Increasing Wall Thickness

Wall thickness is increased by applying reinforced shotcrete to the wall surface. Shotcrete, a mixture of aggregate, cement, and water sprayed by a pneumatic gun at high velocity, is widely used for strengthening walls because it bonds well with concrete. Some prefer application by the dry-mix method (sometimes called gunite) because the slump and stiffness can be better controlled by the nozzle operator and because gunite is applied at higher nozzle velocities, promoting superior bonding.

Concrete shear walls that lack ductility may fail by crushing of their boundary elements, horizontal sliding along construction joints due to shear, or diagonal cracking caused by combined flexure and shear. Among the most common areas of damage are the coupling beams. These can be repaired by through-bolted side plates extending onto the

Concrete Shear Walls With Openings
Figure 6.9. Diaphagm strengthening of an existing gymnasium roof with horizontal bracing.

en en

Short Pier Shear Wall
(N) 1/2°R.W/2-3/4' 0 A325SC bolts
Shear Key Dimensions

for size Detail B

Figure 6.9. (Continued)

for size Detail B

Figure 6.9. (Continued)

faces of the walls. Short and rigid piers between walls openings also tend to attract an inordinate amount of seismic loading and are therefore prone to damage.

The key to shotcreting walls lies in the surface preparation of the wall because existing concrete may be counted as part of the strengthened wall. All loose and cracked concrete must be removed from the existing wall, and its surface cleaned and roughened by sandblasting or other means. To assure composite action, the overlay is mechanically connected to the wall by closely spaced shear dowels. In addition, steel reinforcement placed in shotcrete is developed at the ends by grouted-in dowels or by continuation into

(E) Roof metal deck


Detail D

Detail D

Figure 6.9. (Continued)

an adjacent overlay space. This involves drilling through the perimeter beams or columns, filling the drilled openings with epoxy, and splicing the bars with those in the adjoining overlay areas. If the existing wall openings must be filled, the infill should be connected to the roughened edges of the opening with perimeter dowels set in epoxy.

When interior shotcreting is used, attention must be directed toward stabilizing the exterior walls and any exterior ornamental elements of the structure. These may have to be tied back into the new shotcrete by drilled-in dowels set at regular intervals. Dowels placed in exterior elements that are exposed to moisture should be given a measure of corrosion protection, such as galvanizing.

In cases where it is desirable not to increase the wall size, the outer course of bricks can be removed and replaced with shotcrete. The same can be done with interior shotcreting, except that any members framing into the wall may have to be shored during this operation. The added bonus of this approach is that the vertical load on the existing wall foundations changes very little, and they may not require the otherwise necessary enlargement.

Figure 6.9. (Continued) Increasing Shear Strength of Wall

Increasing the shear strength of the web of a shear wall by casting additional reinforced concrete adjacent to the wall web may be an effective rehabilitation measure. The new concrete should be at least 4-in. thick, contain horizontal and vertical reinforcement, and be properly bonded to the existing web of the shear wall. The use of composite fiber sheets, epoxied to the concrete surface, is another method of increasing the shear capacity of a shear wall. The use of confinement jackets as a rehabilitation measure for wall boundaries may also be effective in increasing both the shear capacity and deformation capacity of coupling beams and columns supporting discontinuous shear walls. Infilling Between Columns

Where a discontinuous shear wall is supported on columns that lack either sufficient strength or deformation capacity, making the wall continuous by infilling the opening between these columns may be an effective rehabilitation measure. The infill and existing columns should be designed to satisfy all the requirements for new wall construction, including any strengthening of the existing columns required by adding a composite fiber jacket or a concrete or steel jacket for strength and increased confinement. The opening below a discontinuous shear wall may also be infilled with steel bracing. The bracing members should be sized to satisfy all design requirements for new construction and the columns should be strengthened with a steel or a concrete jacket. All of these rehabilitation measures require an evaluation of the wall foundation, diaphragms, and connections between existing structural elements and any elements added for rehabilitation purposes.

