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SEISMIC REHABILITATION OF EXISTING BUILDINGS

DURGESH C. RAI Assistant Professor

Department of Civil Engineering, Indian Institute of Technology Kanpur

Kanpur 208 016

INTRODUCTION

In earthquake engineering many terms are used to describe the construction activities whose primary aim is to reduce the vulnerability of the structures in future earthquakes. The terms such as “repair”, “retrofit”, “upgrading” and “strengthening” can be collectively and comprehensively be described by the term “rehabilitation”. A large number of structures in earthquake-prone regions need seismic rehabilitation due to a number of reasons, such as, changes in codes due to advances in knowledge or earthquake damage. Earthquake damaged structures may need strengthening along with the repair of damaged parts (post-earthquake rehabilitation) so that their improved seismic performance may meet expected performance of the current code. Also, structures not conforming to the current code may also require rehabilitation (pre-earthquake rehabilitation) so that they can satisfy the provisions of the current code

Many techniques have been developed for seismic rehabilitation which vary from the conventional infilling, bracing and jacketing for improved strength and ductility to more modern techniques of seismic isolation and supplemental damping (Sugano 1996). This article will primarily concern with those techniques that have been typically used for reinforced concrete (RC) and unreinforced masonry (URM) structures.

SEISMIC REHABILITATION STRATEGY & MEASURES

Strategies used in seismic rehabilitation efforts can be broadly grouped in three categories as follows:

1. Recovering original performance, for which the damaged or deteriorated portions of the structures can be simply repaired or replaced with new element or material.

2. Upgrading original structural performance, which means that the structures will be strengthened to reduce excessive displacement, to enhance its lateral strength, to reduce irregularity due to asymmetric distribution of mass and stiffness and to use energy dissipation devices to enhance the ability of structure to dissipate seismic energy.

3. Reducing seismic response of structure by either isolating the structure from the ground or reduce the structure’s masses. It can be very effective but restricted to only important and historic structures for it is very expensive.

Strategy 1 maintains that an original seismic-deficient structure, continues to remain vulnerable to earthquakes even after the rehabilitation where as strategies 2 and 3 aim

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to significantly improve the seismic resistance of a seismic-deficient structure for improved performance in future earthquakes.

REPAIR METHODS

Cosmetic repairs only improve the visual appearance of component damage and may restore non-structural properties (weather protection) but any structural benefit is negligible. Structural repairs intend to restore structural properties (FEMA 1999).

Cosmetic patching is to use surface coating on concrete and masonry wall to conceal the surface appearance of cracks, primarily for aesthetic reasons. It can provide a barrier against moisture. Various materials such as paints, organic polymers, sealants, and cement plaster can be used. Cracks caused by earthquake are dormant and do not move.

Repointing is process of removing deteriorated mortar from masonry joints and replacing it with new mortar. It restores visual and physical integrity of the masonry. Careful design of mortar is required for historic structures to match the colour, compressive stiffness, thermal properties, etc. The joint is prepared by removing loose/disintegrated materials up to depth of 12 to 25 mm.

Crack injection is to applying a structural binding agent into a crack for the purpose of filling the crack and adhering to the substrate material. A low-viscosity grade epoxy for widths less than 3 mm and medium grade for greater than 3 mm and epoxy gel for 6 mm are typically used. Crack widths as small as 0.05 mm can be filled with epoxy, however, widths up to 0.3 mm can be tolerated. Epoxy resins are not recommended for injection into masonry.

STRENGTHENING TECHNIQUES

Lateral strength and ductility are the most essential factors governing the seismic performance of the structure and an effective seismic strengthening schemes aims to improve (a) the ultimate strength of overall structure, (2) inelastic deformability of the system, i.e., ductility and (3) a proper combination of these two features (Figure 1).

An increase in the combination of strength and ductility is generally required to meet life-safety performance level, which requires collapse prevention, maintenance of exit paths and prevention of falling hazards. Providing increased strength is the most promising approach for low-to-medium-rise buildings. Even when sufficient ductility is added, adequate strength is required to reduce inelastic deformation, hence, damage to structural/non-structural components.

Many techniques have been developed for each strategy. For example, RC infilled walls or steel braced frame enhances lateral strength of existing frames, whereas jacketing of structural elements such as beams and columns with steel straps and ductile steel braces enhances strength as well as ductility.

