International Journal of Civil and Structural Engineering

INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING Volume 1, No 3, 2010 Review article © Copyright 2010 All rights reserved Integrated Publishing services ISSN 0976 – 4399 Critical appraisal on steel water tank design using recent and past I. S codes Kala.P1, Vimala.S 2, Ilangovan.R3 1­ Post graduate student, Department of Civil Engineering, AUT, Trichirappalli, Tamil Nadu, India. 2­ Professor, Department of Civil Engineering, PSNA college of engineering & technology, Dindigul. Tamilnadu 3 – Assistant Professor, Department of Civil Engineering, AUT, Trichirappalli, Tamil Nadu, India. kala_pkpm@yahoo.co.in ABSTRACT IS: 800 code is the basic code for general construction in steel structures and the prime document for any structural design and has influence on many other codes governing the design of other special steel structures. IS:800­1984,furnished provisions, for designing the structures, mainly, by “Working Stress Method”. Realizing the necessity to update the standard to the state of the art of the steel construction technology and economy, the current revision of the standard (IS:800­2007) was undertaken. Earlier codes are silent in respect of design with respect to fatigue, corrosion, earthquake and durability etc, whereas in the present code careful revisions have been incorporated. In particular new code has advocated the design aspects with respect to fatigue, corrosion, earthquake and durability etc which place predominant role because of rattling forces created due to earthquake. The new code furnishes provisions, for designing the structures, mainly, by “Limit State Method of Design”. An attempt here is made to explain the basic differences in the IS codes – old (IS:800­1984)4 and revised(IS:800­2007).To reinforce the effect of the additional parameters included in the revised version, a steel elevated water tank is designed, using both the codes separately, and a conclusion is arrived at based on the sizes f the designed members. (For brevity the detailed design calculation are not given here.)An elevated water tank has been chosen because the structure involves the design of all type of members – struts, ties, columns, base plates and girder. Keywords: Working stress method, Limit state method, yielding, block shear failure, buckling. 1. Introduction Steel Structures are preferred because steel offers much better compressive and tensile strength than concrete. Lighter constructions are also resulted. Unlike masonry or reinforced concrete, steel can be easily recycled. Now, the designs to be made should ensure the fundamental requirements of Structural safety, Stability, Stiffness, Durability, Economy, Aesthetics, Functional safety and requirements.‘Structural safety’ ensure that the structure does, not fail below the collapse load, within the, ‘life time’ of the structure. ‘Stability’ ensures safety against tilting or buckling. Adequate ‘stiffness’ promotes safety against deflection and cracking ensuring functional safety. ‘Durability’ ensures that the 390 INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING Volume 1, No 3, 2010 Review article © Copyright 2010 All rights reserved Integrated Publishing services ISSN 0976 – 4399 spalling of concrete leading to corrosion of reinforcements does not occur. ‘Economy’ ensures minimum consumption of materials, exploiting the full capacity of the materials to resist the loads leading to the overall cost reduction. A structural design ensuring this condition is known as ‘optimal design’. A revision to the IS:800 has been made taking into consideration the above fundamental requirements. The code of practice is largely based on limit state method of design. 1.1 Limit state Method of Design The limit state method of design was developed to take account of all condition that can make the structure unfit for use considering actual behaviour of materials and structures. IS 800 – 2007, the relevant code of practice, applicable to the structural use of hot rolled steel is largely based on limit state method of design. However it’s still retains the working stress method which was in use for last several decades. The code recommends the working stress method in situations where limit state method cannot be adopted conveniently and confidently. Both the design philosophies have therefore been incorporated in the body of the text, but with emphasis of limit state method of design being more realistic and resulting in economical designs. There are basically two categories of limit state, strength and serviceability. The accepted limit for the safety and serviceability requirements before failure occurs is called a limit state. 1.1.1 Strength Limit State 1. Strength including yielding bucking and transformation into a mechanism. 2. Stability against overturning and sway. 3. Failure due to excessive deformation or rupture.4.Fracture due to fatigue and 5. Brittle fracture. 