Adding new shear walls or braced frames conforming to current code detailing provisions is among the most common steps taken to strengthen the lateral-load-resisting systems of buildings. The new walls and frames can either: 1) complement the existing elements; or 2) be designed as the sole means of providing vertical rigidity in the building. In the first case, analysis of comparable rigidities must be done to determine what percentage of the total lateral loading the new construction will carry. In the second case, the existing rigid elements that are now considered to be nonstructural must be checked for inelastic deformation compatibility. In any case, new foundations must be provided under the new elements and dowels placed around them for proper transfer of loads.

A common complication of adding shear walls and braced frames is that they tend to interfere with the building layout, circulation, or fenestration. Quite often, shear walls with openings or braced frames of unusual configurations may be needed to accommodate window or door openings. In some rare cases exterior buttresses or counterforts may be considered.

Adding braced frames, usually of structural steel, can be economical in buildings where the existing steel framing will not require strengthening to accommodate the bracing and where the existing framing is readily accessible. Addition of Boundary Elements

Addition of boundary members may be an effective measure in strengthening shear walls or wall segments that have insufficient flexural strength. These members may be either cast-in-place reinforced concrete elements or steel sections. In both cases, proper connections should be made between the existing wall and the added members. The shear capacity of the rehabilitated wall should be re-evaluated. Addition of Confinement Jackets

Increasing the confinement at the wall boundaries with the addition of a steel or reinforced concrete jacket may be effective in improving the flexural deformation capacity of a shear wall. The minimum thickness for a concrete jacket should be 3 in. A composite fiber jacket may be used to improve the confinement of concrete in compression. Repair of Cracked Coupling Beams

These can be repaired by adding side plates extending on the faces of the walls. In this procedure, the plates are attached with both epoxy adhesive and anchor bolts. The plates may be attached to only one face of the wall or can be placed at both faces for extra strength, with the opposite plates through-bolted together. Another possibility for improving coupling beams is by using composite fiber wrapping. This method is least intrusive because the wrapping and the epoxy combined are only 0.25-in. thick. Adding New Walls

Adding new shear walls at a few strategic locations can be a very cost-effective approach to a seismic retrofit. The new wall is connected to the adjoining frame by drilled-in dowels. Its foundations are similarly doweled into the existing column footings. To accommodate wall shrinkage, the wall can stop short some distance—2 in., for example—from the existing concrete at the top. The space can be filled later with nonshrink grout. Precast Concrete Shear Walls

Precast concrete shear wall systems may suffer from some of the same deficiencies as cast-in-place walls. These may include inadequate flexural capacity, inadequate shear capacity with respect to flexural capacity, lack of confinement at wall boundaries, and inadequate splice lengths for longitudinal reinforcement in wall boundaries. Deficiencies unique to precast wall construction are inadequate connections between panels, to the foundation, and to floor or roof diaphragms.

The rehabilitation measures previously described for concrete buildings may also be effective in rehabilitating precast concrete shear walls. In addition, the following rehabilitation measures may be effective:

• Enhancement of connections between adjacent or intersecting precast wall panels. Mechanical connectors such as steel shapes and various types of drilled-in anchors, cast-in-plane strengthening methods, or a combination of the two may be effective in strengthening connections between precast panels. Cast-in-place strengthening methods include exposing the reinforcing steel at the edges of adjacent panels, adding vertical and transverse reinforcement, and placing new concrete.

• Enhancement of connections between precast wall panels and foundations. Increasing the shear capacity of the wall panel-to-foundation connection by using supplemental mechanical connectors or a cast-in place overlay with new dowels into the foundation may be effective rehabilitation measures. Increasing the overturning moment capacity of the panel-to-foundation connection by using drilled-in dowels within a new cast-in-place connection at the edges of the panel is another effective rehabilitation measure. Adding connections to adjacent panels is also an effective rehabilitation measure, eliminating some of the forces transmitted through the panel-to-foundation connection.

