Full Article - Research in Civil & Environmental engineering

ISSN: 2345-3109

RCEE Research in Civil and Environmental Engineering www.jrcee.com Research in Civil and Environmental Engineering 2013 1 (03) 187-194

ISSUES IN THE DESIGN OF STEEL STRUCTURES FOR SEISMIC LOADING S. Saileysh Sivaraja a*and T.S. Thandavamoorthy b a

Department of Civil Engineering, Dr MGR University, Chennai, India

b

Department of Civil Engineering, Adhiparasakthi Engineering College, Melmaruvathur, Tamil Nadu, India

Keywords Earthquake Steel buildings High rise structures Energy absorption P-∆ effect

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A B S T R A C T Earthquake forces mandate the absorption of energy imparted by it on the structures and steel is best suited for this. Multi-storey buildings are generally constructed in steel as framed structures. A ductile frame can undergo important inelastic deformations, localized in the neighborhood of sections with maximum bending moment. Considerable care is also needed in the design of steel structures to check failures due to instability and brittle fracture to ensure the development of full ductility and energy dissipation capacity under earthquake loading. Load and Resistant Factor Design (LRFD) is a design practice globally adopted to take care of the inelastic behavior of steel. Good workmanship is also an essential requirement in the construction of steel structures which has to resist severe earthquake forces. The paper highlights the issues involved in the design of steel buildings for seismic forces as well as the basic requirements of material characteristics.

INTRODUCTION

Steel is a versatile construction material widely used in the construction of high rise structures, bridges, airport hangers, shopping complex, rope car pylons, recreational structures, steel arch, etc. It has high strength and ductility, which is the prime requirement under seismic action because the structure has to absorb the vibration energy imparted to it during shaking of ground. Duggal (2007) discussed on the large ductility and high strength to weight ratio of structural steel, which make it an ideal material for earthquake resistance. The steel buildings are more flexible and have more lateral displacement than RCC Buildings. Hjelmstad et al (1988) have researched on the cross braces which were fabricated by continuously welding of two angles together toe-to-toe to form a structural box. The special steel collars were fabricated and placed at the top and bottom of the each column to facilitate the attachment of the steel cross-braces. These collars were grouted and bolted to both the original concrete column and adjoining slab to smooth

*

Corresponding author (E-mail: [email protected]).

S. Saileysh Sivaraja et al - Research in Civil and Environmental Engineering 2013 1 (03) 187-194

out the transfer of forces between stories. Jara et al (1989) have worked on to eliminate the damaged nonstructural elements, the steel bracing in transverse direction. Bracing consists of angle sections welded together forming a box section. Bhavikatti (2012) discussed on mainly structural steels and their properties. The structural steel is used for the manufacture of rolled steel sections. These rolled steel sections are used to form steel frame works required in the structures. Agarwal and Shrinkhande (2010) have reported on the applications of the most common retrofitting schemes employed to improve the efficiency and proficiency of either the seismically efficient vulnerable Steel/RCC building or earthquake damaged buildings. Thakare and Nandurkar (2012) worked to make the transmission line structures more cost effective by changing their geometry (shape) and behavior (type). This objective of the research is met by choosing a 220 kV Single Circuit Transmission Line carrying Square Base Self Supporting Towers. In this connection Modani and Godbole (2012) have stated that copper only would be used for electrical purposes and the stainless steel may disappear. In this paper an attempt has been made to study the rate of corrosion of steel bars exposed to various aggressive environments and provide justification for effect of these environments on the rate of corrosion of steel. Makesh et al (2012) worked on the materials and the manufacturing processes of cold-formed steel shapes. Typical uses of cold-formed steel shapes in roof, wall and floor systems are also described in details. Recent developments of cold-formed steel shapes are also highlighted. Engineers are encouraged to take full advantages offered by cold-formed steel construction technology to build strong and stiff buildings of high build ability and structural economy. Senthil et al (2012) have stated that strengthening of structures with carbon fiber reinforced polymer (CFRP) has hitherto been traditionally applied to concrete structures, but more recently applied to steel structures. Steel-CFRP composite in hollow section combine the benefits of the high strength to weight ratio and more ductility. The experiment was conducted on long steel columns with CFRP wrapping. It is concluded that the application of CFRP to slender sections increases ductility of the section.

