REINFORCED CONCRETE WALL BOUNDARY ELEMENT LONGITUDINAL REINFORCING TERMINATION

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1 1NCEE Tenth U.S. National Conference on Earthquake Engineering Frontiers of Earthquake Engineering July 1-, 1 Anchorage, Alaska REINFORCED CONCRETE WALL BOUNDARY ELEMENT LONGITUDINAL REINFORCING TERMINATION Sunai Kim 1 and John W. Wallace ABSTRACT ACI Chapter 1 and 1 requirements for termination of boundary longitudinal reinforcement in reinforced concrete structural walls are reviewed and alternative approaches are assessed. Two options for termination of boundary element longitudinal reinforcement, one per -11 provisions and another using a more aggressive termination per NIST Technical Brief No.6, are studied for,, 1, and 16 story structural walls by conducting nonlinear response history analyses. The impacts of the more aggressive reinforcement cut-offs are examined on building responses such as wall moment, shear, lateral displacements, and the distribution of plastic rotations over the wall height. The analysis results indicate that the more aggressive termination of boundary longitudinal reinforcement has very little impact on expected responses; there are insignificant changes to building drifts and the localized yielding that occurs for the more aggressive termination tends to moderately reduce wall moment and shear demands. Based on the findings, appropriate locations for termination of boundary element longitudinal reinforcement are recommended. 1 Ph.D. candidate, Dept. of Civil Engineering, University of California, Los Angeles, CA 99, sunaikim@gmail.com Professor, Dept. of Civil Engineering, University of California, Los Angeles, CA 99, wallacej@ucla.edu Sunai Kim, John W. Wallace. Proceedings of the 1 th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 1.

2 Reinforced Concrete Wall Boundary Element Longitudinal Reinforcing Termination Sunai Kim 1 and John W. Wallace ABSTRACT ACI Chapter 1 and 1 requirements for termination of boundary longitudinal reinforcement in reinforced concrete structural walls are reviewed and alternative approaches are assessed. Two options for termination of boundary element longitudinal reinforcement, one per -11 provisions and another using a more aggressive termination per NIST Technical Brief No.6, are studied for,, 1, and 16 story structural walls by conducting nonlinear response history analyses. The impacts of the more aggressive reinforcement cut-offs are examined on building responses such as wall moment, shear, lateral displacements, and the distribution of plastic rotations over the wall height. The analysis results indicate that the more aggressive termination of boundary longitudinal reinforcement has very little impact on expected responses; there are insignificant changes to building drifts and the localized yielding that occurs for the more aggressive termination tends to moderately reduce wall moment and shear demands. Based on the findings, appropriate locations for termination of boundary element longitudinal reinforcement are recommended. 1. Introduction Reinforced concrete structural shear walls are one of the most widely used vertical elements in seismic-force-resisting systems worldwide. Their widespread use is due to economy of construction, efficiency in separating spaces, and effectiveness in protecting buildings from strong earthquake ground shaking. Provisions for special structural walls are contained in ACI Sections through [1]. Design for flexure and axial loads are accomplished using a plane-section analysis per 1.9., whereas requirements for development and splicing of longitudinal reinforcement over the wall height are contained in The provisions in 1.9. refer to Chapter 1 provisions for beams; therefore, for termination of boundary longitudinal reinforcement, bars must extend d (distance from extreme compression fiber to centroid of tension reinforcement) or 1 d b (nominal diameter of reinforcement) beyond the point at which they are no longer required to resist flexure. For walls, d always controls, and (a) permits the use of d=.l w (length of wall). Thus, for moderate height buildings at less than 1 stories, with wall lengths of 3-ft, this requirement may require all boundary longitudinal reinforcement to extend over the full height of the building, which is excessive. The NIST Technical Brief No.6 [6] suggests an alternative approach, where boundary vertical reinforcement extends development length, l d, above the location where the bars are no longer required to resist flexure. This paper reviews current ACI Chapter 1 and 1 requirements and assesses an alternative approach per NIST recommendations to determine appropriate locations for termination of boundary longitudinal reinforcement by conducting nonlinear response history analyses (NRHA) on,, 1, and 16 story buildings. Special structural shear walls are utilized as the lateral-force-resisting system and termination locations of boundary longitudinal reinforcement are varied to assess the impact of reinforcement cut-offs on expected responses

