Viscous Fluid Damper Retrofit of Pre- Northridge Steel Moment Frame Structures

Transcription

1 Viscous Fluid Damper Retrofit of Pre- Northridge Steel Moment Frame Structures Saif M. Hussain, S.E. Paul Van Benschoten, S.E. Arvind Nerurkar, P.E. Mohamed Al Satari, Ph.D. Tesfaye Guttema, Ph.D. Silian Lin, Ph.D. Coffman Engineers, Inc. Los Angeles, CA Abstract Viscous Fluid Dampers (VFD s) have long been utilized for vibration control in various applications throughout the world. One of the first uses of VFD s as part of earthquake resistant structural systems was in San Bernardino, California over ten years ago (Hussain, et al, 1993). VFD s can be very effective in reducing lateral displacements and dissipating energy from earthquake and wind loading. This paper attempts to give the reader an overall view of some of the seismic applications of these devices. Issues addressed include modeling, analysis, design, procurement, testing as well as construction challenges. The project type specifically dealt with in this paper is the pre-northridge Earthquake (1994) steel moment resisting frame (SMRF) building. Three such projects are described to highlight some of the issues mentioned above. The case study buildings are all located in Southern California and utilize pre-northridge steel moment frames for lateral force resisting systems. The structures were constructed in the 1980 s. These buildings have beam-column connections that use complete joint penetration welds typical of the construction techniques of that era. 1 ASCE 31 and 2 FEMA 351 analyses indicated that the existing moment frames would perform poorly during a Design Basis Earthquake 1 American Society of Civil Engineers 2 Federal Emergency Management Agency (DBE, 10% chance of exceedance in 50 years). The buildings performance would be characterized by excessive drifts, likely brittle fracture of beam-column connections, as well as significant interior and exterior damage, including the possibility of partial collapse in some cases. The experience with these projects shows that installing braced frames with properly designed nonlinear VFD s could reduce building interstory drifts by about 50%. Additionally the relatively lower base shear associated with a soft structure is maintained since VFD s have effectively zero spring (displacement proportional) stiffness. The reduced drift causes the inelastic rotational demands on the existing rigid beam-column connections to be significantly reduced. A VFD retrofit scheme can significantly enhance the seismic performance of a SMRF building while causing minimal impact on its operations at relatively low construction cost and thus can be quite cost effective. VFD retrofits may necessitate some localized strengthening of the existing structural and/or foundation systems. In one of the case study buildings, the existing concrete pile foundation system was deemed incapable of transferring the additional overturning forces induced by the new braces. Helical piers with grouted shafts were used to increase the existing foundations capacities. The overall success of the subject projects caused the owner to embark on a

2 system wide study of the applicability of this strengthening technique. Introduction The 1994 Northridge Earthquake brought to light defects in the welded beam-column connections in Steel Moment Resisting Frames (SMRF s) that surprised most structural engineers. Numerous SMRF buildings in areas close to the epicenter were found to have damaged connections that ranged from complete weld fracture to minor cracks in the weld metal apparent only upon ultrasonic or magnetic particle inspections. In some case divots or pieces of columns and beams separated from the members and there were also a number of cases of columns shearing right through the section. It is important to point out that there are no reports of loss of life or serious injury caused exclusively due to such structural damage to the SMRF s, even though the costs of subsequent investigations, evaluations, repair and retrofit were very high. An emergency code change was enacted in the Uniform Building Code immediately following the Northridge Earthquake, which prohibited the use of the commonly used prescriptive beam to column welded moment frame connection. Following the discovery of this rather widespread damage that resulted from the Northridge Earthquake, a consortium of SEAOC 1, ATC 2 and CUREE 3 took on the acronym SAC and obtained FEMA 4 funding to study the problem. After years of documentary, analytical and experimental research, countless committee meetings, conferences, workshops and seminars preliminary documents were produced by SAC and published by FEMA. After a fairly rigorous feedback and revision process FEMA produced a set of final guideline documents that provide recommendations on the evaluation, analysis and design of new and existing SMRF buildings (FEMA 350, 351, 352). FEMA 351 offers a comprehensive set of guidelines on the evaluation of existing moment frame buildings along with statistical data to enable the calculation of 1 Structural Engineers Association of California 2 Applied Technology Council 3 Consortium of Universities for Research in Earthquake Engineering 4 Federal Emergency Management Agency expected damage from various levels of earthquake ground shaking for a particular SMRF building. In the case of at least three 1980 s vintage SMRF buildings evaluated by the author(s) in the past couple of years, FEMA 351 was used to estimate the expected damage to the connections and resulting economic losses for the building owner. The expected structural performance of the building in comparison