6.5.3. Reinforcing of Steel-Braced Frames

Reinforcement of existing braced frames is relatively straightforward and is often preferable to adding new ones. The work includes adding cover plates, angles, or similar shapes and new welded or bolted connections. For existing bolted connections of the bearing type, new welds can be designed to carry the entire load, or the existing fasteners can be removed and replaced with new, stronger ones. When welded reinforcement is contemplated, it is wise to check the existing steel for weldability, unless some other welding to that steel is already in place.

6.5.4. Infilling of Moment Frames

In many cases, the existing concrete or steel skeleton is stiffened by filling in the space between the beams and columns with masonry or cast-in-place concrete. These infill walls can be a cost-effective method of increasing the lateral strength and rigidity of the building.

Designers should avoid counting on some of the infill walls in structural analysis but not on others, because the stiffness of the frames filled with this nonstructural masonry will increase, whether the designers realize this fact or not. In an earthquake, these panels attract large lateral forces and are damaged, or the perimeter columns, beams, and their connections fail. When a frame, however well designed, is filled with rigid material, however brittle and weak, the fundamental behavior of this structural element is changed from that of a frame to that of a shear wall.

Rehabilitation measures commonly used for concrete frames with masonry infills may also be effective in rehabilitating concrete frames with concrete infills. Additionally, application of shotcrete to the face of an existing wall to increase the thickness and shear strength may be effective. For this purpose, the face of the existing wall should be roughened, a mat of reinforcing steel doweled into the existing structure, and shotcreate applied to the desired thickness.

6.5.5. Reinforced Concrete Moment Frames

Earthquake damage sometimes results in sheared-off columns that formerly were parts of a frame. Typically, the concrete cover is spalled, column bars buckled, and concrete inside broken up. Most problems in concrete frames involve bar splices and failures of beam-column joints that lack confinement and in which reinforcement is stopped prematurely.

Many old buildings with flat-slab and flat-plate floor systems, even those constructed after 1973 (and presumably reflecting the post-San Fernando earthquake code changes), are vulnerable to earthquakes.

Methods available for strengthening traditional concrete frames include encasing the beam-column joints in steel or high-strength fiber jackets. One such design uses jackets consisting of four U-shaped corrugated-metal parts, two around the beam and two around the column. The column jackets are bolted to the end of the beam, the pieces are welded together, and the space between the jackets and the frame is filled with grout.

Frame joints damaged during earthquakes can be repaired with epoxy injection, and badly fractured concrete can be removed and replaced. To minimize shrinkage, the replacement concrete should be made with shrinkage-compensating (type K) cement, or should utilize a shrinkage-reducing admixture. Frame members that have been pushed out of alignment during an earthquake should be jacked back into the proper position before repair. Damaged columns can also be strengthened with fiber-reinforced plastic wraps or other methods of exterior concrete confinement. This is common practice for seismic strengthening of building and bridge columns in California. Another structural issue that requires consideration is the transfer of load from the floor diaphragms to the frames and walls. This may require new drag struts. These elements can be added by attaching new concrete or structural steel sections to the underside of existing floors. They are typically placed against cleaned and roughened concrete surfaces and anchored to the floors and to frames by drilled-in dowels or through-bolts.

Connections between new and existing materials should be designed to transfer the forces anticipated for the design load combinations. Where the existing concrete frame columns and beams act as boundary elements and collectors for the new shear wall or braced frame, these should be checked for adequacy, considering strength, reinforcement development, and deformability. Diaphragms, including drag struts and collectors, should be evaluated and rehabilitated to ensure a complete load path to the new shear wall or braced frame element, if necessary.

Another method of seismic rehabilitation is to jacket existing beams, columns, or joints with new reinforced concrete, steel, or fiber-wrap overlays. The new materials should be designed and constructed to act compositely with the existing concrete. Where reinforced concrete jackets are used, the design should provide detailing to enhance ductility and the jackets should be designed to provide increased connection strength and improved continuity between adjacent components.

Post-tensioning existing beams, columns, or joints using external post-tensioned reinforcement is an effective strategy of seismic rehabilitation. Post-tensioned reinforcement should be unbounded within a distance equal twice the effective depth from sections where inelastic action is expected. Anchors should be located away from regions where inelastic action is anticipated, and be designed considering possible force variations due to earthquake loading.

