Reinforcement Detailing In Non Engineering And Engineering E

REINFORCEMENT DETAILING IN NON ENGINEERING AND ENGINEERING EARTHQUAKE RESISTANT STRUCTURES ABSTRACT Most losses of life and property during earthquakes have occurred due to collapse of structure. Lack of understanding of earthquake resistant features reinforcement detailing, developments in the philosophy of ductility bases design has aggravated the problem. One of the most important factor causing damage to structure during earthquakes is lack of adequate structural connections along with the other factors. The observations of structural performance in earthquake damaged areas in the past also identifies the need for proper reinforcement detailing in non engineering as well as Engineering structures. This paper is aimed to discuss and describes the reinforcement detailing in earthquake resistant Non Engineering and Engineering Structures. The material requirements, I.S. code provisions for designing and reinforcement detailing in the different component parts of structure along with illustrations are briefly discussed.

1) INTRODUCTION Amongst all the natural disasters, earthquakes can prove most deadly as it has the least duration of occurance but causing huge loss to human lives and property. Increasing magnitude of seismic risk to the life could be imagined by the fact that in India more than 55 % of areas lies in active seismic zones. Considering the fact that in future we have to live with the earthquake tremors it becomes essential to introduce earthquake resistant features in the construction of structures. The main factors contributing to the damage of structures during earthquakes have been their heavy weight, low tensile and shearing resistance, lack of adequate structural connections, poor quality of construction and deterioration of strength with the passage of time. This paper highlights one of the major aspect of inadequate detailing of reinforcement and describes the designing and detailing consideration in non engineering and engineering construction along with I.S.code provisions, material requirements, and general design consideration. The term Non engineering building may only be vaguely defined as a building whose analysis for lateral earthquake forces would defy reasonable mathematical solutions and the design will be based mostly on a set of specifications derived from observed behaviour of such buildings during past earthquakes and trained engineering judgement. Load bearing masonary wall buildings studwalls and brick nogged constructions in wood composite constructions using load bearing walls and piers in masonary, reinforced concrete, steel or wooden posts are the examples of Non Engineering constructions. Where as Reinforced concrete or steel frame buildings, tall buildings using different types of structural system involving the dynamic analysis for seismic loads and ductility design considerations can be termed as Engineering constructions. 2 WHAT HAPPENS TO BUILDING AND STRUCTURES IN AN EARTHQUAKE 1) Seismic forces are very irregular and buildings are variously affected due to complex nature of structures. Therefore, standardization of seismic forces which act and provisions required to be made in structures to withstand them are very difficult. 2) Seismic forces act horizontally, vertically and in a vibratory and oscillatory manner. The result is that unless the structure is well-knit, homogeneous and all its components securely anchored and connected with one another, it is shaken to pieces to a degree depending on the severity of the seismic forces.

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3) The structures act like an inverted pendulum, and unless it is rigid, yet flexible enough, and can provide friction to dampen the oscillations and bring the structure to rest early, the damage increases. 4) The horizontal seismic force acting on the structure is directly proportional to the weight of the structure. Lighter structures suffer less damage. 5) The vertical seismic forces can topple a structure unless it is well founded. 6) The oscillatory and inverted pendulum - like effects also cause overturning movements unless the structure has its centre of gravity well near the ground. 7) The vibratory effect loosens superficial and poorly anchored parts of structures like parapets, plaster on walls and ceilings, and rubble and bricks from walls unless they are reinforced adequately to act as whole units. Tiles cascade down, roofing sheets and trusses get dislodged and partitions collapse. In case of modern steel or R.C.C. framed buildings well founded, the damage is mostly of this nature. 8) Commonsense design and honest construction are necessary. 3. DESIGN CONSIDERATIONS 3.1 General design criteria As discussed earlier during an earthquake ground motions occur in a random fashion, both horizontally and vertically, in all directions radiating from the epi-centre. These ground motions cause structures to vibrate and induce inertia forces on them. Hence, structures whether Non engineering or Engineering in such locations need to be suitably designed and detailed to ensure stability, strength and serviceability with acceptable levels of safety under seismic effect. The criteria adopted by codes for fixing the level of the design seismic loadings can be summarised as below i) Structure should be able to resist minor earthquakes without damage. ii) Structure should be able to resist moderate earthquakes without structural damage but with some non structural damage. iii) Structures should be able to resist major earthquakes without collapse but with some structural and non structural damage. The probability of collapse of structures during earthquake can be minimised by improving damping, ductility and energy dissipation capacity of the structures. The developments in the philosophy of ductility based design is bound to reduce earthquake hazard and protect structures with minor damage. The ductility approach is to provide energy absorbing and dissipating capability of the structure since reinforced concrete is relatively less ductile in compression and shear, dissipation of seismic energy is best achieved by flexural yielding. In engineering construction ductile moment resisting frame i.e. a frame of continuous construction, comprising flexural members and columns designed and detailed to accommodate reversible lateral displacement after formation of plastic hinge, will satisfy the ductility considerations. While in Non engineering constructions simple constructional detailing and adopting appropriate design these objectives can be achieved. 3.2 General Design Objectives The objective of special design and detailing provisions in different I.S.codes is to ensure the overall ductile behaviour of structure and it's component member. Some important design considerations in providing ductility include i) Using low tensile steel ratio ( with low grade steel and using compression steel) ii) Providing adequate stirrups to ensure that shear failure does not precede flexural member. iii) Confining concrete and compression steel by closely spaced hoops or Spirals

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iv)

Proper detailing with regard to anchorage, splicing, minimum reinforcement, bending, termination, extension etc.