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Figure 1. Typical strengthening methods (Sugano 1996)

Determination of a suitable strengthening approach requires careful consideration of the expected performance of the structure in a future earthquake. Selection of a particular seismic strengthening scheme depends on several factors:

1. The scheme must correct known seismic deficiencies of the system and new strengthening elements should be compatible with existing system

2. It should be functionally and sometimes aesthetically compatible and complimentary to the existing building.

3. It should meet the expected performance goal whether they are life-safety for residential units or limited damage for quick return for full operation for an essential facility.

4. It should minimize the disruption to occupants, if it has to be occupied during the strengthening work. In occupied buildings, strengthening work done from the exterior on the building’s perimeter are the most satisfactory.

In following sections some of the most common strengthening schemes will be discussed in detail with respect to application areas, advantages, disadvantages and special design issues.

Adding New Shear Walls

New shear walls can be added to strengthen RC frames, especially open storeys. These new shear walls will be complete with boundary elements and foundation. This intervention greatly adds to strength and stiffness to framed structures. The primary

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disadvantages include considerable increase in mass of the existing structure and expensive and cumbersome new footings. They can be a major problem on soft soils and in pile-supported structures.

Location of new shear walls should be chosen such that they (a) align full height of the building, (b) minimize torsion and (c) can be easily tied with existing frame. Furthermore, they should be able to maximize the dead weight which it can mobilize to resist overturning uplift. Functional consideration dictates the location as they break up the interior space.

It is economical to locate shear walls along existing framing lines in order to provide boundary members, collectors and dead load to help resist overturning forces. On the interior the shear wall continues through the slab and it should be cast in 2 pours 48 hours apart to avoid sagging away of concrete from the underside of the concrete slab. The initial pour is stopped at 450 mm from the slab soffit to allow enough space to form shear keys and prepare the surface for next pour up to the top of the slab.

Adding Infill Walls

Masonry infill walls can be used to increase lateral strength strengthening of RC frames, especially open storeys. However, this should be limited to buildings up to 5 storey tall. It is applicable to most RC framed buildings and can significantly improve lateral strength and stiffness. It adds considerable mass to the structure and need new footings between existing spread footings. It is necessary that concrete/mortar is placed tight to overhead beam (i.e. no gap exists) otherwise shearing of column may result. Further, if proper care is not exercised the existing columns may become weak link and suffer extensive damage. Similar to new shear walls, the new masonry infills should be placed in the building so that torsion is minimized and they should continue to full height of the building. Moreover, for economic design they should be located such that it can mobilize maximize dead weight of the building to resist overturning uplift.

Filling Openings

URM buildings are significantly weak in in-plane shear strength due to presence of openings for windows. Filling the openings with masonry, if permissible, can easily enhance the in-plane strength. Proper consideration should be given to the fact that it substantially increases the mass of the building and hence the seismic forces. Further, this may require new footings between existing spread footings over the increased shear wall. Openings can be filled with reinforced concrete or masonry. The technique is very economical if no foundation enhancement is required. Concrete overlay (shotcrete) on the entire wall may be necessary after filling the opening.

Adding Shotcrete (Gunite) to Existing Masonry

Shotcreting or guniting is ideal for URM when masonry is not strong or its in-plane shear strength is weakened by large openings. It provides comparable stiffness to existing URM walls. With epoxied dowels at about 600 mm each way, shotcrete and URM will work compositely enhancing its out-of-plane stability as well. The process

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itself is messy due to rebound and may not acceptable inside building. Also it is difficult to transfer it through floor system may require review of foundation details due to increased mass of structure. Enough shotcrete is provided so that failure of unreinforced section can be prevented. Thickness of shotcrete and reinforcement should be designed for expected shear demand ignoring masonry contribution.

Adding Jackets to RC Frame Members

Jacketing is an effective technique for strengthening non-ductile RC frame members where functional use prohibits new shear walls. The primary objective of these techniques is to improve the shear capacity and/or confinement of the compression concrete to enhance ductility than lateral strength. It causes minimum loss to floor area and interference to openings as opposed to shear walls. Moreover, there are number of choices for jacketing materials, such as steel, RC, carbon fibers, etc. Jacketing follows an easy procedure for columns whereas it is cumbersome for beams and joints. It does add to the strength and ductility of members but causes no significant increase in building stiffness.

