Reinforcement detailing for openings in Reinforced Concrete Beams:

Transcription

1 Reinforcement detailing for openings in Reinforced Concrete Beams: Openings in beam becomes necessary to provide service lines like water supply lines, air conditioning ducts etc to pass through in order to save the height of the room. Services and structural engineers have to work hand in hand so that proper decision is made in advance to avoid any undesirable damage to the concrete beams. Below are some of the guidance after reviewing the references listed at the end of this short notes for design and the detailing for the provision of holes in concrete beams. An attempt made to collect useful technical information from the available literatures, journals and book. The information furnished relates to the preplanned holes in concrete beams. In general, the presence of web openings leads to a decrease in both cracking and ultimate strength, as well as the post cracking stiffness of continuous beams. Torsional strength and stiffness of a beam decreases with an increase in opening size. For the openings in steel beams reference should be made with steel books, manuals etc. Circular holes are preferable as square and rectangular holes can induce stress concentrations around their corners, increasing the risk of cracking. Whether the hole is formed in an element at the precast factory or formed in-situ makes no difference. There are three forms of pre-planned holes: 1. Holes that are cast at the point of construction of the element and are left open ready to receive services. 2. Services that are cast into element and remain in position. 3. Areas within a concrete element that are designed for holes to be cut retrospectively. Classification of Openings By geometry: Small opening if 0.25 x beam depth and Length is depth. Large openings otherwise. General Guide lines: 1. Sufficient area to develop ultimate compression block in flexure. 2. Openings to be located more than D/2 from the supports, concentrated loads and adjacent openings. D= depth of beam. 3. Opening depth D/2. 4. Opening length: a. stability of chord member. b. deflection requirement of beam. 5. Shear concentration factor of 2 (i.e.2vu) is to be used for design of stirrups. 6. Diagonal bars to carry 50-75% of total shear; vertical stirrups to carry the balance.

2 Diagonal bars around the opening are necessary primarily to control the crack width at service loads. A liberal amount of diagonal bars therefore be provided until a rational quantity could be established. Diagonal bars for corner reinforcement are more effective in controlling crack width and reducing beam deflection. Using a suitable combination of diagonal bars and full depth vertical stirrups as corner reinforcement, the serviceability criterion of maximum crack width can be satisfied. Diagonal bars also help to increase the ultimate strength of beams. The following guidelines can be used to facilitate the selection of the size and location of web openings (refer to Fig. above): 1. For T-beams, openings should preferably be positioned flush with the flange for ease in construction. In the case of rectangular beams, openings are commonly placed at mid depth of the section, but they may also be placed eccentrically with respect to depth. Care must be exercised to provide sufficient concrete cover to the reinforcement for the chord members above and below the opening. The compression chord should also have sufficient concrete area to develop the ultimate compression block in flexure and have adequate depth to provide effective shear reinforcement. 2. Openings should not be located closer than one-half the beam depth D to the supports to avoid the critical region for shear failure and reinforcement congestion. Similarly, positioning of an opening closer than 0.5D to any concentrated load should be avoided. 3. Depth of openings should be limited to 50 percent of overall beam depth. 4. The factors that limit the length of an opening are the stability of the chord members, in particular the compression chord, and the serviceability requirement of deflection. When the opening gets bigger, it is preferable to use multiple openings providing the same passageway instead of using a single opening. 5. When multiple openings are used, the post separating two adjacent openings should not be less than 0.5D to insure that each opening behaves independently. CIRCULAR OPENINGS: Within the scope of the investigation conducted herein on concrete beams with small circular openings under bending and shear, the following conclusions can be made. A beam with circular openings can be conservatively designed using the modified ACI Code approach.this method incorporates the reduction in concrete contribution to shear strength

3 due to the opening and considers two different types of failure; (a) Beam-type failure (b) Frame-type failure 2. Diagonal bars are essential for achieving adequate crack control. The amount of these diagonal bars should be sufficient enough to carry at least 50% of the applied shear with a shear concentration factor of at least When a sufficient quantity of diagonal reinforcement is provided, the applied shear may be distributed between the chord members in proportion to their cross-sectional areas; and 4. The ACI Code20 limit on maximum shear force for solid beam may also be applied to beams with small openings provided that the calculation is based on net section. BEAMS WITH SMALL OPENINGS Openings that are circular, square, or nearly square in shape may be considered as small openings provided that the depth (or diameter) of the opening is in a realistic proportion to the beam size, say, about less than 40% of the overall beam depth. In such a case, beam action may be assumed to prevail. Therefore, analysis and design of a beam with small openings may follow the similar course of action as that of a solid beam. The provision of openings, however, produces discontinuities or disturbances in the normal flow of stresses, thus leading to stress concentration and early cracking around the opening region. Similar to any discontinuity, special reinforcement, enclosing the opening close to its periphery, should therefore be provided in sufficient quantity to control crack widths and prevent possible premature failure of the beam. DETAILING: Reinforcement details at opening segment.

4 Reinforcement details around a small opening. For complete design of R.C.C beam with opening reference 1 shall be referred. References: 1. Beams with Openings: and Design by Mansur, M.A. and Kiang Hwee Tan. 2. Mansur, M.A., Tan, K.H. and Lee, S.L., "A Design Method for Reinforced Concrete Beams with Large Openings", Journal of the American Concrete Institute, Proceedings, Vol. 82, No. 4, USA, July/August 1985, pp Tan, K.H. and Mansur, M.A., "Design Procedure for Reinforced Concrete Beams with Web Openings", ACI Structural Journal, Vol. 93, No. 4, USA, JulyAugust 1996, pp Mansur, M.A., Huang, L.M., Tan, K.H. and Lee, S.L., "Analysis and Design of R/C Beams with Large Web Openings", Journal of the Institution of Engineers, Singapore, Vol. 31, No. 4, July/August 1991, pp

5 5. Advanced Structural concrete Design Design & Detailing of openings in RC Flexural members by Prof.tan Kiang Hwee /Dept. of civil Engg, National university of Singapore. 6. Reinforced concrete Beams with small opening in Bending and torsion by M.A.Mansoor and P.Paramasivam. 7. Reinforced concrete T-beams with Large web openings in positive and negative moment regions by Kiang-Hwee Tan, M.A.Mansoor and Loon-Meng Huang. 8. Design of Reinforced Concrete beams with circular openings by Kiang-Hwee Tan, M.A.Mansoor, and Weng Wei. 9. Services Integration with Concrete Buildings from concrete society, UK. Compiled By.T.RangaRajan.