The Collapse of the Alto Río Building during the 27 February 2010 Maule, Chile, Earthquake

This is the published version. Copyright 2012 Earthquake Engineering Research InstituteThe Alto Río Building, a 15-story building located in Concepción, Chile, collapsed during the 2010 Maule earthquake. Construction of the building was completed in 2009 following the Chilean building code of 1996. The building was provided with reinforced concrete structural walls (occupying nearly 7% of the floor area) to resist lateral and vertical loads. The walls failed in the first story, causing the overturning of the entire building. This paper provides detailed field observations and discusses plausible causes of the collapse

Recommended Provisions and Commentary on Development Length for High-Strength Reinforcement in Tension

Design provisions on development length for straight reinforcing bars in tension are presented in code format and compared with those in ACI 318-14 (Building Code Requirements for Structural Concrete) and fib MC2010 (fib Model Code for Concrete Structures). The proposed provisions are based on a simplified version of the design equation in ACI 408R-03 (Bond and Development of Straight Reinforcing Bars in Tension) extended to apply to high-strength concrete up to 110 MPa (16,000 psi) and high-strength reinforcement up to 1070 MPa (155,000 psi). Compared with those in ACI 318-14 and fib MC2010, the recommended provisions produce designs with improved reliability and longer development lengths for conditions of low confinement or low concrete cover, but generally shorter development lengths for bars with higher degrees of confinement and wider spacing between bars. The recommended development length design equation gives values within 10% of those obtained from the design equation in ACI 408R-03

Diagonally Reinforced Concrete Coupling Beams: Effects of Axial Restraint

Diagonally reinforced concrete coupling beams are commonly used in mid- and high-rise buildings to connect adjacent structural walls separated by openings. Under lateral loading, these beams can undergo large inelastic deformations but must retain their strength for the system to behave as desired. It is not known how or whether resistance to axial elongation of the beams, provided by the floor diaphragms and stiff structural walls, affects the strength and deformation capacity of coupling beams. The influence of axial restraint on diagonally reinforced concrete coupling beams was investigated by comparing the results of an axially-restrained coupling beam specimen with those of a nominally equivalent control specimen tested without axial restraint but using the same loading protocol. The presence of axial restraint increased the strength of the coupling beam specimen by approximately 30%, decreased the chord rotation capacity by approximately 10%, and resulted in buckling of diagonal reinforcement at smaller chord rotations

Diagonally Reinforced Concrete Coupling Beams with Grade 120 (830) High-Strength Steel Bars

Four large-scale coupling beams were tested under fully reversed cyclic loads to investigate the effects on behavior of diagonal bar grade (60 or 120 [420 or 830]), beam shear stress (9.5 or 14√fc′, psi [0.79 or 1.17√fc′, MPa]), and longitudinal bar detailing (either terminated near the beam-wall interface or developed into the walls). Coupling beam chord rotation capacity was 7.1% for the beam with Grade 60 (420) bars and between 5.1 and 5.6% for the beams with Grade 120 (830) bars, a difference likely due to having hoops spaced at 3.4db and 4db for Grade 60 and 120 (420 and 830) bars, where db is the diagonal bar diameter. Effective stiffness, energy dissipation, and residual chord rotations were approximately inversely proportional to bar grade. Developing the secondary longitudinal reinforcement reduced rotation demands at beam ends but did not improve deformation capacity. Beam shear stress did not affect beam chord rotation capacity