1.1.2 Serviceability Limit State This limit state refers to the performance of the structure under service load and includes, 1.Deflection 2.Vibrations 3.Deteriorations 4. Corrosion and 5. Ponding 1.2 Probabilistic Basis for Design Safety of structure is of prime importance for a designer. Safety margins in the form of permissible in working stress design and load factors in plastic design have been provided to ensure safety against the risk of failure – the collapse or un­serviceability.The main parameters in analysis and design – the loads, the material properties and the dimension – are random variables. The statistical variation of these design variables is usually ignored in conventional practice. Actually, magnitude and frequency relationship for both load and strength must be considered to avoid unrealistic results. Therefore, any realistic, rational and qualitative representation of safety must be based on statistical and probabilistic analysis. 391 INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING Volume 1, No 3, 2010 Review article © Copyright 2010 All rights reserved Integrated Publishing services ISSN 0976 – 4399 An illustration of the statistical meaning of safety is given in Fig 1. Let us consider a structural component (say, a beam) designed to carry a given nominal load. Bending moments (B.M.) produced by characteristic loads are first computed. These are to be compared with the characteristic resistance or strength (R.M.) of the beam. But the characteristic resistance (R.M.) itself is not a fixed quantity, due to variations in material strengths that might occur between nominally same elements. The actual resistance of these elements can be expected to vary as a consequence. The statistical distribution of these member strengths (or resistances) will be as sketched in (a). Similarly, the variation in the maximum loads and therefore load effects (such as bending moment) which different structural elements (all nominally the same) might encounter in their service life would have a distribution shown in (b). The uncertainty here is both due to variability of the loads applied to the structure, and also due to the variability of the load distribution through the structure. Thus if a particularly weak structural component is subjected to a heavy load which exceeds the strength of the structural component, clearly failure could occur. Unfortunately it is not practicable to define the probability distributions of loads and strengths, as it will involve hundreds of tests on samples of components. Normal design calculations are made using a single value for each load and for each material property and making appropriate safety factor into the design calculations. The value used is termed as “Characteristic Strength or Resistance” or “Characteristic Load”. Figure 1: Statistical meaning of safety As per IS: 800, suitable provisions in the design are required to be made for the dynamic effects of live loads, impact loads and vibration due to machinery operating loads. In severe cases possibility of resonance, fatigue or unacceptable vibrations shall be investigated. Unusually flexible structures (generally the height to effective width of lateral load resistance system exceeding 5:1) need to be investigated for lateral vibration 392 INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING Volume 1, No 3, 2010 © Copyright 2010 All rights reserved Integrated Publishing services Review article ISSN 0976 – 4399 under dynamic wind loads. Structures subjected to large number of cycles of loading shall be designed against fatigue failure.Durability or Corrosion resistance of a structure is generally, under conditions relevant to their intended life as are listed below: a) The environment b) The degree of exposure c) The shape of the member and the structural detail d) The protective measure and e) Ease of maintenance. Fire resistance of a steel member is a function of its mass, its geometry, the actions to which it is subjected, its structural support condition, fire protection measures adopted and the fire to which it is exposed.The revised code has taken into consideration all the above factors. 3. Design Procedure of Various types of Members as per IS:800­2007 3.1 Design of Tension Members Mode of Failure The different Modes of failure of members are: 1. Cross section yielding. 2. Net section rapture and 3. Block shear failure Strength as governed by yielding of gross section T = A f /γm0 (7 b) dg g y Where, A is the gross area of the angle section. g Strength as governed by tearing at net section T = 0.9A f / γm1+ β A f / γm0 (7a) dn nc u go y Where, f and f are the yield and ultimate stress of the material, respectively. A and A , y u nc o are the net area of the connected leg and the gross area of the outstanding leg, respectively. The partial safety factors γ = 1.10 and γ = 1.25. β, accounts for the end fastener restraint effect and is given by, m0 m1 β = 1.4 – 0.076 (w/t) (f/f) (b/L ) ≤ (f .γ / f .γ ) and β 0.7 ≥ c u mo yus where w and b are as shown in Figure 2. y m1 s 393 INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING Volume 1, No 3, 2010 © Copyright 2010 All rights reserved Integrated Publishing services Review article ISSN 0976 – 4399 Figure 2: Angles with end connection Strength as governed by block shear failure A tension member may fail along end connection due to block shear as shown in Figure 3. The corresponding design strength can be evaluated using the following equations. If the centroid of bolt pattern is not located between the heel of the angle and the centre line of the connected leg, the connection shall be checked for block shear strength given by T = ( A f /(3γ ) + 0.9A f /γ ) db vg y m0 tn u m1 or T = (0.9A f /(3γ ) + A f /γ ) (7c) db vn u m1 tg y m0 where, A and A = minimum gross and net area in shear along a line of transmitted