Engineering Structures 26 (2004) 2081–2099 www.elsevier.com/locate/engstruct Seismic vulnerability,behavior and design of tunnel form building structures Can Balkaya a,Erol Kalkan b,à a Department of Civil Engineering, Middle East Technical University, Ankara 06531, Turkey b Department of Civil and Environmental Engineering, University of California Davis, Davis, CA 95616, USA Received 28 March 2003; received in revised form 3 July 2004; accepted 14 July 2004 Abstract Multi-story reinforced concrete tunnel form buildings are one of the common structural types in regions prone to high seismic risk due to the buildings inherent earthquake resistance and ease of construction. Despite their good performance during earth- quakes in 1999 in Turkey,and abundance of such structures scattered worldwide,current seismic codes and design provisions provide insufficient guidelines for their seismic design. As a compensatory measure,a series of modal and nonlinear static analy- ses are conducted by emphasizing the characteristic dynamic behavior of tunnel form buildings including impacts of wall-to-wall and wall-to-slab interaction and effects of torsion and wall-openings on the load transfer mechanism and seismic performance. A new formula for explicit determination of their fundamental period is developed in addition to a recommended response reduction factor and reinforcement detailing around shear-wall openings. # 2004 Elsevier Ltd. All rights reserved. Keywords: Tunnel form building; Inelasticity; Shear-wall; Pushover analysis; Response modification factor; Fundamental period; Reinforcement 1. Introduction torsion. Such a strict shear-wall configuration in the plan and throughout the height of the building may Multi-story reinforced concrete (RC) tunnel form limit the interior space use from an architectural point buildings (i.e.,box type buildings) are been increasingly of view,and this is one of the disadvantages of tunnel constructed worldwide. The main components of a tun- form buildings. During construction,walls and slabs, nel form system are its relatively thinner shear-walls having almost the same thickness,are cast in a single and flat-slabs compared to those of traditional RC operation. This process reduces not only the number of buildings. Shear-walls in tunnel form buildings are uti- cold-formed joints,but also the assembly time. The lized as the primary lateral load resisting and vertical simultaneous casting of walls and slabs results in load carrying members due to the absence of beams monolithic structures unlike any other frame-type RC and columns. Typical implementation of the tunnel buildings. Consequently,tunnel form buildings gain form system and its details are exhibited in Fig. 1.Ina enhanced seismic performance by retarding plastic tunnel form system,load carrying pre-cast members hinge formations at the most critical locations,such as are avoided,whereas nonstructural pre-cast elements slab–wall connections and around wall openings. such as RC stairs and outside facade panels are com- Seismic performances of tunnel form buildings have monly used to expedite construction. Continuity of been observed during earthquakes (MW 7.4 Kocaeli shear-walls throughout the height is recommended to and MW 7.2 Duzce) in Turkey in 1999. These earth- avoid local stress concentrations and to minimize quakes struck the most populated areas,and caused substantial structural damage,casualties and economic loss. However,in the aftermath of these events,neither à Corresponding author. Tel.: +1-530-754-4958; fax: +1-530-752- 7872. demolished nor damaged tunnel form buildings located E-mail address: [email protected] (E. Kalkan). in the vicinity of the worst-hit regions were reported in 0141-0296/$ - see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.engstruct.2004.07.005 2082 C. Balkaya, E. Kalkan / Engineering Structures 26 (2004) 2081–2099 Fig. 1. Tunnel form construction technique and its special formwork system. contrast to the severely damaged conditions of many of the 3D behavior,diaphragm flexibility,slab–wall conventional RC buildings. Such a creditable perform- interaction and torsion. The stress concentration and ance of tunnel form buildings has aided their construc- shear flow around the shear-wall openings and their tion in Turkey as a replacement of many severely reinforcement detailing were also studied. The results damaged and collapsed RC buildings. Not only in obtained from the 3D models were compared with