3.3. Materials Reinforcement and concrete Ductility requires the use of low grades of steel having well defined and longer yield plateau so that plastic hinges formed will have larger rotation capacities leading to greater energy dissipation. As lower the grade of steel the higher is the ratio of the ultimate tensile strength to yield strength. A higher ratio is desirable as it results in increased length of plastic hinge and thereby increased plastic rotation capacity. From these considerations mild steel ( Fe250) is best suited for flexural reinforcement in earthquake resistant design but will require larger cross sections. I.S.code 13920 1993 permits use of Fe415 steel but prohibits the use of higher grades than Fe415. I.S. code limits the minimum grade of concrete to M20, use of very high strength is undesirable due to it's lower compressive strain which inturn affect ductility. The ACI and Canadian codes limits cylinder strength to 30 Mpa. 4. REINFORCEMENT DETAILING IN NON ENGINEERING CONSTRUCTION Non engineering constructions can be made enough earthquake resistant through some simple principles of planning and reinforcing techniques. In this paper discussion is limited to only reinforcement details at critical sections in the structures. The extent of reinforcing depends upon the seismic intensity. 4.1. Horizontal Reinforcement in masonary Walls. For imparting horizontal bending strength against plane action for out of plane inertia load and for tying the perpendicular walls together. The following reinforcing arrangements are provided.

Fig No. 1 3

4.1.1. Horizontal Band or Ring Beam. The most important horizontal reinforcing is through reinforced concrete bands provided continuously through all load bearing longitudinal and transverse Walls. A band consists of two or four longitudinal steel bars with links embedded in thick concrete ( fig No.1). The steel bars are located closed to the Wall faces and full continuity is provided at corners and junctions. Such bands are provided at critical levels of the buildings i.e. plinth lintel, roof and gables according to requirements. 4.1.2 Dowels at Corners and Junctions As an alternative to the bands, steel dowel bar may be used at corners and T. junctions to integrate the box action of Walls. Dowels are placed in every fourth course or at about 50 cm interval and taken into the Walls to sufficient length to provide full bond strength (fig No 2 a,b,c,)

Fig 2a

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Fig 2b

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Fig 2c

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4.1.3. Reinforcement of partitions and Infills. Partitions and infill panels are necessarily thin Walls and reinforcement is needed for there out of plane stability. Thin steel mesh or Welded Wire fabric is provided every fourth course or 30 cm height.( fig No.3)

Fig No. 3 4.2. Vertical reinforcement in Walls The vertical reinforcing steel shall be provided at the critical sections i.e. at jambs of opening and corners of the Walls. The amount of vertical reinforcing steel will depend upon number of factors like number of storeys, storey height, seismic Zone importance of building, soil foundation type etc. Typical arrangements of placing the vertical steel in brickwork are shown in fig No 4. The total arrangement of providing reinforcing steel in masonary wall construction is illustrated in fig No.5.

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Fig No.4

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Fig No. 5

4.3 Framing of thin masonry walls When masonory walls are thinner than 20 cm, then provision of columns and collar beams becomes necessary as shown in fig No 6. The columns are located at all the corners and junctions of walls as well as both sides of door openings. The collar beams are provided at top and bottom of storeys. The fig No 7 illustrates typical reinforcing details between beams and columns.

Fig No. 6

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Fig No. 7

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5. REINFORCEMENT DETAILING IN ENGINEERING CONSTRUCTION For achieving good performance of R.C. Frames, along with proper design considerations, suitable details for steel reinforcement must be adopted so as to obtain adequate ductility. The ductility requirements will be deemed to be satisfied if the codal provisions given below are achieved. 5.1. Flexural members in ductile frames The codal provisions for design and detailing of flexural members in earthquake resistant design are as below i) The Factored axial stress under earthquake loading should not exceed 0.1 fck. Overall depth 'D' should not exceed 1/4th of clear span and width 'b' should not be less than 200 mm. with b/D ratio more than 0.3 ii) To achieve ductile behaviour, to avoid congestion of steel and to limit shear stresses, the tensile reinforcement ratio is limited to 0.025 i.e. Pt max = 2.5 iii) To avoid sudden brittle failure of beam, a minimum reinforcement ratio p min = 0.24 √fck/fy must be provided at both the top and bottom for entire length of the member with at least two bars placed at each face. iv) To ensure proper development of reversible plastic hinges near beam - column connections. * the positive moment reinforcement at a joint face must not be less than half the negative movement reinforcement at that joint face. * the top and bottom steel at any section along the length of the member should not be less than 1/4th of the negative moment reinforcement at the joint face on either side. * both top and bottom bars must be taken through the column and made continuous wherever possible, in case of interior joint. In other cases they may be extended to the far face of the confined column core and provided an anchorage length of Ld + 10 φ. Where Ld is the development length of bars in tension. fig No. 8 illustrates different support detailing.