to an acceptable threshold, e.g. Life-Safety, was also calculated based on FEMA 351 methodology. A sample damageability curve along with tabulation for one of the case study projects is given in Figure 1 and Table 1 Figure 1. Connection Damage Ratio vs. Interstory Drift Ratio, FEMA 351 below. Table 1. Estimated Number of Damaged Connections based on FEMA 351 Story Number of Connections Percent Damaged Connections Estimated Number of Damaged Connections Roof rd nd Total Estimated Damaged Connections 116 It is to be noted that many SMRF buildings designed in the early 1980 s, mostly to the 1979 or 1982 UBC, seem to have approximately half the required strength and stiffness for that same building based on today s (2001 CBC/ 1997 UBC) codes. This was quite apparent in most of the buildings evaluated by the authors. One case, of a building in San Diego, was a stark anomaly. This was due to several factors, including a change in Seismic Zones (from 3 to 4) for San Diego as well as

3 near fault factors and redundancy factors that are now required by the current code. The lack of redundancy in that particular building was quite extreme. The building had an under strength ratio of about three and an under stiffness ratio of about eight. The soft 1980 s vintage SMRFs therefore make ideal candidates for the introduction of velocity dependent VFD s. Significant damping response is obtained from such dampers involving interstory drifts of around 2 to 4 and drift velocities of about 10 to 20 in/sec. Various Retrofit Options Every seismic retrofit project for an existing building begins with the determination of a performance objective that is ultimately decided upon by the owner with advice from the engineer of record. Even though it is increasingly the practice to use national standards or guidelines such as ASCE 31 and FEMA 356 as the basis for an explicit performance based seismic evaluation and upgrade, implicit upgrade objectives such as bringing the building up to code, though not used by the authors, are still used in the industry. FEMA 356, defines building performance in terms of the level and extent of structural and nonstructural damage that may be sustained by the building during a given earthquake. The earthquake hazard level at the site in question is typically defined in terms of the probability of exceedance in a certain period of time. For example the Basic Safety Earthquake 1 (BSE-1) can be defined as the level of ground shaking that has a probability of exceedance of 10% over a period of 50 years or a return period of 475 years. This is the most commonly used probabilistic seismic hazard that serves as the Design Basis Earthquake or DBE for most new and retrofit building projects in California. Other hazard levels such as the 2% in 50 years level may be used to define a BSE-2 level hazard which is then multiplied by a 2/3 factor to define the BSE-1 level of earthquake hazard at the subject site. On the other hand deterministic calculations based on a specific scenario for a particular fault may also be used to define the seismic hazard level at a given site. On the vulnerability side of the equation, it is virtually impossible to accurately predict exactly how much damage a building will actually sustain in response to a given level of strong ground motion. The performance level of a building subjected to a given level of earthquake ground motion generally depends on the strength and stiffness of structural components, stiffness of nonstructural elements, the quality of material used and type of construction. It is impractical to accurately quantify all the aforementioned parameters that affect the performance level of a building. Both the performance level of an existing building in a seismic event and the seismic hazard level to which the building may be subjected are commonly determined by reliability-based probabilistic approaches which in turn use a preset confidence levels (say high, medium and low). The standards which are currently used for seismic evaluation and retrofitting of existing buildings such as the ASCE 31/03, FEMA 356 and FEMA 351, explicitly acknowledge these inherent uncertainties. The FEMA 356 standard gives detailed descriptions of the building performance level. As defined by FEMA 356, the building performance level is obtained by combining the structural and non structural performance levels. Building performance levels used in FEMA 356 are the Operational (1-A), Immediate Occupancy (1-B), Life Safety (3-c) and Collapse Prevention safety (5-E). The principal goal of seismic rehabilitation is to reduce the risk of death or injury and property damage during a seismic event. This is usually achieved by modifying the structural systems of buildings, which are evaluated and found to have inherent seismic deficiencies (ASCE 31-03). Improvement of a building s seismic performance can be accomplished through strengthening existing structural elements or adding new elements. In the particular case of pre-1994 SMRF buildings, where the concern is usually with overall system flexibility and unsatisfactory performance of moment connections, rehabilitation options may include: Adding system strength and stiffness by adding new lateral load resisting elements such as concrete structural steel braced frames and/or shear walls Eliminating soft and weak stories by adding new frames or walls