Diagonally-Reinforced Concrete Coupling Beams with High-Strength Steel Bars

The use of high-strength steel in diagonally reinforced coupling beams was investigated with the aims of minimizing reinforcement congestion and increasing the maximum permissible design shear stress without compromising behavior under large displacement reversals. Five large-scale diagonally reinforced concrete coupling beam specimens with clear span-to-depth ratios of 1.9 were tested under fully reversed cyclic loads. The primary variables were yield stress of the diagonal reinforcement (60 and 120 ksi [420 and 830 MPa]), target beam shear stress (1.0 and 1.5 times the ACI Building Code limit), length of the secondary (non-diagonal) longitudinal reinforcement, and axial restraint. All specimens had the same nominal concrete compressive strength and beam dimensions. Chord rotation capacities exhibited by the specimens with Grade 120 (830) reinforcement were between 5.1 and 5.6%, less than that of the control specimen with Grade 60 (420) diagonal reinforcement (7.1%). Neither development of secondary reinforcement nor increases in design shear stress affected specimen chord rotation capacity. The axially-restrained specimen with Grade 120 (830) diagonal reinforcement showed the same chord rotation capacity as a similar specimen without axial restraint, but 14% larger strength. In specimens with secondary longitudinal reinforcement extended into the wall (such that the embedment length exceeded the calculated development length), the localization of damage evident along the beam-wall interface in tests of specimens with bars terminating near the wall face was not observed. Although damage was more distributed throughout the beam span, deformation capacity was not increased. Among the specimens, it was shown that the initial stiffness, area of the shear force-chord rotation hysteresis cycles, and residual chord rotation at zero shear force changed in inverse proportion to the diagonal bar yield stress. A database of results from tests of diagonally reinforced coupling beams was compiled and used to evaluate the sensitivity of coupling beam chord rotation capacity to a range of variables. Variables included aspect ratio, reinforcement grade, transverse confinement reinforcement (type, spacing, and ratio), shear stress, and length of secondary (non-diagonal) reinforcement (whether terminated near the beam-wall interface or developed into the wall). An equation was proposed for calculating coupling beam chord rotation capacity as a function of beam clear span-to-height ratio and the ratio of hoop spacing to diagonal bar diameter. Chord rotation capacity was not correlated with other variables. Modifications are also proposed to the stiffness and deformation capacity modeling parameters recommended in ASCE 41-17 and ACI 369.1-17 for diagonally reinforced coupling beams to account for reinforcement grade

Headed Bars in Beam-Column Joints Subjected to Reversed Cyclic Loading

Descriptive equations developed for the anchorage strength of headed bars in beam-column joints under monotonic load are evaluated for beam-column joints subjected to reversed cyclic loading. Test results from 23 studies that include 84 exterior and seven roof-level interior beam-column joints are used in the evaluation. Concrete compressive strengths and reinforcement yield strengths ranged from 3480 to 21,500 psi (24 to 148 MPa) and 53,700 to 150,000 psi (370 to 1030 MPa), respectively. Headed bar sizes ranged from slightly smaller than a No. 4 (No. 13) to No. 11 (No. 36) with net-bearing areas ranging from 1.7 to 8.6 times the bar area. The embedment lengths and center-to-center spacing between the headed bars ranged from eight to 18 bar diameters and from two to eight bar diameters, respectively. Analysis of the test data shows that descriptive equations based on headed bars under monotonic loading are also applicable to headed bars in beam-column joints subjected to reversed cyclic loading. These comparisons were used to justify the single approach used within the ACI Building Code for calculating the development length of headed bars

Earthquake-Resistant T-Shaped Concrete Walls with High-Strength Steel Bars

This study examined the effects of the mechanical properties of high-strength reinforcement on the seismic behavior of concrete walls. The primary variables were the nominal yield strength fy, 100 ksi (690 MPa) or 120 ksi (830 MPa), and the tensile-to-yield strength ratio ft/fy, nominally 1.2 or 1.3. Two large-scale T-shaped structural walls were subjected to reversed cyclic loading to assess their strength and deformation capacity. Test results were compared with data from four walls tested by Huq et al. (2017) at The University of Kansas to evaluate the influence of the uniform elongation εsu, and the fracture elongation εsf, in addition to fy and ft/fy, of high-strength reinforcement on the deformation capacity of concrete walls subjected to reversed cyclic displacements. The four walls tested by Huq et al. (2017) had nearly identical geometry, detailing, test setup, and loading protocol to the two walls of this study, but had different reinforcement mechanical properties. Two walls were tested, one with Grade 120 (830) reinforcement (Wall T5), the other with Grade 100 (690) reinforcement (Wall T6). Confined boundary elements were provided at the three tips of the T section consisting of the main flexural reinforcement (No. 6 or 19 mm bars) enclosed by No. 3 (10 mm) hoops. Outside the boundary elements, No. 4 (13 mm) bars were used as longitudinal and transverse reinforcement. The nominal concrete compressive strength of 8 ksi (55 MPa) and wall dimensions were kept constant in both specimens. The walls had a thickness of 10 in. (25 mm) and height-to-length ratio of 3. Wall stem and flanges were 100-in. (2540-mm) long. The axial load was only the self-weight and the weight of the testing apparatus. The T-shaped cross section allowed a shallow neutral axis depth (within the flange) at flexural nominal strength and induced high tensile strain demands in the main flexural reinforcement (within the stem). The walls were designed such that flexural behavior controlled their strength inducing a maximum shear stress of approximately 4√f’c, psi (0.33√f’c, MPa). The design complied with the ACI Building Code (ACI 318-14) and incorporated the additional detailing recommendations in ATC 115 for Grade 100 reinforcement. Wall T6 with Grade 100 (690) reinforcement had similar strength and deformation capacity to the four walls tested by Huq et al. (2017) at The University of Kansas with Grade 60 (420) reinforcement in T1 and Grade 100 (690) reinforcement in T2, T3, and T4. These walls had a drift ratio capacity not less than 3% if the tensile-to-yield strength ratio (ft/fy) of the flexural reinforcement was greater than 1.18, the uniform elongation (εsu) was greater than 6%, and the fracture elongation (εsf) was greater than 10%. Wall T5 had a drift ratio capacity of 2.3% with Grade 120 (830) flexural reinforcement having ft/fy = 1.32, εsu = 5.3%, and εsf = 8.6%. Moment-curvature analyses were conducted to support the development of closed-form solutions for estimating the deformation capacity of the walls and strain demands on reinforcing bars and concrete. Formulations were derived to include deformations due to shear and strain penetration (or bond slip) to provide conservative (safe) estimates of deformation capacity and strain demands.Commercial Metals CompanyMMFX Technologies Corporatio