6.5.6. Steel Moment Frames

The following measures are effective in rehabilitating existing steel moment frames:

1. Adding steel braces to one or more bays of each story to form concentric or eccentric braced frames to increase the stiffness of the frames. The location of added braces should be selected so as not to increase torsion in the system.

2. Adding ductile concrete shear walls or infill walls to one or more bays of each story to increase the stiffness and strength of the structure. The location of added walls should be selected so as not to increase torsion in the system.

3. Attaching new steel frames to the exterior of the building. The rehabilitated structure should be checked for the effects of the change in the distribution of stiffness, the seismic load path, and the connections between the new and existing frames. The rehabilitation scheme of attaching new steel frames to the exterior of the building has been used in the past and has been shown to be very effective under certain conditions. This rehabilitation approach may be structurally efficient, but it changes the architectural appearance of the building. Its advantage is that rehabilitation may take place without disrupting use of building.

4. Adding energy dissipation devices.

5. Increasing the strength and stiffness of existing frames by welding steel plates or shapes to selected members.

6. Reinforcing moment-resisting connections to force plastic hinge locations in the beam away from the joint region. This reduces the stresses in the welded connection, thereby reducing the probability of brittle fractures. This scheme is not recommended if the full-penetration connection of the existing structure does not use weld material of sufficient toughness to avoid fracture at stresses lower than yield or when strain hardening at the new hinge location produces larger stresses than existing at the weld. Rehabilitation measures to reinforce selected moment-resisting connections may consist of providing horizontal cover plates, vertical stiffeners, or haunches. In regions of high seismicity, pre-Northridge earthquake welded moment connections have typically been found to need strengthening. The upgraded connection must be not only strong enough to resist the stresses resulting from gravity and seismic loading, but also flexible enough to have plastic rotation capacity of at least 0.025 to 0.03 radians.

Repairing connections usually involves, in addition to structural work, removal of wall and ceiling finishes and some disruption of operations, even when the repair is done after working hours. Repair costs can exceed $20,000 per connection (2002 dollars). Further information on seismic upgrade of pre-Northridge welded moment connections is given in the AISC Design Guide 12, Modification of Existing Welded Steel Moment Frames for Seismic Resistance. The guide provides information on three designs: reduced beam section, welded haunch, and bolted bracket. In addition to the technical discussion, it also covers practical implementation issues such as reducing tenant disruption in occupied buildings and safety issues.

6.5.7. Open Storefront

The deficiency in a building with an open storefront is the lack of a vertical line of resistance along one or two sides of a building. This results in a lateral system that is excessively soft at one end of the building, causing significant torsional response and potential instability.

The most effective method of correcting this deficiency is to install a new stiff vertical element in the line of the open-front side or sides. If the open-front appearance is desired, the steel frames may be located directly behind the storefront windows. Shear walls may also be used to provide adequate strength. In both cases collectors are required to adequately distribute the loads from the diaphragm into the vertical lateral-load-resisting element. Adequate anchorage of vertical elements into the foundation is also required to resist overturning forces. Steel moment frames instead of brace frames can also be utilized to provide adequate strength, provided that inelastic deformations of the frame under severe seismic loads are carefully considered to ensure that displacements are controlled. Common methods for upgrading buildings with open storefronts are shown in Fig. 6.10.

6.5.8. Clerestory

A clerestory, typically designed to produce an open airy feeling, can result in significant discontinuity in a horizontal diaphragm. A common method of correcting the diaphragm discontinuity is to add a horizontal steel truss. Steel members can be designed to transfer diaphragm shears while minimizing the visual obstruction of the clerestory.

An alternate approach is to reduce the demands on the diaphragm through the addition of new vertical lateral-force-resisting elements such as shear walls or braced frames.

6.5.9. Shallow Foundations

The following rehabilitation measures may be considered for shallow foundations:

1. Enlarging the existing footing to resist the design loads. Care must be taken to provide adequate shear and moment transfer capacity across the joint between the existing footing and the additions.