vg vn force, respectively, and A and A = minimum gross and net area in tension from the hole tg tn to the toe of the angle, perpendicular to the line of force, respectively. Block Shear Failure Block shear plane L = Length of the end connection, i.e., distance between the outermost bolts in the end c joint measured along the length direction or length of the weld along the length direction and t = thickness of the leg Alternatively, the rupture strength of net section may be taken as T = α A f /γ dn n u m1 394 INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING Volume 1, No 3, 2010 Review article © Copyright 2010 All rights reserved Integrated Publishing services ISSN 0976 – 4399 Figure 3: Block Shear Failure 3.2 Design of Compression Members 3.2.1 Possible Failure Modes The possible failure modes of an auxiliary loaded may be given as follows: 1. Local Buckling: Failure occurs by buckling of one or more individual plate elements, e.g., flange or web, with no overall deflection normal to the applied load. This failure mode may be prevented by selecting suitable width­to­thickness ratios of component plates. Alternatively when slender plates are used, the design strength may be reduced. 2. Squashing: When the length is relatively small and its component plate elements are prevented from local buckling, then the column will be able to attains its full strength or “squash load”(yield stress x area of cross­section). 3. Overall Flexural Buckling: This mode of failure normally controls the design of most compression members. In this mode, failure of the member occurs by excessive deflection in the plane of the weaker principal axis. An increase in the length of the column, results in the column resisting less loads progressively. Torsional And Flexural­torsional buckling Torsional buckling failure occurs by twisting about the shear centre in the longitudinal axis. A combination of flexure and twisting, called flexural­torsional buckling is also possible.In addition to the above failure modes, in compound members, failure of a component member may occur, if the joints between members are sparsely placed. Codes and specifications usually have rules to prevent such failures. 3.3 Basis The plate elements of a cross section may buckle locally due to compressive stresses. The local buckling can be avoided before the limit state is achieved by limiting the width to 395 INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING Volume 1, No 3, 2010 Review article © Copyright 2010 All rights reserved Integrated Publishing services ISSN 0976 – 4399 thickness ratio of each element of a cross section, subjected to compression due to axial force, moment or shear.When plastic analysis is used, the members shall be capable of forming plastic hinges with sufficient rotation capacity (ductility) without local buckling to enable the redistribution of bending moment required before formation of the failure mechanism.When elastic analysis is used, the member shall be capable of developing the yield stress under compression without local buckling. On the above basis, four classes of sections are defined as follows: a.Plastic – Cross section, which can develop plastic hinges and have the rotation capacity required for failure of the structure by formation of a plastic mechanism. b) Compact – cross sections, which can develop plastic moments of resistance, but have inadequate plastic hinge rotation capacity for formation of a plastic mechanism. c) Semi­compact – Cross sections, in which the extreme fiber in compression can reach, yield stress, but cannot, develop the plastic moment of resistance, due to local buckling. d) Slender – Cross sections in which the elements buckle locally even before reaching yield stress. In such cases, the effective sections for design shall be calculated by deducting width of the compression plate element in excess of the semi­ compact section limit. When different elements of a cross section fall under different classification, the section shall be classified as governed by the critical element. The limiting width to thickness ratios of elements for different classifications of sections: Figure 4: Section classification based on Moment­ Rotation characteristics 396 INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING Volume 1, No 3, 2010 Review article © Copyright 2010 All rights reserved Integrated Publishing services ISSN 0976 – 4399 3.3 Design of Beams – Girders Unrestrained beams that are loaded in their stiffer planes may undergo lateral torsional buckling. The prime factors that influence the buckling strength of beams are · Un braced span · Cross sectional shape · Type of end restraint · Distribution of moment · The effects of various parameters that affect buckling strength have been accounted for in the design by appropriate correction factors. The behavior of real beams (which do not comply with the theoretical assumptions) has also been described. In order to increase the lateral strength of suitable stiffness and strength has to be provided. When a beam is transversely loaded in such a manner that the resultant force passes through the longitudinal shear centre axis, the beam only bends and no torsion will occur. When the resultant acts away from the shear centre axis, then the beam will not only bend but also twist. 