Flexural capacity of a frame is increased with jacket and longitudinal and transverse reinforcement. Beam jackets provide confinement, enhance shear capacity and provide for missing longitudinal bars. However, it is difficult to jacket the top of beam and slab may have to be drilled.

Column jackets provide confinement and can remedy short lap splices of existing column reinforcement. A narrow gap at the end column jacket ensures undesired increase of shear forces resulting from increase flexural capacity. The gaps made at the end of the columns not only provide higher shear strength but also help keep constant flexural strength. This increases the ductility of member due to high shear-to-flexural strength ratio. Further, they confine columns with inadequate lap splice of longitudinal bars and enhance flexural strength and ductility.

Jacketing a few members in a building can correct their deficiencies but may cause other components vulnerable. Therefore, it is necessary that the overall behaviour of the structure should be carefully examined: indiscriminate jacketing of frame members may do more harm than good.

Adding Wing (Side) Walls

Columns of non-ductile RC frames can be strengthened with addition of side or wing walls. The design characteristics of these wing walls are similar to addition of new shear wall as discussed above.

Adding Buttresses

Non-ductile RC and URM structures, which are significantly weak in shear strength, can immensely benefit from installation of buttress walls. All exterior work results in minimal disruption to functional use of the facility and occupants. However, a large vacant space is required adjacent to the building for the construction of buttress wall.

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Further, it significantly affects the aesthetics of the strengthened structure. Also, large resistance from the piles or foundation of the buttress will be required because the buttress will not be able to mobilize the dead weight of the building to resist the overturning forces. A load path to transfer shear forces from the building to buttress is required, which means horizontal members (shear collectors) have to be installed on the interior of the building.

Adding Braces

Steel braces can be used to strengthen almost all types of RC, URM and steel structures. They are lightweight system causing minimum influence on foundation and structures mass. Many bracing configurations (such as Chevron, V, K, X, etc.) are possible, which can allow for openings, passages, services, etc. Steel bracing is usually less stiff than masonry or concrete buildings, therefore, they have to crack significantly before steel braces are effective. The new braces are placed in such a way that significant dead weight of the building can be mobilized to overcome overturning forces. Bracing bays will require columns as well horizontal members as collectors to form complete truss. Bracing configurations with tension only braces must be avoided. Figure 9 also shows a RC building damaged in Northridge earthquake, which was subsequently strengthened by installing steel braces.

Strengthening Foundation Structures

Strengthening of footing is typically required if bearing capacity is not adequate for excessive soil pressure due to seismic overturning forces. As discussed above, certain strengthening schemes for superstructure will result in enormous increase in overturning forces that need to be safely transferred to subsoils. In such cases the existing footing may not be adequate and may require extensive modification.

Underpinning the footing and adding new drilled piers are two such schemes. Many configurations are possible, which can allow for openings, passages, services, etc. The new footing should be constructed in staggered increments each increment should be preloaded by jacking prior to transfer of load from the existing footing. Drilled piers are usually provided to increase vertical capacity of footing when soil bearing pressure and uplift due to overturning forces is excessive. RC piers should be cast-in-situ in uncased holes so as to develop both tension or compression else use under-rimmed piles. It is important that each RC pier extend above the existing footing and is connected by RC beam through the existing wall. Similarly pile foundation when found deficient may be upgraded. New piles can driven adjacent to existing footing. A large footing overlay will be required to create new pile cap so that forces can be transferred to new piles.

Strengthening foundation is not only expensive but a very disruptive process. It is usually cost effective to change strengthening scheme so that foundation strengthening is not required.

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EFFICACY OF STRUCTURAL ENHANCEMENTS

Figure 2 summarizes the effectiveness of seismic strengthening of various infilled units under lateral loads. It provides qualitatative indication of improvement in strength and ductility as observed during various laboratory tests. It indicates:

1. When adequate connections are provided, infill walls provided the same strength as monolithic wall.

2. Multiple precast panels have large ductility but much less strength

3. Steel braced frame possess significantly higher strength and ductility

4. Concrete clocks and bricks masonry also significantly enhanced lateral strength

Similarly, Figure 3 presents efficacy of various column enhancements. It is clear that any jacketing resulted in significant increase in strength and ductility. All jacketing techniques with end gaps resulted in considerable increase in strength and ductility. Steel jacketing without end gaps reached much higher lateral strength but there is decrease in strength after reaching the peak strength. Separation of spandrel walls considerably increased ductility (i.e. increasing length of a “short” column) while the strength is significantly reduced.

