INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING Volume 1, No 3, 2010

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1 Critical appraisal on steel water tank design using recent and past I. S codes Kala.P 1, Vimala.S 2, Ilangovan.R 3 1 Post graduate student, Department o Civil Engineering, AUT, Trichirappalli, Tamil Nadu, India. 2 Proessor, Department o Civil Engineering, PSNA college o engineering & technology, Dindigul. Tamilnadu 3 Assistant Proessor, Department o Civil Engineering, AUT, Trichirappalli, Tamil Nadu, India. kala_pkpm@yahoo.co.in ABSTRACT IS: 800 code is the basic code or general construction in steel structures and the prime document or any structural design and has inluence on many other codes governing the design o other special steel structures. IS: ,urnished provisions, or designing the structures, mainly, by Working Stress Method. Realizing the necessity to update the standard to the state o the art o the steel construction technology and economy, the current revision o the standard (IS: ) was undertaken. Earlier codes are silent in respect o design with respect to atigue, corrosion, earthquake and durability etc, whereas in the present code careul revisions have been incorporated. In particular new code has advocated the design aspects with respect to atigue, corrosion, earthquake and durability etc which place predominant role because o rattling orces created due to earthquake. The new code urnishes provisions, or designing the structures, mainly, by Limit State Method o Design. An attempt here is made to explain the basic dierences in the IS codes old (IS: ) 4 and revised(is: ).To reinorce the eect o 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 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 o all type o members struts, ties, columns, base plates and girder. Keywords: Working stress method, Limit state method, yielding, block shear ailure, buckling. 1. Introduction Steel Structures are preerred because steel oers much better compressive and tensile strength than concrete. Lighter constructions are also resulted. Unlike masonry or reinorced concrete, steel can be easily recycled. Now, the designs to be made should ensure the undamental requirements o Structural saety, Stability, Stiness, Durability, Economy, Aesthetics, Functional saety and requirements. Structural saety ensure that the structure does, not ail below the collapse load, within the, lie time o the structure. Stability ensures saety against tilting or buckling. Adequate stiness promotes saety against delection and cracking ensuring unctional saety. Durability ensures that the 390

2 spalling o concrete leading to corrosion o reinorcements does not occur. Economy ensures minimum consumption o materials, exploiting the ull capacity o 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 undamental requirements. The code o practice is largely based on limit state method o design. 1.1 Limit state Method o Design The limit state method o design was developed to take account o all condition that can make the structure unit or use considering actual behaviour o materials and structures. IS , the relevant code o practice, applicable to the structural use o hot rolled steel is largely based on limit state method o design. However it s still retains the working stress method which was in use or last several decades. The code recommends the working stress method in situations where limit state method cannot be adopted conveniently and conidently. Both the design philosophies have thereore been incorporated in the body o the text, but with emphasis o limit state method o design being more realistic and resulting in economical designs. There are basically two categories o limit state, strength and serviceability. The accepted limit or the saety and serviceability requirements beore ailure occurs is called a limit state Strength Limit State 1. Strength including yielding bucking and transormation into a mechanism. 2. Stability against overturning and sway. 3. Failure due to excessive deormation or rupture.4.fracture due to atigue and 5. Brittle racture Serviceability Limit State This limit state reers to the perormance o the structure under service load and includes, 1.Delection 2.Vibrations 3.Deteriorations 4. Corrosion and 5. Ponding 1.2 Probabilistic Basis or Design Saety o structure is o prime importance or a designer. Saety margins in the orm o permissible in working stress design and load actors in plastic design have been provided to ensure saety against the risk o ailure the collapse or un serviceability.the main parameters in analysis and design the loads, the material properties and the dimension are random variables. The statistical variation o these design variables is usually ignored in conventional practice. Actually, magnitude and requency relationship or both load and strength must be considered to avoid unrealistic results. Thereore, any realistic, rational and qualitative representation o saety must be based on statistical and probabilistic analysis. 391

3 An illustration o the statistical meaning o saety 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 irst computed. These are to be compared with the characteristic resistance or strength (R.M.) o the beam. But the characteristic resistance (R.M.) itsel is not a ixed quantity, due to variations in material strengths that might occur between nominally same elements. The actual resistance o these elements can be expected to vary as a consequence. The statistical distribution o these member strengths (or resistances) will be as sketched in (a). Similarly, the variation in the maximum loads and thereore load eects (such as bending moment) which dierent structural elements (all nominally the same) might encounter in their service lie would have a distribution shown in (b). The uncertainty here is both due to variability o the loads applied to the structure, and also due to the variability o the load distribution through the structure. Thus i a particularly weak structural component is subjected to a heavy load which exceeds the strength o the structural component, clearly ailure could occur. Unortunately it is not practicable to deine the probability distributions o loads and strengths, as it will involve hundreds o tests on samples o components. Normal design calculations are made using a single value or each load and or each material property and making appropriate saety actor into the design calculations. The value used is termed as Characteristic Strength or Resistance or Characteristic Load. Figure 1: Statistical meaning o saety As per IS: 800, suitable provisions in the design are required to be made or the dynamic eects o live loads, impact loads and vibration due to machinery operating loads. In severe cases possibility o resonance, atigue or unacceptable vibrations shall be investigated. Unusually lexible structures (generally the height to eective width o lateral load resistance system exceeding 5:1) need to be investigated or lateral vibration 392