3.4 Design approach as per New IS: 800 The New IS: 800 follows the same design philosophy with certain alterations in the parameters for calculating design bending strength governed by lateral torsional buckling. Figure 5: Bending strength for rolled sections of design strength according to BS 5950 The step by step design procedure has been detailed below: 397 INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING Volume 1, No 3, 2010 Review article © Copyright 2010 All rights reserved Integrated Publishing services ISSN 0976 – 4399 The design bending strength of laterally unsupported beams as governed by lateral torsional buckling is given be Md = βbZp fbd βb = 1.0 for plastic and compact sections = Ze / Zp for semi­compact sections Ze , Zp = plastic section modulus and elastic section modulus with respect to extreme compression fibre.] fbd = design bending compressive stress, obtained as given below: fbd = χLT fy / Ym0 3.4.1 Bolted connections Bolts can be used for making end connections in tension and compression members. IS: 800 limits the use of punched holes only in material with yield stress less than 360 MPa and where the thickness (in mm) does not exceed 5600/fy mm. Structurally drilled holes are better and should be recommended as far as possible. 3.4.2 Base Plate for concentrically Loaded Columns: The design compressive stress in a concrete footing is much smaller than it is in a steel column. So it becomes necessary that a suitable base plate should be provided below the column to distribute the load from it evenly to the footing below. For a purely axial load, a plain square steel plate or a slab attached to the column is adequate. If uplift or overturning forces are present, a more positive attachment is necessary. These base plates can be welded directly to the columns or they can be fastened by means of bolted or welded lug angles. 4. Design Of Water Tank 4.1 General guidelines Steel tanks are used for the storage of water and other liquids, e.g. acids, alcohols, gasoline etc. Steel plates are used to form the container. The plates are designed and detailed so as to be readily made liquid tight by ordinary shop and erection methods. Following codes should be referred to for the given design of steel water storage tanks. 1. IS 800 – 2007 code of practice for general construction in steel. 2. IS 804 – 1998code of practice for rectangular pressed steel tanks. 3. IS 805 – 1995 code of practice for use of steel in gravity water tanks. 4.2 General Requirements Size of the tank: It is economical to build large diameter and low height tanks when the stand­pipes are at such an elevation that these can supply water a sufficient pressure to meet local requirements. 398 INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING Volume 1, No 3, 2010 Review article © Copyright 2010 All rights reserved Integrated Publishing services ISSN 0976 – 4399 Usual heights to diameter ratio are as follows: H = 0.4D Where, H is the height of stand­pipe, and D is its diameter. Size of plates: For a pleasant look, the net width of the various rings of the plates is kept same (about 2m). the length of the plates is kept 5­6m Having selected the diameter of the stand­pipe, its height, the number of rings of plates, and the number of plates per ring, the thickness of plates can be found by the hoop stress theory as applicable to thin cylinders. t = γHD/2σatwhere γ = unit weight of the liquid(9.81 x10­6 N/mm3 for water) To account for the efficiency of the joint (η) t = γHD/2ησat Since the steel plates will be in contact with water, which may lead to corrosion, the thickness so found is increased by 1.5mm. But in no case the thickness of the plates should be less than 6 mm. Tanks elevated on staging or towers are generally provided for water storage and supply. The capacity and height to the bottom are determined from a consideration of the service of the tank. The hemispherical bottom is the most common. 4.3 Circular tanks The hemispherical bottom consists of a dished circular plate in the bottom called a saucer plate and plates with radial seams make up the rest of the bottom. In hemispherical bottom tanks, the ratio of the height of the cylindrical shell to the diameter is approximately 1:1 for small capacities and 1.25:1 for capacities over 5 x 105 liters.Tanks up to 9m in height commonly have vertical columns. For higher tanks, the columns in the tower supporting the tank are battered. The batter is about 1.25 to 1.5:12 for hemispherical bottom tanks up to 25 x 105 litre capacity and 1:12 for larger capacities. This batter decreases wind stresses in the tower.The top of the tank is generally covered with thin plates with a pitch of 1 to 6. For tanks up to 7.0m diameter the roof plates are assumed to be self­supporting and for larger diameters angle rafters are used to support the plates. A free board of 15cm is provided. 4.4Circular girder A circular girder of an angle or channel section is provided at the junction of the cylindrical shell and suspended bottom. It has to support weight of the tank, the weight of the water stored and its own weight. The total load acts as a uniformly distributed load 399 INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING Volume 1, No 3, 2010 Review article © Copyright 2010 All rights reserved Integrated Publishing services ISSN 0976 – 4399 over the girder. The girder behaves as a beam curved in plan subjected to shear, bending moment and torsion. In turns it transmits loads to the vertical column over which it is simply supported. Even number of columns, spaced at an equal distance along the circumference are provided. The following expressions may be used to obtain the forces in the circular girder. Maximum bending moment M1 = M 0 cosφ /2 + WR/2n [Sin φ ­ (2sin2 φ/2)/ (φ/2)] Maximum torsion (at a point x at the angle φ’ from the column) T = m0 sin φ’­(1­cos φ’) +Wφ’R /4[1­(sin φ’/ φ)] Where W = total vertical load N= number of columns (4, 6, 8, 12) R = radius of circular girder. 