4 Upgrading the existing steel moment connections in deficient SMRF buildings to lower the probability of damaged or failed connections in a major earthquake Using Energy Dissipating elements such as Fluid Viscous Dampers (FVD), Viscoelastic Dampers (VED), Friction Devices (Adding Damping And Stiffness -ADAS dampers) which add supplemental damping and/or stiffness to the lateral load resisting system Modifying existing steel moment frames using yielding steel elements such as the Buckling Restrained Braces (BRB s) Three different existing SMRF building projects recently worked on by the authors are presented below as case studies to illustrate the process of seismic evaluation and upgrade design for this class of building where VFDs in new braced frames were judged to be the desired upgrade option. First Case Study: Retail Building, Costa Mesa, California Building description The building is a 225,000 s.f. high-end retail store located in a Southern California. It is a four story steel framed structure with a brick façade exterior. All the floors are a 3¼ composite lightweight concrete slab over a 1¼ 20-gauge Verco Deck. The roof is 22-gauge Verco Deck with a layered insulation system weather surface. The insulation system consists of lightweight concrete combined with vermiculite and has an approximate total thickness of five inches. The lateral force resisting system is ordinary steel moment resisting frames (SMRF) with two frame lines in each direction. Each frame line extends from grade to the roof and has five bays. The foundation system consists of driven square precast concrete piles with caps and grade beams. The columns transfer all forces to the ground through the deep foundation system. Seismic Hazard of the Site The building is located south of Los Angeles in Costa Mesa California approximately nine kilometers from the Newport - Inglewood fault. This fault is classified as a type B fault that results in a 2001 CBC equivalent static base shear of 0.13g. Site-specific response spectra for the Design Basis Earthquake were provided by the geotechnical engineer. Time-histories, scaled to the site specific DBE, from Loma Prieta-Gilroy #2, Northridge- Newhall fire station, and Landers-Yermo fire station records were used for the time-history analysis. Structural Deficiencies The building lateral force resisting system was found to be deficient as a result of an ASCE 31, Tier I and Tier II evaluation. The performance of the building, as originally constructed, subjected to the design earthquake is expected to be poor. The evaluation indicated that the pre-northridge beam to column connections of the moment frames are likely to suffer damage rendering many if not most of these connections unable to fulfill their intended function of providing continuity of the flexural load path from beam to column. In addition, when checked using the current CBC design forces, the beams and columns of the moment frames had Demand-Capacity-Ratios (DCR s) of two or greater. The building interstory drifts were calculated to be twice the amount allowed by the current building code. This drift can be expected to cause severe damage to the exterior brick façade, including the likelihood of falling hazards in and around the building. The drift is likely to result in extensive damage to interior nonstructural elements and finishes. Moreover, the excessive drift combined with the inadequate seismic separation from the adjacent buildings makes pounding a likely outcome of strong ground shaking at the site. Retrofit Objectives The overall objective for the project was to carry out a voluntary seismic retrofit of the building to meet Life- Safety performance goals in a DBE as much as practically possible given the various time, budget and construction constraints. This was to be done in conjunction with a pre-planned major renovation and remodel project for the facility. Drift demands were to be reduced to significantly decrease the likelihood of pounding, as well as to reduce the inelastic rotation demands on the pre-1995

5 SMRF beam-column connections. It was found that the additional damping introduced by the VFD s essentially eliminates the inelastic demands on the moment frames, thereby significantly reducing the expected consequent damage to the connections during a seismic event. Modeling Assumptions Computer modeling for the project involved the use of both ETABS and RAM-Perform analysis software. A three dimensional linear-elastic ETABS model was used to perform the static and linear dynamic analysis for the project. The columns were modeled with a pin at the base. All of the diaphragms were considered rigid for force distribution purposes. RAM-Perform was used to run the nonlinear time-history analysis, in which nonlinear elements for frame members as well as dampers were used. Performed Analyses A linear static analysis using the 1979 UBC was performed to determine if the building met the original building code requirements. Additionally, linear static and dynamic analyses were performed in accordance with the 2001 CBC to make a comparison to the current code requirements. Static and dynamic analysis was done to evaluate the structure and determine deficiencies based on the ASCE 31 standard which included Tier I and Tier II checks required for steel moment frame buildings. The relatively long period associated with original moment frames is maintained thereby preserving the advantage of a system that generates lower base shear in an earthquake. The added damping reduced the unscaled dynamic base shear response of the building by a factor of about 2. Construction Issues The project was completed while the store remained in operation. The retrofit included installation of 20 braced frames with VFD s on three of the four floors of the building. The existing deep foundation system was also retrofitted in order to resist the increased localized forces from the new damper braced