Figure 6.10. Common methods for upgrading buildings with open storefronts.

2. Underpinning the existing footing, removing of unsuitable soil underneath and replacing it with concrete, soil cement, or another suitable material. Underpinning should be staged in small increments to prevent endangering the stability of the structure. This technique may be used to enlarge an existing footing or to extend it to a more competent soil stratum.

3. Providing tension hold-downs to resist uplift. Tension ties consisting of soil and rock anchors with or without prestress may be drilled and grouted into competent soils and anchored in the existing footing. Piles or drilled piers may also be effective in providing tension hold-downs for existing footings.

4. Increasing the effective depth of the existing footing by placing new concrete to increase shear and moment capacity. The new concrete must be adequately doweled or otherwise connected so that it is integral with the existing footing. New horizontal reinforcement should be provided, if required, to resist increased moments.

5. Increasing the effective depth of a concrete mat foundation with a reinforced concrete overlay. This method involves placing an integral topping slab over the existing mat to increase shear and moment capacity.

6. Providing pile supports for concrete footings or mat foundations. Adding new piles may be effective in providing support for existing concrete footing or mat foundations, provided the pile locations and spacing are designed to avoid overstressing the existing foundations.

7. Changing the building structural characteristics to reduce the demand on the existing elements. This may be accomplished by removing mass or height from the building or adding other elements such as energy dissipation devices to reduce the load transfer at the base. New shear walls or braces may be provided to reduce the demand on foundations.

8. Adding new grade beams to tie existing footings together when soil conditions are poor. This method is useful for providing fixity to column bases, and to distribute lateral loads between individual footings, pile caps, or foundation walls.

9. Grouting techniques to improve existing soil.

6.5.10. Rehabilitation Measures for Deep Foundations

The following rehabilitation measures may be considered for deep foundations:

1. Providing additional piles or piers to increase the load bearing capacity of the existing foundations.

2. Increasing the effective depth of a pile cap by adding concrete and reinforcement to its top. This method is effective in increasing its shear and moment capacity, provided the interface is designed to transfer loads between the existing and new materials.

3. Improving the soil adjacent to an existing pile cap by injection-grouting.

4. Increasing the passive pressure bearing area of a pile cap by addition of new reinforced concrete extensions.

5. Changing the building system to reduce the demands on the existing elements by adding new lateral-load-resisting elements.

6. Adding batter piles or piers to the existing pile or pier foundation to increase resistance to lateral loads. It should be noted that batter piles have performed poorly in recent earthquakes when liquefiable soils were present. This is especially important to consider near wharf structures and in areas with a high water table.

7. Increasing tension tie capacity from a pile or pier to the superstructure.

6.5.11. Nonstructural Elements Nonload-Bearing Walls

The performance of buildings with nonstructural walls that adversely affect the seismic response of a building may be improved by removing and replacing them with walls constructed of relatively flexible materials such as gypsum board sheathing or modifying the wall connections so that they will not resist lateral loads. Removal and replacement of existing hollow clay tile, concrete, or brick masonry partitions is the preferred method of addressing the inadequate out-of-plane capacity of nonstructural partitions. Alternatively, steel strongbacks can provide the out-of-plane support. Steel members are installed at regular intervals and secured to the masonry with drilled and grouted anchors. The masonry spans between the steel members, which span either vertically between floor diaphragms or horizontally between columns. A third method for mitigating masonry walls with inadequate out-of-plane capacity is to provide a structural overlay. The overlay may be constructed of plaster with welded wire mesh reinforcement or concrete with reinforcing steel or welded wire mesh. This approach is used at times merely to provide containment of the masonry. Nonstructural masonry walls are frequently used as firewalls around means of egress. Egress walls with deficient out-of-plane capacity can fail, resulting in rubble blocking the egress. Containment of the masonry with a plaster or concrete overlay can maintain egress, although the walls may need to be replaced following a major seismic event. Precast Concrete Cladding

Precast concrete cladding panels with rigid connections may not have the flexibility or ductility to accommodate large building deformations. Failure of the connection may result in heavy panels falling away from the building. Complete correction of this deficiency is likely to be costly, since numerous panel connections would need to be modified to accommodate anticipated building drifts. This may require removal and reinstallation or replacement of the panels. A more economical solution is to install redundant flexible/ ductile connections that will keep the panels from falling, should the existing connections fail.