Figure 2. typical load-deformations curves of strengthened frames using various techniques (Sugano 1989)

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Figure 3. Typical load-deformation curves of strengthened columns using various techniques (Sugano 1996)

USING SUPPLEMENTAL DAMPING FOR SEISMIC UPGRADING: A CASE STUDY

Typically building structures possess an ambient damping of 2 to 5% of critical when their primary structural members are not loaded into the inelastic regime. In the traditional deisgns of earthquake-resistance, portions of primary structural members (such as, portion of beams adjacent to joints in moment resisting frames, braces in concentric braced steel frames) are specially detailed to undergo large inelastic deformation and thereby dissipating energy induced by earthquakes. This mode of energy dissipation results in damage to primary structural members and also causes significant non-structural damage due to resulting large inter-storey deformations. Supplemental damping is a technique of passive structural control in which the inherent damping of the structure is enhanced by incorporating suitable energy dissipation devices (EDDs) in the structure. These devices can be used either in series or in parallel with the primary structural system. The presence of EDDs reduces the seismic energy demand on the primary structural system, which may be deficient because of older non-seismic design practices.

It has been shown that significant reduction in response of the structure due to earthquakes is possible if the damping can be increased. The response of higher damped system will be less as effect of individual earthquake pulses will damp out quickly and will not accumulate. As shown in Figure 14, the effectiveness of damping in reducing response decreases at higher damping levels and differential gains are rather marginal for damping larger than, say 30% of critical.

A conceptual design for upgrading a landmark building, San Francisco City Hall, using supplemental damping will be discussed. The City Hall is a beautiful Renaissance architecture located in Civic Centre National Historic Landmark District area of San Francisco. It was built in 1913 at the same site where the previous city hall was

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destroyed in the devastating 1906 San Francisco earthquake. The structure is a 5-storey office building, 94 by 124 m in plan with central dome rising 90 m above main floor, as shown in Figure 15. The massive dome, 27.5 m in diameter, acts as a pendulum for seismic loads. The building was damaged in the moderate 1989 Loma Prieta earthquake.

The structure is primarily has a steel frame as the skeleton with brittle brick masonry, hollow clay tiles as infills and partitions. The building is characterized with a non-uniform distribution of lateral strength and stiffness with the second storey being a soft storey, i.e., its stiffness is much less than the first storey. The developed scheme could meet the performance requirements at nearly half the total costs (direct and indirect) than required by a base-isolation technique (Rai 1999).

REFERENCES CEB (1995). Fastenings for Seismic Retrofitting: State-of-the-Report, Comite Euro-International Du Beton, Thomas Telford, London

BSSC (1992). NEHRP handbook for Seismic Rehabilitation of Existing Buildings, FEMA-172, Building Seismic Safety Council, Washington, D.C.

FEMA (1999). FEMA 308, Repair of Earthquake Damaged Concrete and Masonry Wall Buildings. Applied Technology Council, Redwood City, CA.

BIS (1993). IS:13935-1993 Repair and Seismic Strengthening of Buildings-Guidelines, Bureau of Indian Standards, New Delhi

JBDPA/Japan Building Disaster Prevention Association (1990). Guideline for seismic retrofitting design of existing reinforced concrete buildings. Akasaka, Tokyo.

Rai, D. C. (1999). “Use of supplemental damping in seismic retrofitting: a case study.” Engineering Structures, Elsevier, vol. 21, no. 7: 603-614.

Sugano S. (1996). “State-of-the-Art in Techniques for Rehabilitation of Buildings,” 11 WCEE, Acapulco, Mexico, Paper no. 2179 on CD-ROM, Elsevier.

Sugano S. (1989) “Study of the seismic behaviour of retrofitted reinforced concrete buildings.” Proc. ASCE ’89 Structures’ Congress, San Francisco, CA.

Wyllie, L.A.(1996). “Strengthening Strategies for Improved Seismic Performance,” 11th WCEE, Acapulco, Mexico, Paper no. 1424 on CD-ROM, Elsevier.