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EFFECT OF STEEL STRUCTURES

The large ductility and the high strength-to-weight ratio of structural steel make it an ideal material for earthquake resistance. In general, steel buildings are more flexible than RCC buildings, but also they display more lateral displacement than RCC buildings. Structural planning of steel buildings should conform to that the beams yield prior to the columns, and the strength of a connection should be greater than the strength of beams and columns framing into the connection members and connections should guarantee high strength, ductility, and energy dissipation capacity, and an excessive lateral sway should be avoided. Multistorey buildings are generally constructed in steel as framed structures. A ductile frame can undergo important inelastic deformations, localized in the neighborhood of sections with maximum bending moment. This eventually leads to the formation and rotation of plastic hinges and redistribution of plastic moments, allowing the structure to resist higher loads than those predicted by the elastic analysis. The steel frames may either be un-braced or braced. Un-braced steel buildings are ductile and possess large energy dissipation capacity but tend to deform greatly, causing serious damage to non-structural elements during small to medium-size earthquakes. Braced frames can resist large amounts of lateral forces and have reduced lateral deflection and thus reduced P-∆ effect. However, a uniform distribution of bracing throughout the structure is desirable. This versatile material is widely used in the construction of roofs (Fig. 1), high rise structures, bridges, airport hangers, shopping complex, rope car pylons (Fig. 2), recreational structures (Fig. 3) and steel arches (Fig. 4), etc.

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Fig. 1 Steel truss

Fig. 2 Rope car pylons

Fig. 3 London eye

Fig. 4 Steel arches

FAILURE MODES OF STEEL STRUCTURES

Although steel is highly ductile, inelastic ductility is not necessarily retained in the finished structure. Hence, care must be taken during design and construction to avoid losing this property. Considerable care is also needed to check failures due to instability and brittle fracture to ensure the development of full ductility and energy dissipation capacity under earthquake loading. The causes of instability are: (i)

Local buckling of plate elements (e.g., web, flange, etc.) with large width–to-thickness ratios: A steel member containing plate elements with a large width-to thickness ratio is unable to reach its yield strength, because of prior local buckling. Even if the yield strength is attained, ductility will be inadequate. Under cyclic loading, it is observed that strength and ductility decrease with increasing width-to-thickness ratio, and local buckling of web causes further degradation.

(ii) Flexural buckling of long columns and braces: Long columns may fail by buckling. This mode of instability is sudden and can occur when the axial load in a column may never reach the yield. Even a small lateral force in such condition will produce a substantial deflection leading to instability and the phenomenon is called flexural buckling. The capacity of slender columns is, therefore, limited by the stiffness of the member rather than the strength of the material. The lateral stiffness of the frames, therefore, is increased by bracing the frames. However, buckling of braces is a potential source of instability of steel frames. Steel bracing dissipates considerable energy by yielding under tension but buckle without much energy dissipation in compression. Therefore, the energy dissipation capacity of concentrically braced frames is marked less, due to buckling of braces than that of the moment frames. 189


(iii) Lateral-torsional buckling of beams: During moderate to strong shaking of the ground, additional forces are developed in various members of a structure. For a beam loaded in flexure, the load bearing the side (generally the top) carries the load in compression, whereas the non-load bearing side (generally the bottom) will be in tension. If the beam is not supported in the opposite direction of bending, and the flexural load increases to a critical limit, the beam will fail due to local buckling on the compression side in wide-flange sections designed for flexure only. If the top flange buckles laterally, the rest of the section will twist resulting in a failure mode known as lateral-torsional buckling. (iv) P-∆ effects in frames subjected to large vertical loads: If the lateral stiffness is inadequately high, the building as a whole, or one or more stories, can fail due to the P-∆ effect. This is because of the secondary effect on shears and moments of the frame members, due to the action of the vertical loads, which interact with the lateral displacement of the building resulting from seismic forces. (v) Uplift of braced frames: Earthquakes have a vertical component of movement in addition to the traditionally considered horizontal effects. The stresses produced due to vertical motion are generally considered not to be significant to cause instability. However, due to the horizontal component of movement, the overturning moments produce additional longitudinal stresses in walls and columns and additional upward (uplifting) and downward (thrust) forces in foundations causing instability. (vi) Connection failure: The failures of bolted and welded connections are to be avoided.

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CAUSES OF FAILURE

The causes of brittle failure in steel buildings are that brittle failure is more frequent in welded steel structures, particularly, those that are fillet welded, than it is in structures connected by mechanical fasteners. This is due to a combination of possible weld defects, high residual stresses, stress concentration, which reduce the possibility of crack arrest, tension failure at net sections of bolted or riveted connection, and Lamellar tearing of plates in which the through-thickness strain due to weld metal shrinkage is large and highly restrained. It is evident that the main objectives to achieve adequate performance of steel buildings are: the use of sufficiently ductile steel, and the ductile design and fabrication of framed members and connections. All frame instability, especially the excessive sway leading to higher levels of damage to non-structural components and to higher secondary stresses due to P-∆ effect, should be avoided; all forms of brittle failures should be avoided; and also failure mechanism should provide maximum redundancy, i.e., the possibility of failure by local collapse should be avoided. All portions of the building should be tied well together. The relevant Indian codes of practice, IS 800 (2007) and IS 801 (1975) applicable to the structural use of hot-rolled steel and cold-rolled steel, are largely based on the working stress or allowable stress method and the plastic design method. The limit-state design approach, also known as capacity design approach, developed in the 1970 is in use in most part of the world. The method based on this approach is called Load and Resistance Factor Design (LRFD) method. Since the latest revision of Indian Standard Codes for steel construction based on limit state design is slowly coming into vogue in design offices in the country because of its recent introduction, the scope of this paper is limited to the discussion relating to the 190


fundamental issues of design only. However, seismic provisions for structural steel buildings, prescribed by the American Institute of Steel Constructions (AISC) have been used to explain the concepts and specifications incorporated with regard to LRFD method, wherever required.