4 under dynamic wind loads. Structures subjected to large number o cycles o loading shall be designed against atigue ailure.durability or Corrosion resistance o a structure is generally, under conditions relevant to their intended lie as are listed below: a) The environment b) The degree o exposure c) The shape o the member and the structural detail d) The protective measure and e) Ease o maintenance. Fire resistance o a steel member is a unction o its mass, its geometry, the actions to which it is subjected, its structural support condition, ire protection measures adopted and the ire to which it is exposed.the revised code has taken into consideration all the above actors. 3. Design Procedure o Various types o Members as per IS: Design o Tension Members Mode o Failure The dierent Modes o ailure o members are: 1. Cross section yielding. 2. Net section rapture and 3. Block shear ailure Strength as governed by yielding o gross section T dg = A g y /γ m0 (7 b) Where, A g is the gross area o the angle section. Strength as governed by tearing at net section T dn = 0.9A nc u / γ m1 + β A go y / γ m0 (7a) Where, y and u are the yield and ultimate stress o the material, respectively. A nc and A o, are the net area o the connected leg and the gross area o the outstanding leg, respectively. The partial saety actors γ m0 = 1.10 and γ m1 = β, accounts or the end astener restraint eect and is given by, β = (w/t) (/) (b/l c ) ( u.γ mo / y.γ m1 ) and β 0.7 yus where w and b s are as shown in Figure

5 Figure 2: Angles with end connection Strength as governed by block shear ailure A tension member may ail along end connection due to block shear as shown in Figure 3. The corresponding design strength can be evaluated using the ollowing equations. I the centroid o bolt pattern is not located between the heel o the angle and the centre line o the connected leg, the connection shall be checked or block shear strength given by T db = ( A vg y /(3γ m0 ) + 0.9A tn u /γ m1 ) or T db = (0.9A vn u /(3γ m1 ) + A tg y /γ m0 ) (7c) where, A vg and A vn = minimum gross and net area in shear along a line o transmitted orce, respectively, and A tg and A tn = minimum gross and net area in tension rom the hole to the toe o the angle, perpendicular to the line o orce, respectively. Block Shear Failure Block shear plane L c = Length o the end connection, i.e., distance between the outermost bolts in the end joint measured along the length direction or length o the weld along the length direction and t = thickness o the leg Alternatively, the rupture strength o net section may be taken as T dn = α A n u /γ m1 394

6 3.2 Design o Compression Members Possible Failure Modes Figure 3: Block Shear Failure The possible ailure modes o an auxiliary loaded may be given as ollows: 1. Local Buckling: Failure occurs by buckling o one or more individual plate elements, e.g., lange or web, with no overall delection normal to the applied load. This ailure mode may be prevented by selecting suitable width to thickness ratios o 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 rom local buckling, then the column will be able to attains its ull strength or squash load (yield stress x area o cross section). 3. Overall Flexural Buckling: This mode o ailure normally controls the design o most compression members. In this mode, ailure o the member occurs by excessive delection in the plane o the weaker principal axis. An increase in the length o the column, results in the column resisting less loads progressively. Torsional And Flexural torsional buckling Torsional buckling ailure occurs by twisting about the shear centre in the longitudinal axis. A combination o lexure and twisting, called lexural torsional buckling is also possible.in addition to the above ailure modes, in compound members, ailure o a component member may occur, i the joints between members are sparsely placed. Codes and speciications usually have rules to prevent such ailures. 3.3 Basis The plate elements o a cross section may buckle locally due to compressive stresses. The local buckling can be avoided beore the limit state is achieved by limiting the width to 395