Diagonally Reinforced Concrete Coupling Beams: Effects of Axial Restraint

Two pairs of nominally identical large-scale coupling beam specimens were tested under reversed cyclic displacements. Within each pair, one specimen was free to elongate and the other had resistance to elongation during testing. The specimens had clear span-to-overall-depth ratios of 1.9, a nominal concrete compressive strength of 6000 psi (42 MPa), Grade 60 or 120 (420 or 830) diagonal bars, and nominal shear stresses near the ACI Building Code (ACI 318) limit of 10√fc′ psi (0.83√fc′ MPa). Passive axial restraint resulted in beam axial forces and was correlated with higher coupling beam strength, lower chord rotation capacity, earlier diagonal bar buckling, and greater damage. The importance of these effects increased with the magnitude of the induced axial force. The ACI equation for coupling beam nominal strength (based on the area, yield stress, and inclination of diagonal bars) underestimated beam strength by up to 80%, whereas estimates based on flexural strength were substantially more accurate and allowed consideration of axial force effects

Reinforced Concrete Coupling Beams with High-Strength Steel Bars

The use of high-strength steel bars in reinforced concrete coupling beams has the potential to reduce reinforcement congestion and support more efficient design and construction methods. A series of tests was conducted to investigate the effects of high-strength reinforcement on coupling beam behavior. Eleven large-scale coupling beam specimens were tested under fully reversed cyclic displacements of increasing magnitude. The main variables of the test program included: yield stress of the primary longitudinal reinforcement (Grades 80, 100, and 120 [550, 690, and 830]), span-to-depth (aspect) ratio (1.5, 2.5, and 3.5), and layout of the primary longitudinal reinforcement (diagonal [D] and parallel [P]). All beams had the same nominal concrete compressive strength (8,000 psi [55 MPa]) and cross-sectional dimensions (12 by 18 in. [310 by 460 mm]). Beams were designed for target shear stresses of 8√f’c psi (0.67√f’c) for D-type beams and 6√f’c psi (0.5√f’c) for P- type beams. Transverse reinforcement was Grade 80 (550) in all but one beam, which had Grade 120 (830) reinforcement. The test program is documented by presenting the details of specimen construction, test setup, instrumentation, and loading protocol. Documentation of test data includes material properties, cyclic force-deformation response, progression of damage, calculated and measured strengths, initial stiffness, and measured reinforcement strains. Analysis of test data includes hysteretic energy, changes in beam length and depth, components of chord rotation, and the development of an equation for estimating chord rotation capacity.Charles Pankow FoundationACI Foundation’s Concrete Research CouncilConcrete Reinforcing Steel Institut

Lap Splicing of Large High-Strength Steel Reinforcing Bars

Electric Power Research InstituteCommercial Metals CompanyDayton Superio