Results of an experimental program on the anchorage strength of headed reinforcing bars are presented. Two hundred and two exterior beam-column joint specimens with concrete compressive strengths ranging from 3960 to 16,030 psi (27.3 to 110.6 MPa) were tested under monotonic loading. Key parameters included concrete compressive strength, embedment length, bar size, head size, spacing between headed bars, and confining reinforcement within the joint region. Bar stresses at failure ranged from 26,100 to 153,200 psi (180 to 1057 MPa). Specimens exhibited concrete breakout, side-face blowout, or a combination of the two failure modes, with concrete breakout being the dominant failure mode. A comparison of bar stress at anchorage failure with the stress calculated based on ACI 318-14 shows that ACI 318-14 provides a very conservative estimate of anchorage strength for No. 5 (No. 16) bars and low concrete compressive strengths. The estimate becomes progressively less conservative with increasing bar size and concrete compressive strength.
Equations to characterize the anchorage strength of headed bars were developed, incorporating key factors affecting anchorage strength: concrete compressive strength; embedment length; bar diameter; spacing between the bars; and confining reinforcement parallel to the headed bars. Results from tests of 138 exterior beam-column joints, 64 without and 74 with confining reinforcement within the joint region, were used to develop the equations. Concrete compressive strengths ranged from 4050 to 16,030 psi (27.9 to 110.6 MPa) and bar stresses at failure ranged from 33,100 to 153,160 psi (228 to 1056 MPa). The bearing area of the headed bars ranged from 3.8 to 9.5 times the area of the bar. Some headed bars contained obstructions adjacent to the head that exceeded the dimensions permitted for HA heads in ACI 318-14 and ASTM A970-13a but are now permitted by ASTM A970-18. The test results show that headed bar anchorage strength is proportional to the concrete compressive strength raised to the power 0.24. The contribution of confining reinforcement is proportional to the area of confining reinforcement parallel to the headed bar within eight to 10 bar diameters of the headed bar. Headed bars with obstructions larger than those permitted in ACI 318-14 that meet the provisions in ASTM A970-18 exhibit anchorage strengths that are similar to those that meet the provisions in ACI 318-14.
In most of the countries, the irregular building construction is popular for fulfilling both aesthetic and functional requirements. However, the evidence of past earthquakes in Nepal and the globe demonstrated the higher level of seismic vulnerability of the buildings due to irregularities. Considering this fact, the present study highlighted the common irregularities and its effect on reinforced concrete building response. The effect of structural irregularities was studied through numerical analysis. The geometrical, mass and stiffness irregularities were created by removing bays in different floor levels and removing the columns at different sections respectively. In this study, the numerical models were created in finite element program SAP2000. The structural performance was studied using both non-linear static pushover and dynamic time history analysis. The results indicate that the level of irregularities significantly influenced the behavior of structures.
Irregular building structure is frequently constructed across the globe for fulfilling aesthetic as well as functional requirements. The structures with irregularities are the common building type in earthquake-prone country like Nepal. However, a post-earthquake reconnaissance survey reports revealed the high seismic vulnerability of the building with structural irregularities. In this context, the present study explores the influence of structural irregularities on performance of reinforced concrete (RC) frame structure. To this end, the structural irregularities are created in in the building structures. The geometrical irregularities are created by removing the bays in different floor levels. Likewise, the effect due to mass irregularities are studied by considering the swimming pool and game house at different floor levels. Furthermore, the stiffness irregularities are formulated by removing the building columns at different sections. All these irregularities are studied analytically in finite element program with 3-D structural models. The numerical analysis is done with non-linear static pushover and time history analysis. The results are analyzed in terms of fundamental time period, storey shear, storey displacement, drift and overturning moment. The results indicate that the level of irregularities significantly influenced the behavior of structures.
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