Retrofitting By Means Of Post Tensioning Khaled Nahlawi 1 Abstract An analytical program was prepared to retrofit the Holy Cross Church in Santa Cruz, California. An inelastic analysis was perfonned on the church buttresses and tower to obtain the overall behavior. Post tensioned tendons were placed in core drilled holes in the buttresses and in the comers of the end walls to improve the behavior of the structure; higher shear capacity and higher ductility leveis were achieved. A fusing element, which yields at a predetermined load was introduced to prevent the buttress from reaching its ultimate capacity and subsequently preventing it from failure. It is connected to a steel truss placed on top of the existing roof. The soil - buttress and buttress - roof interactions were considered in the inelastic analysis. Introduction The Holy Cross Church was built around the tum of the century (circa 1890). It is an unreinforced masonry (URM) structure consisting of three connected sections. There is a tall tower at the south end of the structure, an apse at the north end houses the altar and sacristy, and the middle section of the structure - the nave - connects the tower and the appendix. The 660 mm thick 5.5 x 5.5 m square URM tower rises 27 m above the ground levei. A 17.0 m high wood framed steeple rests on top ofthe URM tower (see Fig.l). The nave is Keywords: Post-tensioning; shear; ductility; buttress; bond length. IKhaled Nahlawi, Ph.D., PE., Senior Project Engineer with BFL, Redwood City, California, USA 479
16.7 x 31 m in plan dimensions. The two 530 mm thick east and west side walls are 12 m high (see Fig. 2). Eight tapered buttresses at approximately 4.4 m spacing rise on the outside face of the nave's side walls. Their thickness varies from 2.1 m at ground levei to 0.64 m at the rooflevel (see Fig. 3). The appendix is 5.1 x 16.7 m in plan. Its walls are 0.53 m thick and 9.2 m high. The URM structure was built on a continuous 1.70 m deep stone rubble foundation, which rests on hard soi!. Timber trusses with a top chord slope of 45 degrees span approximately 17 m between facing buttresses. Structural Deficiencies The church survived two major earthquakes: The 1906 San Francisco Earthquake and the 1989 Loma Prieta Earthquake. No information is available on the extent of damage that resulted from the San Francisco Earthquake. However, the 1989 Loma Prieta Earthquake revealed several deficiencies in the structure. The top portion of the tower above the roof levei ofthe nave, which extended an additional 5.50 m, was severely cracked and had to be removed along with the 17 m high steeple (see Fig. 4). Medium size cracks were observed around window openings in the weak east - west direction. The front south wall, where the tower and the south nave wall meet, was very weak in resisting the lateral force because ofthe openings. Wide cracks were observed in the 1.2 m deep spandrel beam between the window and door openings. The gable outward movement was not sufficiently restrained. Structural Intgrity was lost and numerous flexural cracks developed (see Fig. 4). However, in the strong direction (North - South direction) no cracks were visible in the nave or altar. Proposed Retrofit The maio purpose of the Holy Cross Church retrofit is: 1) to strengthen the structure, 2) to reduce any potential for loss of!ife in future earthquakes, and 3) to preserve its historical appearance. Post tensioning was proposed to retrofit the church as an alternate to conventional methods of retrofitting URM structures. A diamond steel truss was proposed to transfer lateral loading from buttresses to end walls. It was further proposed to rebuild the tower and steeple of steel frames covered with wood frarning. Post Tensioning Twelve-12.5 mm strands were placed in a 180 mm core drilled hole in each buttress. The two front buttresses at the south end required fourteen-12.5 mm strands. The holes were drilled with an incline to the vertical to follow the centerline of the buttress cross section to reduce eccentricity (see Fig. 3). The four corners of the tower, the two buttresses of the appendix, and the north walls ofthe nave and appendix each received seven-12.5 mm 480
strands in a straight core drilled 100 rnrn diameter hole, respectively. Figure 2 shows the location ot the post tensioning. The bottom portion of each tendon was bonded while the upper part was unbonded. Hence, the tendons could be re-jacked if stress was lost in a future earthquake. The bottom portion of each tendon was anchored using primary grout of 34.4 MPa compressive strength. Grout for the unbonded length was used for corrosion protection. At the top anchorage, a 2.1 m long and 0.6 m deep brick section was replaced with a reinforced concrete beam to distribute the stresses into the URM wall. The strands were jacked to 71 % of ultimate (Guts). Two seven-12.5 rnrn strand tendons were monitored with load cells over a six month period to measure the stress losses in the post tensioned strands. Tower and Steeple The existing roofwas not disturbed. An in-plane diamond shaped steel truss was built to increase the diaphragm's shear resistance and stifiness, and was placed on top of the existing roof. The truss members served to resist the shear forces and transfer them to the end wails. A single steel member