Rational design of deep beams with openings

Abstract

Deep beam is a very important structural element in various types of concrete structures such as transfer girder, panel beams, foundation walls of rectangular tanks, bins, shear walls, etc. Beams whose span to depth ratio is about 5 or less are categorized by the ACI Building Code as deep beams. The current ACI Building Code has provisions for the design of deep beams subjected to flexure and shear only. Several new lines of thinking have developed in the recent past in order to unify the design of various structural members. In the design of reinforced concrete deep beams, it is sometimes necessary to provide opening for service or access. Deep beam with web opening is not yet covered by the current code of practice. A very limited study on deep beams with openings and some studies on beams with openings are available in the literature. In the present study a laboratory investigation has been carried out on seven model deep beam specimens having shear-span-to-depth ratio equal to 1.0. Whereas only one of the deep beams was solid in nature, the other six deep beams had openings. All the seven model deep beams were of 1600 mm x 400 mm x 130 mm size. Three types of web openings having sizes ISO mm x 150 mm, 300 mm x 160 mm and 450 mm x 160 mm were kept in the deep beam model specimens. The solid deep beam was designed in compliance to the concept of compressive force path (CFP). The other six deep beams were designed following the tenets of the CFP method coupled with the recommendations of Mansur and Tan in their works related to the analysis and design of concrete beams with web openings. Out of the two groups of deep beams having openings, the top and bottom chord above and below the openings of the first group were provided with special confinement reinforcement at a spacing half the effective chord depth. The other set of deep beams with openings were very much similar to the first group, but had additional horizontal confinement steel and diagonal steel around the openings. The results of the study revealed that deep beams with or without openings may be designed following the method adopted in the study as all the deep beams attained similar initial cracking load as well as final failure load. The beneficial effect of confining steel in arresting early crack propagation and attainment of higher failure load could also be ascertained from the study. For all the deep beams tested, the compressive force path has played a significant role in carrying the applied loads from the loading points to the supports. A limited numerical investigation has displayed that numerical tools may essentially playa vital role in determing the CFP in a reinforced concrete member.