frames. Maintaining day to day operations of the store proved to be a challenging goal. All of the work was performed at night between 10:30pm and 8:00am and required that the sales floor be clean and operational every day following a night of construction activity in the building. Although the owner allowed some areas to be curtained off during the day, no work was allowed in the moment frame core area of the building. Once the deficiencies were determined, various strengthening schemes were considered to address those deficiencies. These included the addition of conventional braced frames, retrofit on the moment connections and addition of supplemental dampers. Results As a result of the aforementioned analyses, it was determined that VFD s installed in chevron braced frames were very well suited to reduce the drift of this moment frame building and consequently the expected damage and disruption from major earthquakes. The supplemental damping from VFD s reduced the building drifts by about 50%. Since the retrofit scheme does not add any spring stiffness to the building, the natural frequency is virtually not affected by the retrofit. Figure 2. Typical Damper Braced Frame and Helical Piers. The braced bay with viscous damper configuration was well suited for this challenge because the system is confined to one or two bays per side, per floor. Moreover, the retrofitted bays could be located outside the sales floor area. The work associated with the installation of the braces was conducive to short work

6 periods and relatively easy clean up. The work was localized with minimal impact to surrounding architectural finishes and the materials were handled using elevators, escalators, and man doors. pumped concrete in an operating store were considered to be too great to allow concrete pumping for this project. Figure 4. Installed Dampers Behind Storage Areas Cost Issues Figure 3. Installed Chevron Braced Frame with Dampers in Utilized Space The foundation retrofit was expected to be the most challenging aspect of the work given the existing deep foundation system, space constraints and the other limitations imposed by the building owner/manager on the design and construction team. It was found that helical piers are very well suited for this type of challenging project because they were compatible with the existing deep foundation system, require a relatively small footprint, and were installed in the building using small electric equipment that could access the store interior without demolishing large access holes. This equipment vented no exhaust inside the building. Since the helical piers are installed in seven foot increments, they only require foot ceiling clearance for installation and thus allowed all of the foundation work to be carried out from the interior. This satisfied another constraint imposed by both the building owner and the mall operator who did not allow any construction work on the exterior of the building. The small footprint of the helical piers required minimal concrete to construct the retrofitted foundation configuration. Consequently, all of the concrete on the project was conveyed through the store via wheel barrows. The risks and problems associated with The initial cost of the VFDs is relatively modest and the devices are claimed by the manufacturer to be virtually maintenance-free throughout the expected service life of about 50 years. Coffman Engineers, based on prior experience and additional research, had estimated the selected retrofit scheme to cost in the range of $10 per square foot of building floor area. The actual construction cost for the upgrade work came out to be very close to this number. To put this in perspective, the total renovation project cost for this store was multiple times the seismic upgrade costs. The comprehensive success of this retrofit project convinced the owner to launch a system wide voluntary seismic evaluation and upgrade program for their stores in high seismic risk areas. It must be noted that programs such as these can be expected to result in greater public safety as well as reduction of financial losses due to major earthquakes. Second Case Study: Three Story Retail Building, Redondo Beach, California Building description The building is a three story steel structure clad with a brick and concrete masonry unit façade. It was designed in accordance with the 1982 Uniform Building Code and was originally constructed in The building is

7 located adjacent to a mall structure and a parking access structure with a seismic separation of just four inches. The building footprint is about 273 feet by 183 feet. Each floor is approximately 48,000 square feet in area. There are three floors above grade and one 8000 s.f. partial basement below grade. The total floor area of the building is approximately 150,000 square feet with several mezzanines framing levels and miscellaneous mechanical penthouses located on the roof level. Typical story heights are 18 feet. The building is framed with steel wide flange columns and beams. Typical floors are 3 deep 20 gauge structural metal decks with 3¼ thick reinforced light weight concrete topping (6¼ total depth). The roof level is constructed of 1½ deep structural metal deck with 2 thick vermiculite concrete topping over rigid insulation (varying total depth). The mechanical penthouse roofs are constructed of one and one half inch deep 18 gauge structural metal deck with two and one quarter inch thick reinforced lightweight concrete over rigid insulation. Figure 5. Three Dimensional Model of the Lateral System The lateral resisting system is an ordinary steel moment frame system (SMRF) in each