Improper design or installation of precast concrete cladding may also be more than just a connection problem. The cladding may act as an unintended lateral-load-resisting element, should the connections be rigid or insufficient gaps be present between panels. Correcting this deficiency can be accomplished by installing occasional seismic joints in the panels to minimize their stiffness or by stiffening the existing lateral-force-resisting system.

If an entirely new precast cladding system is installed, the connections should be designed to

• Carry gravity loads of precast panels.

• Transfer the in-plane and out-of-plane inertia forces of the panels into the building.

• Isolate the panels from the inelastic drift likely to be experineced by the building in a large earthquake. Stone or Masonry Veneers

Stone or masonry veneers may become falling hazards unless their anchorage can accommodate the inelastic deformation of the building. Removal and replacement by veneer with adequate anchorage is one option. A second option is to decrease the deformation of the supporting wall by adding stiffness to the structure. Building ornamentation

Building ornamentation such as parapets, cornices, signs, and other appendages are another potential falling hazard during strong ground shaking. Unreinforced masonry parapets with heights greater than 11/2 times their width are particularly vulnerable to damage. Parapets are commonly retrofit by providing bracing back to the roof framing.

Cornices and other stone or masonry appendages may be retrofitted by installing drilled and grouted anchors at regular intervals. Sometimes they may be replaced with a lightweight substitute material such as plastic, fiberglass, or metal. Acoustical Ceiling

Unbraced suspended acoustical tile ceilings are significantly more flexible than the floors or roofs to which they are attached. The ceilings sway independently from the floor or roof, typically resulting in their connections being broken. This deficiency can be reduced by stiffening the suspended ceiling system with diagonal wires between the ceiling grid and the structural floor or roof members. Vertical compression struts are also required at the location of the diagonal wires to resist the upward component of force caused by the lateral loads. Current code standards can be used for the upgrade of existing ceiling systems.


This standard endorses the use of performance-based design solutions for seismic rehabilitation of buildings. The chosen performance of the building may vary from preventing collapse to a near-perfect building that would survive an expected earthquake without a scratch. The standard allows owners to select their desired performance level and permits designers to choose their own approaches to achieve the desired results rather than strictly adhering to the prescriptive requirements of codes. Instead of dictating how to achieve a given design goal, performance-based design emphasizes the goals that must be met and sets the criteria for acceptance. This way, engineers are free to innovate without running afoul of specific code provisions, within certain limits.

The FEMA documents outline criteria and methods for ensuring the desired performance of buildings at various performance levels selected by the owners with input from their design professionals. The guidelines allow owners to select a level of seismic upgrade that not only protects lives, a goal of all building codes, but also protects their investment.

FEMA 356 is a radical departure from current practice in that it seeks to provide the structural engineering profession with tools to explicitly, rather than implicitly, design for multiple, specifically defined, levels of performance. These performance levels are defined in terms of specifically limiting damage states, against which a structure's performance can be objectively measured. Recommendations are developed as to which performance levels should be attained by buildings of different occupancies and use. This tiered specification of performance levels at predetermined earthquake hazard levels becomes the design performance objective and a basis for design. It recognizes the importance of the performance of all the various component systems to the overall building performance and defines a uniform methodology of design to obtain the desired performance.

6.6.1. Overview of Performance Levels

FEMA 356 sets forth a menu of four rehabilitation objectives associated with four earthquake hazard levels. The rehabilitation objectives are

• Operational performance.

• Immediate occupancy performance.

• Life safety performance.

• Collapse prevention performance.