7 thickness ratio o each element o a cross section, subjected to compression due to axial orce, moment or shear.when plastic analysis is used, the members shall be capable o orming plastic hinges with suicient rotation capacity (ductility) without local buckling to enable the redistribution o bending moment required beore ormation o the ailure mechanism.when elastic analysis is used, the member shall be capable o developing the yield stress under compression without local buckling. On the above basis, our classes o sections are deined as ollows: a.plastic Cross section, which can develop plastic hinges and have the rotation capacity required or ailure o the structure by ormation o a plastic mechanism. b) Compact cross sections, which can develop plastic moments o resistance, but have inadequate plastic hinge rotation capacity or ormation o a plastic mechanism. c) Semi compact Cross sections, in which the extreme iber in compression can reach, yield stress, but cannot, develop the plastic moment o resistance, due to local buckling. d) Slender Cross sections in which the elements buckle locally even beore reaching yield stress. In such cases, the eective sections or design shall be calculated by deducting width o the compression plate element in excess o the semi compact section limit. When dierent elements o a cross section all under dierent classiication, the section shall be classiied as governed by the critical element. The limiting width to thickness ratios o elements or dierent classiications o sections: Figure 4: Section classiication based on Moment Rotation characteristics 396

8 3.3 Design o Beams Girders Unrestrained beams that are loaded in their stier planes may undergo lateral torsional buckling. The prime actors that inluence the buckling strength o beams are Un braced span Cross sectional shape Type o end restraint Distribution o moment The eects o various parameters that aect buckling strength have been accounted or in the design by appropriate correction actors. The behavior o real beams (which do not comply with the theoretical assumptions) has also been described. In order to increase the lateral strength o suitable stiness and strength has to be provided. When a beam is transversely loaded in such a manner that the resultant orce passes through the longitudinal shear centre axis, the beam only bends and no torsion will occur. When the resultant acts away rom 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 ollows the same design philosophy with certain alterations in the parameters or calculating design bending strength governed by lateral torsional buckling. Figure 5: Bending strength or rolled sections o design strength according to BS 5950 The step by step design procedure has been detailed below: 397

9 The design bending strength o laterally unsupported beams as governed by lateral torsional buckling is given be M d = β b Z p bd β b = 1.0 or plastic and compact sections = Z e / Z p or semi compact sections Z e, Z p = plastic section modulus and elastic section modulus with respect to extreme compression ibre.] bd = design bending compressive stress, obtained as given below: bd = χ LT y / Y m Bolted connections Bolts can be used or making end connections in tension and compression members. IS: 800 limits the use o punched holes only in material with yield stress less than 360 MPa and where the thickness (in mm) does not exceed 5600/ y mm. Structurally drilled holes are better and should be recommended as ar as possible Base Plate or concentrically Loaded Columns: The design compressive stress in a concrete ooting 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 rom it evenly to the ooting below. For a purely axial load, a plain square steel plate or a slab attached to the column is adequate. I uplit or overturning orces are present, a more positive attachment is necessary. These base plates can be welded directly to the columns or they can be astened by means o bolted or welded lug angles. 4. Design O Water Tank 4.1 General guidelines Steel tanks are used or the storage o water and other liquids, e.g. acids, alcohols, gasoline etc. Steel plates are used to orm 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 reerred to or the given design o steel water storage tanks. 1. IS code o practice or general construction in steel. 2. IS code o practice or rectangular pressed steel tanks. 3. IS code o practice or use o steel in gravity water tanks. 4.2 General Requirements Size o the tank: It is economical to build large diameter and low height tanks when the stand pipes are at such an elevation that these can supply water a suicient pressure to meet local requirements. 398

10 Usual heights to diameter ratio are as ollows: H = 0.4D Where, H is the height o stand pipe, and D is its diameter. Size o plates: For a pleasant look, the net width o the various rings o the plates is kept same (about 2m). the length o the plates is kept 5 6m Having selected the diameter o the stand pipe, its height, the number o rings o plates, and the number o plates per ring, the thickness o plates can be ound by the hoop stress theory as applicable to thin cylinders. t = γhd/2σ at where γ = unit weight o the liquid(9.81 x10 6 N/mm 3 or water) To account or the eiciency o the joint (η) t = γhd/2ησ at Since the steel plates will be in contact with water, which may lead to corrosion, the thickness so ound is increased by 1.5mm. But in no case the thickness o the plates should be less than 6 mm. Tanks elevated on staging or towers are generally provided or water storage and supply. The capacity and height to the bottom are determined rom a consideration o the service o the tank. The hemispherical bottom is the most common. 4.3 Circular tanks The hemispherical bottom consists o a dished circular plate in the bottom called a saucer plate and plates with radial seams make up the rest o the bottom. In hemispherical bottom tanks, the ratio o the height o the cylindrical shell to the diameter is approximately 1:1 or small capacities and 1.25:1 or capacities over 5 x 10 5 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 or hemispherical bottom tanks up to 25 x 10 5 litre capacity and 1:12 or larger capacities. This batter decreases wind stresses in the tower.the top o the tank is generally covered with thin plates with a pitch o 1 to 6. For tanks up to 7.0m diameter the roo plates are assumed to be sel supporting and or larger diameters angle raters are used to support the plates. A ree board o 15cm is provided. 4.4Circular girder A circular girder o an angle or channel section is provided at the junction o the cylindrical shell and suspended bottom. It has to support weight o the tank, the weight o the water stored and its own weight. The total load acts as a uniormly distributed load 399