orthogonal direction. The primary frames are located on interior gridlines. In the north/south direction, the building comprises of 6 frames and in the east/west direction 4 frames extending the full length of the building. Typically, each frame column extends from the first floor on grade to the roof. The steel moment frame system utilized knee braces on the first level, which is not allowed in the current building codes. CMU walls were used for the elevator shaft enclosures. The frame columns transfer all forces to the foundation system through the concrete basement walls, footings and a concrete slab on grade. The foundation consists of individual and continuous reinforced concrete spread footings. Seismic Hazard of the Site The site soil consists of silty sand to depths of 4 to 27 feet below the ground surface underlain primarily by sand. Based on the local soil and geological conditions, the site is classified as soil profile type S D. The Project Site is located at a distance of 4.5 kilometers from the Type B (2001 CBC) Palos Verdes Fault. Structural Deficiencies & Retrofit Objectives The building was evaluated in accordance with ASCE and FEMA 356 standards. The insufficient separation between the building and the adjacent mall building may result in pounding during a major earthquake. The inelastic demand on the knee braces during a moderate to high seismic event may result in the formation of plastic hinges in the columns which may contribute to a development of a collapse mechanism in the steel frame. The already low stiffness of the SMRF is further reduced when these problematic knee braces are removed thereby causing a significant increase in the calculated number of damaged steel moment frame connections (FEMA 351). One other complicated problem was the elevator shaft enclosure CMU walls which were not detailed to accommodate the building drift that may occur in an earthquake. The aforementioned structural deficiencies of the building were investigated and addressed during the seismic evaluation and retrofitting design phase of the building. FEMA 351 was used to estimate the expected damage to the connections and resulting economic losses. The results of the evaluation for the estimated connection damage ratio vs. building pseudo interstory drift ratio and the corresponding estimated number of connections damaged at each floor are shown in Table 1 above.

8 Modeling Assumptions A three dimensional mathematical model of the building that represents the distribution of the mass and stiffness of the structure to capture the relevant dynamic responses was developed using ETABS software. The floor and the roof diaphragms were modeled as rigid diaphragms with lumped lateral- and rotational-masses at each floor level. The building was retrofitted using sixteen 200 kip fluid viscous dampers which were modeled as nonlinear link elements. The force-velocity relationship of the dampers was given by F = CV α (C is the damping constant; V is the relative velocity between the damper ends, and α is the damping exponent). This relationship is graphically illustrated in Figure 6 below. Damper Force (Kips) Performed Analyses Nominal Lower Bound Upper Bound Velocity (in/s) Figure 6. Force-Velocity Characteristics of the 200 kips Fluid Viscous Damper Two site-specific response spectra for the Basic Safety Earthquakes BSE-1 and BSE-2 (475 and 950 years return periods, respectively) were developed for the project by the geotechnical engineer. Based on the USGS/NEHRP mapping, the short period response design acceleration S DS = 1.22g and spectral response design acceleration at a one-second period S D1 = 0.74g. Three pairs of representative earthquake horizontal ground motion time-histories were scaled to match the site-specific response spectra. The time-history records used were from the 1992 Landers, the 1989 Loma Prieta and the 1999 Duzce earthquakes. The scaling of the response spectrum and time-histories was in accordance with the provisions of FEMA 356. Nonlinear timehistory analyses using ETABS NL software were performed using the scaled earthquake records. The primary frames were modeled with linear-elastic elements whereas the dampers, as mentioned above, were nonlinear link elements. Results A comparison of the base shear of the as-built structure with knee braces and the retrofitted building without knee braces shows that the base shear is reduced by about 34%. Similarly, story drifts were reduced by 22% at the roof. The results of this case study show that the addition of the fluid viscous dampers is very effective in reducing both the story drifts and base shear. Table 2. Summary of Retrofit Scheme Results Case Study No. 2 Story Story Disp. (in) Story Drift (in) Story Shear (kips) X-Dir. Y-Dir. X-Dir. Y-Dir. X-Dir. Y-Dir. Existing Building with Knee Braces (MRF) Remarks Roof T 1 = 1.7sec 3rd nd Removed Knee Braces Added Viscous Fluid Dampers Roof T 1 = 2.2sec 3rd nd W = 11,393 kips, V 1982 UBC = 0.14W, V 2001 CBC = 0.26W Third Case Study: Retail Building, San Diego, California Building description This building is a five-story steel structure with a combination of stucco façade and concrete masonry unit fire rated assemblies. It was designed in accordance with the 1982 Uniform Building Code (UBC) and constructed in The building footprint is 286 feet by 156 feet. Each floor is approximately 40,000 square feet in area. There are 5 floors above grade and three partial mezzanine areas located on the third and fifth floors. The total floor area of the building is approximately 150,000 square feet with several storage mezzanines and miscellaneous mechanical penthouses located on the roof level.

