Strength and Serviceability of Concrete Elements Reinforced with High-Strength Steel

The potential use of high-strength steel reinforcement (HSSR) with yield stress () larger than 80 ksi requires reconsideration of design criteria and limits used to avoid sudden failure because they were conceived for elements with Gr. 60 and 80 (=60 and 80 ksi) reinforcement. The objective of this study is to investigate the feasibility of using HSSR for applications not related to seismic demands, with a focus on three topics: (i) minimum flexural reinforcement in slabs and walls, (ii) maximum flexural reinforcement in beams, and (iii) crack widths and deflections. Nine slabs with gross longitudinal reinforcement ratio () between 0.07% and 0.18%, four walls with between 0.07% and 0.24%, and six continuous beams with longitudinal reinforcement ratio between 0.8% and 2.5% and a net tensile strain of approximately 0.005 were tested to study the strength, toughness, and serviceability of elements with HSSR. The test data indicated that it is feasible to use HSSR (with up to 120 ksi). The following conclusions are made in relation to the three specific topics investigated: 1) Minimum longitudinal reinforcement ratio for slabs can be reduced in inverse proportion to increase in yield stress. Test slabs with HSSR and as small as 0.09% had rotational capacities larger than 4%. Nevertheless prudence is due because brittle failure may occur at small displacements in lightly reinforced elements in which cracking moment approaches or exceeds yield moment and because low shear strength has been observed to be associated with low ratios of longitudinal reinforcement. 2) Current minimum longitudinal reinforcement limits in walls with uniformly distributed reinforcement, even for Gr.60 reinforcement, should be reconsidered. Specimens with Gr. 60 and Gr.120 bars, and ranging between 0.07% and 0.24% failed at drift capacities smaller than 1%. In walls with products of gross reinforcement ratio and measured yield stress smaller than approximately 200 psi, failure took place just after first cracking and at loads smaller than those at first cracking suggesting that elements in this range should be avoided in all structures no matter how low the probability of cracking is estimated to be. This is especially critical for ordinary structural walls without confined boundary elements in seismic regions. 3) In the tests conducted, sections with HSSR designed to have net tensile strain () of 0.005 had sufficient rotational capacity for moment redistribution. In design, detailing to accommodate increases in shear and bond demands caused by load redistribution is needed and may be as critical as controlling maximum reinforcement ratio through to avoid brittle failure. 4) Test data suggest that extrapolation of minimum thickness for one-way slabs with HSSR as the thickness required for an element with working stress of 40 ksi multiplied by a factor related to working stress in the reinforcing bars (), 0.4+ 3()200 , would lead to acceptable immediate and long-term deflections. 5) According to the measurements made, reduction in bar spacing (s) with increase in working stress in the reinforcing bars (), determined as =15(40,000())−2.5∗, would be sufficient to avoid intolerable crack widths even in elements with working stresses up to 80 ksi

High Strength Steel in the Reinforced Concrete Structures: Serviceability

The use of high-strength steel (yield stress larger than 100 ksi) in reinforced concrete structures can provide an effective alternative to using conventional steel (yield stress up to 80 ksi). The goal of this study is to investigate if slabs with reduced quantities of high-strength steel reinforcement meet the serviceability criteria. Instantaneous and long-term deflections in slab specimens with conventional steel and reduced amounts of high-strength steel (as compared with conventional steel) were studied. Two sets of two reinforced concrete slabs, each 14 ft. long, 30 in. wide, were built. The depth, quantity, and type of longitudinal reinforcement were varied between specimens in each set. The first set of slabs consisted of a 4 in. thick specimen with conventional steel (Gr. 60) and a 6 in. thick specimen with high-strength steel (Gr. 120). The longitudinal reinforcement ratios were 0.7% and 0.2% respectively. The second set consisted of a 5 in. thick specimen with Gr. 60 steel and an 8 in. thick specimen with Gr. 120 steel. The longitudinal reinforcement ratios were 0.5% and 0.1% respectively. The slabs were designed such that the theoretical deflections at service loads were similar for the specimens within each set. The slabs will be loaded until the working strains are reached in the reinforcement, and then will be tested under this constant service-load

Strength of Reinforced Concrete Beams with High-Strength Steel

Structures are commonly made of reinforced concrete, which is a composite material made of concrete and steel reinforcement. Using high-strength steel, with yield stress larger than 100 ksi, could help reduce the quantity of steel required in structural members, thus reducing costs and improving constructability. The hypothesis being tested is that smaller quantities of high-strength steel reinforcement (HSSR) can be used in place of conventional steel in reinforced concrete beams while maintaining similar strength and deformation at failure. Two reinforced concrete beams with two different types of longitudinal steel reinforcement were constructed. The beams were 18 in. wide, 30 in. deep and 58.5 ft long. The beam with HSSR had approximately half the quantity of longitudinal reinforcement leading to reduced material costs and simpler construction. Numerical analyses indicate that the two beams will have comparable strengths and deformation capacities indicating that conventional steel can be replaced by HSSR

The Institute’s Team for Damage Investigations

ACI Committee 133, Disaster Reconnaissance, was conceived in the aftermath of the 2010 Chilean Earthquake, an event that affected thousands of structures. That event caused extensive damage to an estimated 50 to 100 mid-rise and high-rise reinforced concrete (RC) buildings, including seven that were damaged beyond repair.1-3 Although ACI has had a strong history of publishing assessments of disasters (refer to textbox: Historical Disasters Examined in ACI Publications), the Institute had no formal mechanism in place to deploy a team to investigate and report on critical lessons to its technical committees and membership. Furthermore, the broadening international reach of the ACI 318 Building Code, which has been adopted or referenced in the national code of more than 30 countries, including Chile,4 highlighted the need for ACI liaisons to be on the ground immediately after a disaster to serve as a technical resource to local engineers. Recognizing these needs, former ACI Committee 318 Chair Jack Moehle consulted with former ACI Presidents José Izquierdo-Encarnación and Luis García about the formation of a committee with a disaster reconnaissance directive. In October 2012, a proposal was submitted to the ACI Board of Direction to establish and fund a new committee with the primary objectives of: Providing a mechanism for evaluating the application of ACI documents internationally; and Disseminating deployment findings to ACI technical committees and through ACI publications.5 To date, the Chairs of the resulting committee, ACI Committee 133, have included Jack Moehle, Ken Elwood, Michael Kreger, and Santiago Pujol. This committee has actively engaged a diverse group of practitioners and researchers

Observations About the Seismic Response of RC Buildings in Mexico City

Over 2000 buildings were surveyed by members of the Colegio de Ingenieros (CICM) and Sociedad Mexicana de Ingenieria Estructural (SMIE) in Mexico City following the Puebla-Morelos Earthquake of 2017. This inventory of surveyed buildings included nearly 40 collapses and over 600 buildings deemed to have structural damage. Correlation of damage with peak ground acceleration (PGA), peak ground velocity (PGV), predominant spectral period, building location, and building properties including height, estimated stiffness, and presence of walls or retrofits was investigated for the surveyed buildings. The evidence available suggests that (1) ground motion intensity (PGV) drove the occurrence of damage and (2) buildings with more infill and stiff retrofit systems did better than other buildings