11 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 o columns, spaced at an equal distance along the circumerence are provided. The ollowing expressions may be used to obtain the orces in the circular girder. Maximum bending moment M 1 = M 0 cosφ /2 + WR/2n [Sin φ (2sin 2 φ/2)/ (φ/2)] Maximum torsion (at a point x at the angle φ rom the column) T = m0 sin φ (1 cos φ ) +Wφ R /4[1 (sin φ / φ)] Where W = total vertical load N= number o columns (4, 6, 8, 12) R = radius o circular girder Wind orce Design wind speed is given by V z = V b k 1 k 2 k 3 Where, V b = basic wind speed o the place in m/s k 1 = probability actor (risk coeicient) k 2 = terrain height and structure size actor. K 3 = topography actor The value o k 1 is obtained ater deciding the age o the tower and the zone in which it is to be built.the value o k 2 is ixed by deciding the class o the structure which itsel depends upon the tertian category. The topography actor k 3 is ascertained rom topography o the area and by the use o appendix C o IS Design wind pressure p z = 0.6 V z 2 (at a height z) 2.The solidity ratio (φ)is calculated and corresponding to this the orce coeicient (C ). 3.Wind load on the tower, F = C Ae p z where A e = eective rontal area o the tower normal to the wind direction p z = design wind pressure Wind load on the tank container is the product o the intensity o wind pressure and the longer side area exposed to the wind. This orce will act at mid height o container. 400

12 4.4 Earthquake Force As per IS: (which is still not revised or tanks) Table 1: Designed sections using the old (IS: ) and New(IS:800) codes or the same Water Tank Components used Working Stress Method Sections Used Limit State Method Circular Girder ISA 150*150*18mm ISA200*200*18mm Columns 271 N/m ISHB 271 N/m Remarks Bigger size 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 Base Plate 230*230*16mm 230*230*16mm Same size The details urnished in the above table are or inormation only. These observations no way aect the rationality o the Limit State Method ( ie the authenticity o the revised code). The reasons stated in the conclusion above are more than enough to prove the eicacy o the revised code i.e. IS: Conclusions 1. In the limit state method the partial saety actors on load and material have been derived using the probability concept (using statistical methods ) and thereore the method is more rational and realistic. 2. In limit state method the perectly plastic region up to the onset o strain hardening is used. 3. The riveted joints have lost their importance due advantages o bolted joints 4. Ductility aects the strength o a tension member. An increase in ductility allows a better redistribution o stress concentration over the cross section. 401

13 5. For small geometry actor ( ratio o gauge length to the bolt diameter ), the joint is more eicient and results in higher tensile strength in tension member. 6. The eect o shear lag and block shear ailure should be considered. 7. To avoid shear lag, nowadays equal angles are used. 8. Compression members are more critical than tension members. 9. Four dierent column curves a, b, c and d have been recommended by IS code o practice or dierent cross sections o columns to account or the initial imperections o their geometry. 10. Only plastic sections should be used or indeterminate beams to take the advantages o successive ormation o plastic hinges. 11. The compression lange o beam is subjected to lexural buckling as well as torsion. So, the beam is subjected to lexural torsional buckling. 6. Reerences 1. British Standards Institution : "BS 5950, Part 1 Structural use o steelwork in building", British Standards Institution, London, Code O Practice IS Part I, II,III &IV Calculation o Dead Loads or dierent structures, Bureau o Indian Standards, New Delhi, Code O Practice IS ,Determination o seismic design orces based on actors, Bureau o Indian Standards, New Delhi, IS: 800 (1984), General Construction in Steel Code o Practice, Bureau o Indian Standards, New Delhi, IS: 800 (2007), General Construction in Steel Code o Practice, Bureau o Indian Standards, New Delhi, Code O Practice IS 1367 Part 1 & 3, Requirements o Bolted connections, Bureau o Indian Standards, New Delhi, Code O Practice IS , Requirements o Welded connections, Bureau o Indian Standards, New Delhi,

14 8. Duggal.S.K. Limit State Design o Steel Structrues,Tata McGraw Hill Education Private Limited., New Delhi, 2010., 3 rd edition. 9. Structural steel design course material, NITTTR, Taramani, Chennai, Sep2010, pp