3 such as wall moment and shear, wall lateral drifts and the distribution of wall plastic rotation demands over the wall height. Based on the findings, appropriate termination locations and cutoff lengths for boundary element longitudinal reinforcement are recommended.. Basis of Design Four reinforced concrete shear wall buildings,,, 1 and 16-stories in height, were chosen to systematically assess the impacts boundary element longitudinal reinforcement termination per -11 versus a more aggressive termination per NIST recommendations. Table 1 shows typical floor plan dimensions, number of walls in each direction and wall dimensions for the four buildings. All buildings have a first floor story height of 13 feet, and the floors above have a typical story height of 1 feet. The gravity system consists of two-way concrete slabs that are supported on square concrete columns and bearing shear walls. The lateral system consists of concrete shear walls in each direction. All concrete columns and shear walls are supported on concrete foundations. Table lists the load criteria used for the calculation of self-weight of the structure and superimposed dead and live loads. Table 1. Summary of building and shear wall dimensions Floor length, Floor width, Floor No. of walls Length of L (ft) B (ft) area (ft ) per direction wall, L w (ft) 1 1 1, , , , 3 No. of stories Thickness of wall, t w (in) Table. Load criteria Use Location Dead load (psf) Live load (psf) Office All levels below roof 1 reducible + 1 partition Roof Roof level 1 reducible Cladding Perimeter of building 1 vertical - 3. Code Design & Modeling Parameters The gravity and seismic design of,, 1, and 16-story buildings were conducted in compliance with IBC 9[], which adopts ASCE7-[3] and -11[1]. A linear modal response spectrum analysis was used for the seismic design of the four buildings. A location in downtown Los Angeles, California was chosen as a representative site with soil conditions corresponding to site class B. Table outlines the parameters used for the linear modal response spectrum analysis and to calculate base shear of the four buildings. The concrete strength and stiffness assumptions are listed in Table 3, and reinforcing strength is listed in Table.

4 Table 3. Concrete member strength & stiffness Concrete member Nominal f c Expected f c Nominal E Expected E Shear walls. ksi 6. ksi 3 ksi 9 ksi 1 Expected concrete material strength is 1.3 f c Table. Reinforcing strength Reinforcing member Nominal Expected yield 1 Expected ultimate ASTM A76 Grade 6 F y = 6 ksi F y = 7 ksi F u = 1 ksi 1 Expected reinforcing steel strength is 1.17 F y Table. Linear modal response spectrum analysis parameters and base shears Response spectrum analysis parameters S s.1g S DS 1.g S 1.g S D1.g F a 1. I 1. F v 1. R. -Story -Story 1-Story 16-Story Approximate period, T a (sec) Base shear coefficient, C s Seismic weight, W (kips) 13,3 17, 7,,3 Base shear, V (kips),6 1,97,3,6 Figure 1. Generalized force-deformation relations for concrete elements [] Non-linear Perform 3D [] models of the cantilever shear walls that were representative of lateral force resisting systems, were created for the,, 1 and 16 story buildings. The cantilever shear walls were modeled using beam-column elements with plastic hinges at the bottom and top of each floor. Moment-rotation parameters were based on ASCE1-6 Table 6-1 for shear walls controlled in flexure [], as shown in Table 6 and Fig.1. The seismic mass was lumped at each level, and the walls were constrained in the out of plane direction. The cantilever shear walls were modeled with fixed connections at the base of the building. No. of stories Table 6. Modeling parameters per ASCE1-6 Table 6-1 [] Plastic hinge Confined Failure rotation boundary Residual strength a b c.1. Yes Flexure Yes Flexure Yes Flexure Yes Flexure.1..7

5 . Boundary Element Longitudinal Reinforcing Termination The boundary element longitudinal reinforcement termination provisions are examined per current -11 provisions and per NIST Technical Brief No.6, as outlined in Table. The reinforcement termination lengths and extensions were calculated for the four buildings, and are shown in Table 9 and Figs st cut-off length 1 st extension from theoretical cut-off point All other extensions Table. Summary of -11 & NIST Technical Brief No.6 recommendations for boundary element longitudinal reinforcement termination -11 NIST Technical Brief No.6 ACI & (b) No cut-off within plastic hinge region; use upper bound of plastic hinge length = Max {Story Height,, } ACI.1..3(a) = Max {d, 1 d b } ACI 1.1.3: d =.L w = Max {.L w, 1 d b } Same as above ACI & (b) No cut-off within plastic hinge region; use upper bound of plastic hinge length = Max {Story Height,, } ACI In hinge region, develop tension reinforcing 1. l d Use development length, l d No. of Stories 1 16 Table 9. First cut-off lengths and reinforcing extensions Provisions First cut-off 1 st All other Reinforcement extension length extensions reduction (%) ACI NIST ACI NIST ACI NIST ACI NIST