9 The building is framed with steel columns and beams. Typical floors are 3 deep 20-gauge structural metal decks with 3¼ thick reinforced light weight concrete topping (6¼ total depth). The roof level is constructed of 1⅜ deep structural metal deck with two and one quarter inch thick Zonolite concrete topping over rigid insulation (7¼ total depth). The roof deck under the mechanical roof top units is three inch deep 20 gauge structural metal deck with three and one quarter inch thick reinforced lightweight concrete. The gravity columns along some gridlines are shared with the Mall Building gravity load carrying system. In other locations, girders are supported by common columns at the two buildings interface. The lateral force resisting system is an ordinary steel moment frame system in both principal directions. A shaft wall system is used for the elevators. Each frame column extends from the first floor on grade to the roof. The frame columns transfer all forces to the foundation system through the spread footings and slab on grade. The foundation system consists of individual and continuous reinforced concrete spread footings. Some of the spread footings are shared with the adjacent mall. Individual footings support non-bearing CMU walls used for sound and fire separation. Seismic Hazard at the Site The original design of this building was based on the seismic hazard associated with the 1982 UBC designation of Zone 3. The 2001 California Building Code (CBC) now designates the seismic hazard at that site as Zone 4. This building code change increases the design peak ground acceleration from 0.3g to 0.4g. Additionally, the 2001 CBC stipulates near-fault factors for sites in close proximity to known active faults. Since this particular site is less than 2 km from the Rose Canyon fault the design peak ground motion acceleration is increased to 0.572g, 1.91 times the original design PGA Structural Deficiencies & Retrofit Options Numerous significant deficiencies were identified in the existing lateral force resisting system. Poor geometry and inadequate stiffness of the lateral resisting system resulted in an extremely soft structure with a primary torsional mode of vibration at T 1 = 5.46s and a secondary translational mode of vibration at T 2 = 3.7s. The fundamental mode of vibration for a structure of this type typically should occur in a translational direction at approximately T 1 = 1.0s to 1.3s (T A, 1.3T A : 2001 CBC Method A ) or about eight times stiffer than the existing structure s lateral system. Additionally, most lines of lateral resistance consist of single-bay moment resisting frames. The inherent lack of redundancy is not allowed in the current building codes, the CBC and the International Building Code (IBC). A failure of one connection in the single bay frame could significantly compromise the ability of the structure to resist lateral loads from strong ground shaking. An additional problem in the existing structure results from the deformation incompatibility between the hard connected or insufficiently isolated non-bearing CMU walls in the loading dock, driveway and storage areas and the soft single bay moment frames. The CMU walls, as they are currently detailed, would attract a significant amount of lateral load due to a major seismic event. However, the CMU walls are not designed as lateral resisting elements and would sustain significant damage in a few cycles of earthquake loading. The layout of the CMU walls in the middle and eastern portion of the lower structure could induce destabilizing torsional behavior in the structure until the wall stiffness degraded significantly. Retrofit Objective A significant number of seismic deficiencies were found in the subject building as a result of an ASCE 31 and ASCE 351 evaluation as well as based on common current practice. Significant structural modifications were deemed necessary to meet the life-safety performance objective as outlined in guidelines such as FEMA 356. Both increased stiffness and damping will be required to adequately retrofit the building. Additionally, separation and deformation compatibility issues will need to be addressed and resolved. Modeling Assumptions A three-dimensional computer model of the building was created using ETABS. The following modeling assumptions were made to mathematically simulate the building s dynamic characteristics in its current

10 condition: The mass of the building was distributed across the floor plan and the mechanical units were also simulated as lump masses; All columns were pinned at the base; All connections between columns and beams were pinned except the existing SMRF s; Floors were modeled as shell elements and rigid diaphragms. Retrofit Options Multiple potential seismic retrofit schemes were investigated. However, based on the performance effectiveness and constructability of the retrofit schemes studied, the Fluid Visco-Elastic Dampers system was proposed as the optimum solution for thie building. The Fluid Visco-Elastic Dampers (FVED), or more commonly known as a Spring Damper, was considered because it combines a linear-elastic spring in parallel with a nonlinear velocity dependent viscous damper (see Figure 7). These devices offer increased elastic stiffness as well as supplemental velocitydependant damping simultaneously and can be customized for any particular