Fig No. 8

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Fig No.8 : End support detailing for earthquake moment (a) Both top and bottom bars are taken up (b) Top bars down and bottom bars up (c) Top bars down and bottom bars up but within the beam depth

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When lap splices are provided transverse reinforcement for confining concrete and to support longitudinal bars, in the form of closed stirrups or 'hoops' ( with 1350 hook and 10 φ extension) should be provided over entire splice length at spacing not exceeding 150 mm (fig 9)

Fig. No.9

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The bar extensions must provide for possible shifts in the inflection points which may occur under the combined effects of gravity and seismic loadings.

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vii) viii)

Inclined bars are not allowed as effective shear reinforcement due to changing direction of diagonal stress. Web reinforcement must be in the form of closed stirrups (hoops) placed perpendicular to longitudinal reinforcement and must be provided throughout the length of member. Hoops should have minimum diameter of 8 mm in beams with clear span more than 5 m (Fig No 10).

Fig No. 10

Fig No.11

ix) x)

The spacing of hoops should be in accordance with fig No.11. Fig No. 12 gives the typical bar details for special ductile moment resisting frames.

Fig No. 12

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5.2. Columns subjected to Axial load and bending. i) Minimum dimension should be 200 mm with the ratio of the shortest dimension to perpendicular dimension not less than 0.4. When unsupported length exceeds 4 m, minimum dimension should not be less than 300 m m. ii) Lap splices are not permitted near the ends of column. They are permitted in the central half of member length. Hoops should be provided over the entire splice length at spacing not exceeding 150 mm. At any section not more than 50 percent of the bars should be spliced (fig 13)

Fig No. 13

iii)

iv)

Special confining reinforcement must be provided near the joints and on both sides (extending over a length Lo from joint face) ( fig No 14a). The length Lo should not be less than larger lateral dimension of the member at section where yielding may occur 1/6 of clear height of member (c) 450 mm. The hoop spacing should not exceed 1/4 of minimum member dimension but need be less than 75 mm or more than 100 mm. The area of cross section of the bar to be used as special confining should be taken as Ash ≥ 0.09 s Dk

fck fy

Ag - 1 Ak

for circular hoops/spiral Ash ≥ 0.18 s Dh

fck fy

Ag - 1 Ak

for rectangular hoops Where S = spacing of hoops, Dk = Core diameter Dh = longer dimension of the rectangular hoop not more than 300 mm Ag = gross area of column, Ak = area of concrete core. v)

Special confining reinforcement shall be provided at the locations as shown in fig 14(b,c,d).

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5.3. Beam - column connections i) The special confining reinforcement near column ends should be extended through the joint as well (fig No.14a). If the joint is externally confined the spacing of hoops in joint region may be taken as twice that required at the end of the column but limited to 150 mm. ii) The beam and column bars must be well anchored in the compression zone so as to achieve full strength. (fig No. 8) 15

6. CONCLUSION Considering the fact that in future we have to live with earthquake tremors it becomes important to study, learn and incorporate in actual practice, the simple principles of reinforcement detailings, in Non engineering and Engineering earthquake resistant constructions to avoid huge losses of properties and human lives during an earthquake. 7. REFERENCES 1) I.S. 1893 : 1984 - Criteria for Earthquake Design of structures, Bureau of Indian standards, New Delhi 1984. 2) A Manual of earthquake resistant non engineered construction, Indian society of earthquake Technology Roorkee 1981. 3) I.S. 4236 : 1976 . code of practice for earthquake resistant design and construction of buildings. Bureau of Indian standards New Delhi 1976 4) I.S. 13920 : 1993 - Ductile detailing of Reinforced Concrete structures subjected to Seismic forces - Code of practice, Bureau of Indian standards New Delhi 1993. 5) S.P : 34 - 1987 Handbook on concrete reinforcement and detailing. Bureau of Indian standards New Delhi - 1987 6) SP 22 : 1982 Explanatory handbook for Earthquake Engineering, Bureau of Indian Standards New Delhi 1982. 7) S. Unnikrishna Pillai and Devdas Menon. Reinforced concrete Design, Tata - McGraw Hill publishing Company Ltd. New Delhi 1998. 8) U.H. VARYANI. Structural Design of multi-storeyed buildings, South Asian publishers New Delhi 1999. 9) SIPOREX publication, B.G. Shirke and Company Pune, Dec 1993. 10) PROJECT REPORT submitted by C.O & E.T. final years students under guidance of Prof. P.S.Lande "To study the impact of earthquake on structure damage repair and rehabilitation in Latur" 1995-1996.

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