6 Figure. Shear wall elevations of boundary element longitudinal reinforcing termination (a) -story per -11 (b) -story per NIST Technical Brief No.6 (c) -story per -11 (d) -story per NIST Technical Brief No.6 Figure 3. Shear wall elevations of boundary element longitudinal reinforcing termination (e) 1-story per -11 (f) 1-story per NIST Technical Brief No.6 (g) 16-story per -11 (h) 16-story per NIST Technical Brief No.6. Ground Motion Records Seven ground motion records were chosen to perform NRHA on the,, 1 and 16-story buildings. The ground motions were chosen to exceed the ASCE7- design level ground

7 shaking for building periods between.6 to.6 seconds, but to be less than the maximum considered earthquake (MCE) level ground shaking. Table 7 lists the ground motion records utilized and Figs. - shows pseudo-spectral acceleration response spectra and spectral displacements for all 7 earthquakes, mean and mean ± standard deviation of 7 earthquakes, and ASCE7- MCE and design level earthquakes. Further studies will include NRHA on the four buildings utilizing ground motion records that are scaled to match ASCE7- MCE and design levels. The following load combination was used to conduct NRHA: 1. D + L exp ± E (1) where D, L exp and E represent dead load, % of unreduced live load and effects of ground motions, respectively. Modal damping of % was used for all modes. Table 7. Ground motion records Record Record PGA PGD Earthquake name Station No. ID (g) (in) 1 P9 199 Cape Mendocino 916 Petrolia P Chi-Chi, Taiwan TCU P Duzce, Turkey Duzce..3 P6 19 Imperial Valley 117 El Centro Array #9.31. P Kocaeli, Turkley Yarimca P Loma Prieta 731 Gilroy Array # P7 199 Loma Prieta 71 Capitola Pseudo-Spectral acceleration (g) P9 Cape Mendocino P1 Chi-Chi, Taiwan P1 Duzce, Turkey P6 Imperial Valley P111 Kocaeli, Turkey P736 Loma Prieta P7 Loma Prieta Mean Mean ± σ ASCE7- MCE ASCE7- Design Period (sec) Figure. Pseudo-spectral acceleration response spectra for ASCE7- MCE and design levels, and mean of 7 earthquakes

8 Spectral displacement (in) 1 1 P9 Cape Mendocino P1 Chi-Chi, Taiwan P1 Duzce, Turkey P6 Imperial Valley P111 Kocaeli, Turkey P736 Loma Prieta P7 Loma Prieta Mean Mean±σ ASCE7- MCE ASCE7- Design Period (sec) Figure. Spectral displacement versus period for ASCE7- MCE, design levels and mean of 7 earthquakes 6. Results and Discussion The analysis results indicate that more aggressive termination of boundary longitudinal reinforcement per NIST guidelines has very little impact on expected responses. Table shows the building periods and accumulated mass participation factors per -11 and NIST Technical Brief No.6. Figs. 6-9 show the results of,, 1 and 16-story mean building responses of drift, moment, wall rotations and shear. In general, the localized yielding of the wall has insignificant (less than %) effects on building drifts and tends to moderate wall moment and shear demands (less than % at base). The effects of diagonal cracking and tension shift were not considered in this study since yielding was considered over the height of the walls for all,, 1 and 16-story buildings. Tension shift is only considered for elastic walls by vertically shifting the nominal moment capacity diagrams. No. of stories 1 16 Table. Building periods and accumulated mass participation factors Provisions 1 st Mode Period nd Mode Period 3 rd Mode Period (mass participation) (mass participation) (mass participation) ACI.67 sec (.7).16 sec (.96). sec (.99) NIST.67 sec (.7).16 sec (.96). sec (.99) ACI 1.1 sec (.6). sec (.91).1 sec (.97) NIST 1.1 sec (.6).3 sec (.91).1 sec (.97) ACI 1. sec (.66).3 sec (.).1 sec (.9) NIST 1. sec (.66).3 sec (.).1 sec (.9) ACI.3 sec (.6).3 sec (.).17 sec (.9) NIST.3 sec (.6).3 sec (.).17 sec (.9)