application. A total number of 48 spring dampers (2 per braced bay, 24 bays) in a chevron configuration were proposed to be installed throughout the structure. The total effective damper force can be expressed mathematically in the form: F = KX + CV α, where F is the damper force, X is the stroke, C is the damping coefficient, V is the velocity, and α is the damping exponent. Two parallel link elements were modeled per bay: one link with a linear effective stiffness K e = dampers = 288 kip/in, and another nonlinear link with damping coefficient C=125 2 dampers = 250 kip-(sec/in) 0.5 and exponent α = 0.5. This links configuration provides the equivalent properties of 2 spring dampers per bay. Figure 7. Fluid Visco-Elastic Damper Schematic Cross Section Performed Analyses A site-specific response spectrum was developed for the DBE (475 year return period) event to scale three pairs of representative earthquake horizontal ground motion time-histories. The time-history records used were from the 1979 Imperial Valley, the 1999 Duzce Turkey and the 1940 El Centro Earthquakes. The scaling of the response spectrum and time histories was in accordance with the provisions of FEMA 356. A nonlinear time history analysis was performed for the retrofitted building by using these 3 earthquake time histories where the building structure is modeled as linear-elastic and the viscous fluid dampers are modeled as nonlinear link elements. Results The calculated dynamic responses of the existing and retrofitted building are shown in Tables 3, including the maximum floor level displacements, story drift ratios, and story shears. Based on these analytical results, the following could be concluded: Due to the additional stiffness from the spring dampers which were part of damper braced frames arranged in a fairly well distributed manner on the building footprint, the first mode of building is changed from the torsion mode (5.5 sec) to the translation mode (1.9 sec) in the X - direction without dramatically increasing building base shear. Furthermore, the building inter-story drifts and maximum displacements are reduced significantly, because of the additional viscous damping from the spring damper. The contribution of the spring element stiffness reduces the calculated building roof displacement by 40% from approximately 23 inches to 14 inches. The fluid viscous damper element contribution reduces the roof displacement by an additional 50% from approximately 14 to 7 inches. Therefore, this retrofit scheme and configuration reduces the roof displacement from approximately 23 inches to 7 inches due to the combination of increased building stiffness and viscous damping.

11 Table 3: Summary of Retrofit Scheme Results for Case Study No. 3 Story Story Disp. (in) Drift Ratio Story Shear X-Dir. Y-Dir. X-Dir. Y-Dir. X-Dir. Y-Dir. Existing Building (MRF) Roof th th rd nd T 1 = 5.46 sec (Torsion), T 2 = 3.70 sec (Trans. in Y), V B max = 0.139W Fluid Visco-Elastic Damper Spring Damper Roof th th rd nd T 1 = 1.91 sec (Trans. in X), V B max = 0.166W, W = 16,335 kips Summary Some salient structural engineering aspects of such retrofit schemes can be summarized as follows: VFD s virtually add no stiffness to the building; and therefore, the lower base shear associated with a longer period building can still be taken advantage of. Although the forces may increase due to code revisions and increased ground accelerations, the fundamental period remains essentially the same. The original moment frame system remains intact, i.e; there is no abandonment of the existing system. The moment frames still provide the primary lateral strength and stiffness for the building. Only localized retrofit is required for certain elements such as diaphragms, drag and collector elements, columns and foundations Moment frames dissipate energy based on interstory drifts whereas viscous dampers dissipate energy based on inter-story drift velocities. Since velocity and displacement are out of phase, the maximum loads imparted by the moment frames and braces on the diaphragms, collectors and foundations are not concurrent. This system is both efficient and complementary. VFD s can maintain near-elastic behavior of the existing moment frames in response to the DBE; therefore existing connections need not be upgraded. The above aspects also contribute to cost efficiencies for such projects and are thus more apt to be accepted by owners and financiers. Viscous Fluid Dampers Implementation Issues Because VFDs used for earthquake protection are typically quite sophisticated in terms of the response, performance characteristics, materials, construction and testing of these devices, there are only a handful of well qualified manufacturers of such products in the US or, in fact, even globally. Two such US manufactures are located in upstate New York, Taylor Devices and Enidine Systems. In order to procure the desired VFD for a given project the engineer has to provide clear and complete technical specifications that detail the device, its performance characteristics, duty cycle, testing requirements and dimensional limitations among other items. It is extremely important to ensure, via properly written project specifications, that only adequately qualified manufacturers having a solid history of supplying the requisite devices are allowed to bid on and win the project. Most general contractors in the construction industry do not have the experience or the knowledge that is essential in order to satisfactorily procure these rather sophisticated and unusual elements for a given building project. As such it is incumbent upon the design professional to produce technical specifications that will assist the contractor in proposing only properly qualified subcontractors offering devices that match all the technical requirements of the project. As with highly specialized items such as seismic isolators dampers, if possible, are recommended to be procured separately and directly by the owner and then assigned to the general contractor (GC) for coordination and installation. Such a process greatly decreases the