Each of these performance levels is associated with defined levels of damage to structural, architectural, mechanical, and electrical building components as well as tenant furnishings. The designer is referred to FEMA Tables C1.3-C1.7 for an overview of where each performance level falls within the overall spectrum of possible damage states. From these tables, the designer may infer, for example, a building designed for top-of-the-line performance using higher earthquake hazard levels is likely to come out scratch-free, delivering performance well above the code minimum for life safety level. On the other hand, much less is expected of a building rehabilitated to a collapse prevention performance level. It is deemed to have fulfilled its obligations if it remains standing during and after a large earthquake: Any other damage or loss is acceptable.

The four levels of earthquake levels hazard recognized in the development of design performance objectives are

• Frequent earthquakes, having a 50% chance of exceedence in 30 years (43-year mean return period).

• Occasional earthquakes, having a 50% chance of exceedence in 50 years (72-year mean return period).

• Rare earthquakes, having a 10% change of exceedence in 50 years (475-year mean return period). Also called basic safety earthquake (BSE-1) and design basis earthquake (DBE).

• Very rare earthquakes, having a 10% chance of exceedence in 100 years (950-year return period). Also called basic safety earthquake (BSE-2) and maximum considered earthquake (MCE).

In order to execute a performance-based design, a series of design parameters and acceptance criteria are given for each performance level for the various structural and nonstructural components. Design response parameters are defined at an element level in terms of element forces, interstory drifts, and plastic rotations. These can be derived from a structural analysis of building response to a particular design earthquake. Acceptance criteria are the limiting values for design parameters in order to attain a given performance level. For example, if interstory drift ratio is a design parameter used for a certain class of building, acceptance criteria would be the drift ratios defined for each performance level. Typical drift ratios normally considered in design are 0.020 for the near collapse level, 0.015 for the life safety level, 0.01 for the operational level, and 0.005 for the fully operational level. A wide variety of potential design parameters may need to be defined including deformation, strength, and energy-based parameters. The purpose of FEMA 356 is to provide a consensus-backed, professionally accepted, nationally applicable, seismic rehabilitation standard. It can be used as a tool by design professionals, a reference document by building regulatory officials, and a foundation for the future development and implementation of building code provisions and standards related specifically to existing buildings. The absence of such a standard has been the primary barrier to widespread seismic upgrading of buildings in the United States.

In new buildings, the structural system can be controlled to fit a set of preconditions or a configuration to satisfy the design objectives prescribed by building codes. The degree of nonlinear behavior can be designed to be consistent throughout the structural system, allowing a single seismic reduction factor, R, to be used for the entire building.

Experience in seismic design over the past 100 years has shown that buildings designed to resist ground shaking from an earthquake with a 10% chance of exceedence in 50 years, at a life safety level of performance, have been able to resist the strongest earthquake without collapse. This experience has given structural engineers enough confidence to design new structures in which ductile details are specified, properties of materials used in construction are controlled, and stringent requirements of testing and inspection are specified.

Assessing the seismic vulnerability of existing buildings is an entirely different problem. This is because, for existing buildings, structural details and the properties of materials must be confirmed or assumed from available information augmented by testing and inspection. Conservative assumptions consistent with the quality of the information available must be made prior to seismic evaluation. The engineer has no control over the structural system or its configuration. The existing building may not fit prescriptive details to permit code-type analysis. Nonlinear behavior of the components of the structural system will probably not be consistent. Thus, the properties of each component must be separately studied. Because of the inconsistent levels of reserve capacity in existing buildings and the differences between the 10% in a 50-year earthquake and the maximum considered earthquake (MCE) in various regions of the country, it is inappropriate that rehabilitated buildings be designed to resist a single level of earthquake shaking. Therefore, using an entirely different approach, FEMA 356 provides a basis of rehabilitation designs for a variety of structural performance levels, ranging from enhanced performance to collapse prevention. It emphasizes the idea that seismic rehabilitation should be directed to controlling deformation in order to minimize damage. Use of all existing seismic resistance is permitted in the evaluation. Acceptance criteria tailored to recognize the deformation capacity of all existing as well as enhanced or new components are provided.