connects the in-plane diamond shaped steel truss to the post tensioning anchorage system, which is designed to yield at a predetermined load. An eccentric steel frame was used to rebuild the damaged top portion of the tower. It is covered with wood frarning and brick embossed stucco to preserve the exterior appearance ofthe structure. A steel frame was a1so used to rebuild the dismantied steeple. It is covered with wood frarning and identical thin metal shingies, which were originally used, to preserve its historical appearance. The advantages of rebuilding the tower and steeple out of steel frame are the reduction in dead weight and better resistance to the tension forces created by the overturning moment. The steel tower was connected to the existing masonry tower by post tensioning anchor plates. The small cracks in the structure were repointed. The structural cracks in the front wall between the door and window openings were injected with low-modulus epoxy resin. Analysis URM is a brittie material by nature. Applying post tensioning to the URM will increase its ductility, provided the axial load does not exceed 25% of the bricks' compressive strength(l). To determine the bricks' compressive strength, f m, a total of four masonry panels were tested; three in-situ and one in a testing lab. The post tensioned buttresses were analyzed using a nonlinear computer program that takes the geometric and material non-iinearity into consideration. The buttress was 481
modeled using a column model with an applied. axial load representing post tensioning, which did not exceed 10% ofthe brick's compressive strength capacity. This results in an increase in ductility (1) The roof-buttress and buttress-soil interactions were considered in the computer model by introducing linear and rotational springs, respectively. The lateral load vs. deflection curve is plotted for an increment of 10% of the vertical dead weight applied laterally to the buttress (see Fig. 5). The lateral load deflection curve behavior starts linearly. The fust joint opens at the base to allow for rocking. As the lateralload increases, the secondjoint will open at 5.8 m high in the weak direction and at 9.75 m high in the strong direction, after which the lateral load vs. deflection curve continuous linea11y with a decrease in slope. In the weak direction, the buttress reaches its capacity at a lateral load of approximately 90% of weight. Beyond that, the buttress fails in a brittle manner. In the strong direction, the buttress undergoes large deformations at ultimate compressive strength without any increase in strength (increase in ductility). Due to the presence of post tensioning, ductility increased from approximately zero ductility to a ductility factor of 5. The analysis reveals that post tensioning improves the overall behavior of the buttresses. However, it is not desirable from a practical point of view or for safety reasons for the buttresses to reach their full capacity. A fusing element is introduced to the top connection between the steel box to which the post tensioning is anchored and the steel truss. The fusing element wilj yield at a predetermined load (55% - 75%g), thus preventing extensive damage to the buttresses and eventual collapse. The stressing losses ca1culated from the load cells' readings were 12.7% and 7%, respectively. This translates into an active force of66% and 61% Guts, respectively. The typical buttress was analyzed for an active force of 60.5% Guts or 111 kn. Conclusion The URM structure was checked at working load levei for loads according to UCBC(2), i.e.: 13%g for overall structural behavior. The post tensioned tower and buttresses were analysed using the capacity design method. The advantages of using post tensioning over conventional methods are numerous: 1 -Post tensioning does not alter the interior nor the exterior appearance ofthe structure. It preserves the historical value ofthe architecture. 2 -Forces from post tensioning are permanent and prevent masonry from opening up in future earthquakes. 3 -Post tensioning increases the shear capacity of masonry considerably and therefore earthquake forces are more efiectively resisted. 482
4 -Applied axial force from post tensioning will increase the structural ductility. 5 -Post tensioning is an econornical retrofit / strengthening method. The structural retrofit on the Holy Cross Church started on April 1, 1992. It took seven months to accomplish the project. The project was c\imaxed by lifting the 17.0 m lúgh steeple on the tlúrd anniversary of the Loma Prieta Earthquake, October 17, and placing it on top of the tower. Acknowledgment The author would like to thank Hans Ganz and Franco Lurati with VSL Intemational for their support, William T. Holmes and Bret Lizundia from Rutherford & Chekene, who acted as plan checkers on behalf of the City of Santa Cruz, and Ronald Hamburger from EQE for lús input. REFERENCES 1. "Post - Tensioning Masonry Structures", VSL Report Series No. 2, VSL Intemational Ltd., Beme, 1990, 35 pp. 2. "Uniform Code for Building Conservation", Appendix Chapter I "earthquake Hazard Reduction in Existing Unreinforced Masonry Buildings", Intemational Conference of Building Officials, Whittler, caiifornia, 1987, pp. 2i-33. 483
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