9 (a) (b) (c) Drift 3 1 ASCE1 θ Limit (d) Mu: Mn: Mu: Mn: Moment (kip-in) x Rotation Shear (kips) Figure 6. -Story building responses: comparison of boundary element longitudinal reinforcing termination per -11 and NIST (a) mean building drifts (b) mean moment demands and capacities (c) mean shear wall rotations (d) mean shear demands (a) (b) (c) Drift 6 ASCE1 θ Limit (d) 6 Mu: Mn: Mu: Mn: Moment (kip-in) x Rotation Shear (kips) Figure 7. -Story building responses: comparison of boundary element longitudinal reinforcing termination per -11 and NIST (a) mean building drifts (b) mean moment demands and capacities (c) mean shear wall rotations (d) mean shear demands

10 (a) (c) Drift 1 ASCE1 θ Limit (b) (d) 1 1 Mu: Mn: Mu: Mn: Moment (kip-in) x Rotation Shear (kips) Figure. 1-Story building responses: comparison of boundary element longitudinal reinforcing termination per -11 and NIST (a) mean building drifts (b) mean moment demands and capacities (c) mean shear wall rotations (d) mean shear demands (a) 1 (b) Drift c) 1 (d) 1 ASCE1 θ Limit 1 Mu: Mn: Mu: Mn: Moment (kip-in) x Rotation Shear (kips) Figure Story building responses: comparison of boundary element longitudinal reinforcing termination per -11 and NIST (a) mean building drifts (b) mean moment demands and capacities (c) mean shear wall rotations (d) mean shear demands

11 Conclusions The boundary element longitudinal reinforcement termination requirements are reviewed per -11 provisions and per NIST Technical Brief No.6 recommendations. Both - 11 and the NIST Technical Brief require the first termination location to be above the plastic hinge region, which is taken as the maximum of story height, L w / and M u /( V u ) per ACI From the first termination location, ACI requires reinforcement to be extended to a larger value of d or 1 d b, where d is taken as.l w per -11 Chapter 1, whereas NIST recommends reinforcement to be extended 1. l d in the plastic hinge region, and l d in all other regions. The two procedures are examined for,, 1 and 16-story cantilever shear walls using nonlinear response history analysis. Based on the findings, the following conclusions are reached: Analysis results show that the more aggressive reinforcement termination per NIST recommendations has very little impact on shear wall building drifts (< % change), and the localized yielding throughout the shear wall height tends to moderately reduce shear wall moment and shear demands (reduction less than % at base). For short to moderate height buildings (less than stories), boundary element longitudinal reinforcement termination is recommended per NIST Technical Brief No.6. Reinforcing termination per NIST Technical Brief No.6 reduces the amount of boundary element longitudinal reinforcing by approximately %, compared to reinforcing termination per -11. Further studies are needed to study the impact of more aggressive reinforcement termination in taller buildings, which typically employ coupled shear walls. References 1. American Concrete Institute (11). Building Code Requirements for Structural Concrete (ACI 31-11) and Commentary (R-11), Farmington Hills, MI.. American Society of Civil Engineers (7). ASCE/SEI Standard 1-6, Seismic Rehabilitation of Existing Buildings, Reston, VA. 3. American Society of Civil Engineers (6). ASCE7-, Minimum Design Loads for Buildings and Other Structures, Including Supplement N.1, Reston, VA.. Computers and Structures (11). Perform 3D Version... Nonlinear Analysis and Performance Assessment of 3D Structures, Inc. CSI: Berkeley, CA.. International Code Council (9). 9 International Building Code, Falls Church, VA. 6. Moehle, J.P., Ghodsi, T., Hooper, J.D., Fields, D.C., Gedhada, R., (11). Seismic Design of Cast-in-Place Concrete Special Structural Walls and Coupling Beams, Seismic Design Technical Brief No.6, produced by the Consultants Joint Venture, a partnership of the Applied Technology Council and the Consortium of Universities for Research in Earthquake Engineering, for the National Institute of Standards and Technology, Gaithersburg, MD, NIST GCR REV-1.