The seismic loads used in the evaluation are based on a suite of USGS-developed acceleration maps including four key maps. Two of these are BSE-1 (basic safety earth-quake-1) maps of acceleration response spectra having a 10% probability of exceedence in 50 years. The other two are BSE-2 (basic safety earthquake-2) maps of acceleration response spectra for the MCE—modified 2% probability of exceedence in 50-year maps: Both BSE-1 and BSE-2 maps are given for 0.2-second-period (short period) and 1-second-period buildings.

6.6.2. Permitted Design Methods

Two methods are permitted by FEMA 356, a simplified method and a systematic method. The simplified approach is for the rehabilitation design of small buildings of regular configuration, and is intended to fulfill limited objectives. Partial rehabilitation measures that seek to eliminate high-risk building deficiencies such as exterior falling hazards are included in the technique.

The systematic rehabilitation method discussed at length in this section is applicable to any building. It is a component- and element-based design. In this method, global seismic response of the building is sought with unreduced seismic loads (that is, with a global ^-factor of unity). In the seismic evaluation, all components and seismic elements are considered with their individual deformation and force-resisting characteristics. It is a deformation-based design with the explicit rather than tacit acknowledgment that seismic elements and components behave in a nonlinear manner.

Any of the following analysis procedures may be used in the rehabilitation study and upgrade design:

• Linear static procedure (LSP). This procedure replaces the equivalent lateral force procedure included in most seismic design codes. It incorporates techniques for considering the nonlinear response of individual seismic elements. The distribution of forces is similar to equivalent lateral force procedures for new buildings.

• Linear dynamic procedure (LDP). In this method, the modeling and acceptance criteria are similar to those of LSP. However, calculations are carried out using modal spectra analysis or time history analysis using response spectra or time-history records that are not modified to account for inelastic response for distribution of forces.

• Nonlinear static procedure (NSP). This method is frequently referred to as a pushover analysis. It has been in use for some time without specific guidance from building codes and standards regarding modeling assumptions and acceptance criteria. This is now alleviated to some extent because FEMA 356 sets forth specific procedures.

• Nonlinear dynamic procedure (NDP). The modeling approaches and acceptance criteria for this method are similar to those of NSP. It differs from NSP in that response calculations are made using inelastic time history dynamic analysis to determine distribution of forces and corresponding internal forces and system displacements. Peer review by an independent engineer with experience in seismic design and nonlinear procedures is recommended because this method requires assumptions that are not included in FEMA 356.

6.6.3. Systematic Rehabilitation

The process of arriving at a systematic rehabilitation design includes the following steps:

1. Determination of seismic ground motions.

2. Determination of as-built conditions.

3. Classification of structural components into primary and secondary components.

4. Setting up of analytical models and determination of design forces.

5. Ultimate load combinations; combined gravity and seismic demand.

6. Component capacity calculations, QCE and QCL.

7. Capacity versus demand comparisons.

8. Development of seismic strengthening strategies.

First, the seismic hazard for the site is established by determining the probable ground shaking (spectral acceleration) from either seismic hazard maps or a site-specific investigation. Other site hazards such as liquefaction, lateral spreading, and land sliding are determined from site reconnaissance, existing documentation, or a subsurface investigation.

The desired performance level is then established. This requires close communication with the client, using damage descriptions for each performance level as a tool to get ideas across. The damage descriptions associated with each performance level can be used to inform and assist the client to make a decision of the preferred performance level.

Next, an analysis is performed after classifying building components as either primary or secondary. This distinction is required because the acceptance criteria are different for each type of component. The primary components are parts of the building's lateral-force-resisting system, whereas the secondary components are those not required for lateral-force resistance, although they may actually resist some lateral forces. The analysis is performed by considering general requirements such as PA effects, torsion, overturning, continuity, integrity of elements, and building separations. Cracked properties as given in Table 6.1 are used for concrete buildings.

New or modified components are evaluated using the same standards as existing components, and the designs are completed by comparing capacities with demands for

Seismic Rehabilitation of Existing Buildings TABLE 6.1 Effective Stiffness Values

Component Flexural rigidity3 Shear rigidity Axial rigidity







Columns with compression due to design




gravity loads > 0.5Agfc

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