12 likelihood of misunderstandings, miscoordination and potential project delays and cost impacts that may emanate from lack of knowledge and experience on the part of many GC s. Other than basic engineering, Quality Assurance and Quality Control (QA/QC) measures that provide high confidence levels in the VFD made by one or the other manufacturer, it is essential that the devices be properly and comprehensively tested to verify their robustness and performance characteristics. Every VFD unit produced must undergo QA/QC testing which includes at least one set of cyclic testing that provides the engineer with an assurance as to the performance characteristics and reliability of the device. It is customary practice on most projects to investigate the option of specifying dampers of the type and size that already have a track record of successful usage on similar projects. In such cases extensive product research, development and refinement time is avoided as is the uncertainty associated with custom engineered devices. Prototype testing which is a requirement for custom engineered VFDs can be effectively avoided by using pre-tested damper designs. Figure 8. Prototype Damper Test Force-Velocity Relationship As standard practice in VFD projects, the authors have specified pseudo-prototype testing of pre-tested designs by calling for extended cyclic testing of two devices of each type and size during the dampers production phase. This is adequate to establish the properties of the specified damper and verify that those properties do fall within the allowable limits given in the specifications. The performance specifications for the dampers must accommodate various factors such as ambient temperature variations, age, cycling heat build-up, manufacturing tolerances etc. Therefore the specifications contain an upper bound and lower bound curve which are usually 15% above and below the nominal performance curve, respectively. The engineer must verify the acceptability of the design through bounding analysis using these upper and lower bound properties of the device. Conclusions As demonstrated above, pre-northridge steel moment frame structures can be effectively retrofitted with VFD s. It is not unusual to enable a reduction in calculated drifts by a factor of 2 as a result of the installation of VFD s as supplementary dampers in a SMRF system. Furthermore the relatively lower base shear associated with a soft structure is maintained due to the lack of spring stiffness in VFD s. As a consequence of reducing drifts, the rotational demands on the existing rigid beam-column connections can reduced to a near-elastic level. This virtually eliminates the need to upgrade these potentially brittle connections. Due to the localized nature and low level of intrusiveness of such retrofits, they effectively enhance the seismic performance of a building while allowing it to maintain operation. VFD retrofit schemes may require some localized strengthening of the existing structural and/or foundation systems. When existing deep foundation systems need strengthening, helical piers with grouted shafts could offer an efficient solution. Special measures may be needed to ensure displacement compatibility between the existing and the retrofit foundation systems. In such cases, some in-situ testing should be performed to verify capacity and compatibility. Acknowledgments This work would not have been possible without the assistance of Dennis Firth, S.E. The authors would like to thank him for his efforts.

13 References Hussain, Saif M. et al, 1993, Seismic base isolation design for the San Bernardino County Medical Center replacement project, Structural Engineering in Natural Hazards Mitigation: Proceedings of Papers Presented at the Structures Congress, American Society of Civil Engineers, New York, Vol. 1, pages ASCE/SEI 31, 2003, Seismic Evaluation of Existing Buildings, American Society of Civil Engineers, Reston, Virginia. FEMA 351, 2000, Recommended Seismic Evaluation and Upgrade Criteria for Existing Welded Steel Moment-Frame Buildings, Building Seismic Safety Council for the Federal Emergency Management Agency, Washington D.C. FEMA 356, 2000, Prestandard and Commentary for the Seismic Rehabilitation of Buildings, Building Seismic Safety Council for the Federal Emergency Management Agency, Washington D.C. ICBO, 2001, California Building Code, Structural Engineering Provisions, Vol. 2, 2001 edition, International Conference of Building Officials, Whittier, California. ICC, 2006, International Building Code, 2006 edition, International Code Council, Falls Church, Virginia.