Turkey,but also in many other countries prone to seis- those of the 2D models. In the final part,the value of a mic risk,tunnel form buildings are gaining increasing consistent response modification factor (R-factor) is popularity. This accentuates an urgent need to clarify introduced for a typical tunnel form building. seismic behavior,design and safety issues of these buildings. In this study,consistency of code-based empirical 2. A simple formula development for fundamental formulas to estimate the fundamental period of build- period estimation ings was evaluated for tunnel form buildings. The com- parative analysis results reveal that common formulas It is a customary practice to obtain the lower bound involving the Turkish Seismic Code,TSC [27] and the fundamental period of a structure via code-given Uniform Building Code [23] may yield inaccurate expressions to establish the proper design force level results for explicit determination of their fundamental unless modal analysis based on the detailed finite period. Based on the premise that such formulas are element model is conducted. Therefore,accurate esti- commonly used in engineering practice,a new predic- mation of the fundamental period is inevitably essential tive equation is proposed in this paper. This equation to calculate the reliable design forces. It has long been was developed based on the finite element analysis of recognized that significant errors tend to occur when 140 buildings having a variety of plans,heights and the code-given equations such as those given in the wall-configurations. This equation and the values of its UBC and the TSC are utilized for shear-wall dominant estimating parameters are introduced in the first part of systems [11,25]. To compensate for this deficiency,Lee the paper. The seismic performance evaluation is next et al. [25] proposed a simple formula based on their presented based on the inelastic static analyses of two experimental data to estimate the lower bound funda- representative case studies. To accomplish detailed mental period of tunnel form buildings having stories three-dimensional (3D) analyses on shear-wall domi- 15. A set of new formulas to estimate the period of nant systems,a nonlinear isoparametric shell element such buildings having stories 15 has been recently having opening–closing and rotating crack capabilities developed by Balkaya and Kalkan [11]. The objective was utilized. Thus,the seismic behavior of tunnel form here is to present updated information on the period of buildings was investigated more efficiently without tunnel form buildings using an extended building necessitating any simplifications in the finite element inventory as a continuation of our earlier work. In this models (e.g.,use of a rigid beam as a link element and/ paper,a simpler formula that can be applicable or a wide beam–column element for shear-wall model- for both mid-rise (story level 15) and high-rise ing). This efficiency further facilitated the investigation (story level > 15) tunnel form buildings is developed C. Balkaya, E. Kalkan / Engineering Structures 26 (2004) 2081–2099 2083 Table 1 Structural and dynamic properties of tunnel form buildings Plan No. No. of Height Dimension (m) Shear-wall area (m2) FEM results Predicted period, T (s) story (m) Length Width Length Width T (s) First mode Eq. (1) TSC98 UBC97 1 5 14.0 29.70 15.70 4.78 17.80 0.13 Long. 0.27 0.17 0.17 10 28.0 29.70 15.70 4.78 17.80 0.29 0.53 0.38 0.37 12 33.6 29.70 15.70 4.78 17.80 0.37 0.64 0.45 0.44 15 42.0 29.70 15.70 4.78 17.80 0.49 0.80 0.55 0.54 18 50.4 29.70 15.70 5.98 22.25 0.70 1.05 0.57 0.57 20 56.0 29.70 15.70 7.97 29.67 0.74 1.31 0.54 0.54 25 70.0 29.70 15.70 7.97 29.67 1.03 1.64 0.65 0.64 2 5 14.0 31.04 19.92 3.40 19.92 0.12 Long. 0.20 0.15 0.15 10 28.0 31.04 19.92 3.40 19.92 0.28 0.40 0.35 0.35 12 33.6 31.04 19.92 3.40 19.92 0.35 0.48 0.42 0.42 15 42.0 31.04 19.92 3.40 19.92 0.47 0.60 0.52 0.52 18 50.4 31.04 19.92 4.25 24.90 0.58 0.79 0.55 0.54 20 56.0 31.04 19.92 5.67 33.20 0.64 0.99 0.52 0.52 25 70.0 31.04 19.92 5.67 33.20 0.95 1.24 0.63 0.62 3 5 14.0 38.80 17.03 3.98 19.60 0.14 Long. 0.25 0.18 0.18 10 28.0 38.80 17.03 3.98 19.60 0.31 0.49 0.39 0.39 12 33.6 38.80 17.03 3.98 19.60 0.39 0.59 0.47 0.46 15 42.0 38.80 17.03 3.98 19.60 0.50 0.74 0.57 0.57 18 50.4 38.80 17.03 4.98 24.50 0.59 0.97 0.60 0.59 20 56.0 38.80 17.03 6.64 32.67 0.64 1.21 0.57 0.56 25 70.0 38.80 17.03 6.64 32.67 0.93 1.51 0.68 0.67 4 5 14.0 12.00 8.00 1.44 2.88 0.14 Trans.