4.3.1 Wind force Design wind speed is given by Vz = Vb k1 k2 k3 Where, Vb = basic wind speed of the place in m/s k1 = probability factor (risk coefficient) k2 = terrain height and structure size factor. K3 = topography factor The value of k1 is obtained after deciding the age of the tower and the zone in which it is to be built.The value of k2 is fixed by deciding the class of the structure which itself depends upon the tertian category. The topography factor k3 is ascertained from topography of the area and by the use of appendix C of IS 875 1.Design wind pressure pz = 0.6 Vz2 (at a height z) 2.The solidity ratio (φ)is calculated and corresponding to this the force coefficient (Cf ). 3.Wind load on the tower, F = Cf Ae pz where Ae = effective frontal area of the tower normal to the wind direction pz = design wind pressure Wind load on the tank container is the product of the intensity of wind pressure and the longer side area exposed to the wind. This force will act at mid­height of container. 400 INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING Volume 1, No 3, 2010 Review article © Copyright 2010 All rights reserved Integrated Publishing services ISSN 0976 – 4399 4.4 Earthquake Force As per IS: 1893­1984 (which is still not revised for tanks) Table 1: Designed sections using the old (IS:800­1984) and New(IS:800) codes for the same Water Tank Sections Used Components used Remarks Working Stress Method Limit State Method ISA 150*150*18mm ISA200*200*18mm ISHB150@ 271 N/m ISHB 150@ 271 N/m Same size Bracing Struts ISA 60*60*6mm ISA 65*65*6mm Bigger size Bracing Ties ISA 70*70*6mm ISA 70*70*6mm Same size 230*230*16mm 230*230*16mm Circular Girder Columns Base Plate Bigger size Same size The details furnished in the above table are for information only. These observations no way affect the rationality of the Limit State Method ( ie the authenticity of the revised code). The reasons stated in the conclusion above are more than enough to prove the efficacy of the revised code i.e. IS: 800­2007. 5. Conclusions 1. In the limit state method the partial safety factors on load and material have been derived using the probability concept (using statistical methods ) and therefore the method is more rational and realistic. 2. In limit state method the perfectly plastic region up to the onset of strain hardening is used. 3. The riveted joints have lost their importance due advantages of bolted joints 4. Ductility affects the strength of a tension member. An increase in ductility allows a better redistribution of stress concentration over the cross section. 401 INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING Volume 1, No 3, 2010 Review article © Copyright 2010 All rights reserved Integrated Publishing services ISSN 0976 – 4399 5. For small geometry factor ( ratio of gauge length to the bolt diameter ) , the joint is more efficient and results in higher tensile strength in tension member . 6. The effect of shear lag and block shear failure should be considered. 7. To avoid shear lag, nowadays equal angles are used. 8. Compression members are more critical than tension members. 9. Four different column curves a, b, c and d have been recommended by IS 800­2007 code of practice for different cross sections of columns to account for the initial imperfections of their geometry. 10. Only plastic sections should be used for indeterminate beams to take the advantages of successive formation of plastic hinges. 11. The compression flange of beam is subjected to flexural buckling as well as torsion. So, the beam is subjected to flexural torsional buckling. 6. References 1. British Standards Institution : "BS 5950, Part­1 Structural use of steelwork in building", British Standards Institution, London, 1985 . 2. Code Of Practice–IS 875 ­1987– Part ­ I, II,III &IV Calculation of Dead Loads for different structures, Bureau of Indian Standards, New Delhi, 2007. 3. Code Of Practice–IS 1893 ­2002,Determination of seismic design forces based on factors, Bureau of Indian Standards, New Delhi, 2007. 4. IS: 800 (1984), General Construction in Steel – Code of Practice, Bureau of Indian Standards, New Delhi, 2007. 5. IS: 800 (2007), General Construction in Steel – Code of Practice, Bureau of Indian Standards, New Delhi, 2007. 6. Code Of Practice IS 1367­Part 1 & 3, Requirements of Bolted connections, Bureau of Indian Standards, New Delhi, 2007. 7. Code Of Practice IS 816­1969, Requirements of Welded connections, Bureau of Indian Standards, New Delhi, 2007. 402 INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING Volume 1, No 3, 2010 Review article © Copyright 2010 All rights reserved Integrated Publishing services ISSN 0976 – 4399 8. Duggal.S.K. ‘Limit State Design of Steel Structrues,Tata McGraw Hill Education Private Limited ., New Delhi, 2010., 3rd edition. 9. Structural steel design course material, NITTTR, Taramani, Chennai, Sep2010, pp 20­97. 403