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DESIGN REQUIREMENTS

Behavior of steel buildings under strong earthquake has, generally, been satisfactory from the point of view of strength. Medium-height buildings (up to 40 m) designed for only vertical loads with flexible connections have performed well in past. However, their lateral stiffness being inadequate the windows and non-structural partitions have suffered considerable damage. The actual and idealized stress-strain relationship for steel shown in Fig. 5 (a) and (b) is usually idealized in bilinear form, shown by the dotted lines, although strain hardening (broken lines) is taken into account in some cases. The yield stress (fy) and the ultimate stress, fu, are used for steel sections or plates, and fy is used for reinforcing bars. The value of Young’s modulo (E) is about 2  105 MPa. The hysteretic stress-strain relationship for steel subjected to repeated real and elastic-plastic behavior, is shown in Fig. 6 (a) and (b). The unloading branch shows an incipient slope equal to the elastic slope and is gradually softened owing to the Hysteretic Bi-linear and Bauschinger effect. Examples of simple models are shown in Fig. 6 (c) and (d).

(a) Actual

(b) Idealized Fig. 5 Stress-Strain Relationship

5.1 Materials and Workmanship Steel structures generally perform well in major earthquakes. However, careful detailing and control of material properties are necessary to ensure the development of its full ductility under earthquake loading. The basic steel material must be homogeneous with moderate value of yield stress and of good quality. Steel with minimum yield strength of 340 N/mm2 is specified for members expecting inelastic action under the effects of the design earthquake. In order to obtain adequate ductility from a steel structure proper use of the inelastic properties of the steel are to be ensured. To achieve this, the component material must be such that total elongation up to breaking failure is sufficiently large and the ratio of yield stress to which a tension member with bolt holes breaks on a net section, before yielding takes place in a gross section. In general, the steel that is explicitly permitted for use in seismic design is supposed to meet the following requirements: A ratio of yield stress to tensile stress not greater than 0.85; a pronounced stress-strain plateau at the yield stress; a large inelastic strain capability (e.g., tensile elongation of 20 percent of or greater measured on a gauge length of 5.65 A where A is the cross sectional area) and good weld ability. In addition to the above characteristics, the steel used should have adequate ductility and consistency of 191


mechanical properties to satisfy seismic requirements, besides satisfying four general requirements, such as adequate notch ductility, free from lamination, resistance to lamellar tearing, and good workability.

(a) Real behavior

(c) Bi-linear behavior

(b) Elastic-Plastic behavior (d) Bauchinger behavior Fig. 6 Hysteretic Behavior of Steel

5.2 Ductility Ductility is generally described as the post-elastic behavior of a material. For steel, it may be expressed simply from the results of elongation tests on small samples, or more significantly in terms of momentcurvature or hysteresis relationships. 5.3 Notch ductility Notch ductility is measure of the resistance of steel to brittle fracture. It is generally expressed as the energy required for fracturing a test piece of particular geometry. For ductile elements, steel should be of low carbon and weldable and should have notch ductility. 5.4 Consistency of mechanical properties Practical relation of the fundamental principles is that in the case of beam-column joints in framed structures, it is desirable that beams must fail before columns. This requires that the maximum and minimum strengths of the members are nearly equal in magnitude as possible. This implies that the standard deviation of strengths should be as small as possible. 5.5 Laminations Laminations are large areas of unbounded steel found in the body of a steel plate or section. This implies layering of the steel with little structural connection between layers. The laminated areas originate in the casting and cropping procedures for the steel ingots, and may be as much as several square meters in extent, Steel may be screened ultrasonically for lamination before fabrication.

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5.6 Lamellar tearing Lamellar tearing is a tear or stepped crack which occurs around a weld where sufficiently large shrinkage stresses have been imposed in the through thickness direction of the susceptible material. It commonly occurs in the T butt welds and in other welds. Lamellar tearing also occur at the interface between inclusions and the surrounding material due to the incapacity of the parent metal to accommodate strains imposed by weld shrinkage in the thickness direction. This is very common in thick plates. For example, this failure can occur where butt or fillet welds of 20 mm or over are made on plates of at least 30 mm in thickness, where there is a high degree of a resistant tearing occur in planes parallel to the direction of rolling. The solutions to this problem lie to a degree in the selection of the steel (low sulfur content), as tearing usually occurs during fabrication following cooling of the adjacent weld. Good detailing of welds are some of the recommended construction practices to avoid lamellar tearing. 5.7 Workmanship The detailing and fabrication of ductile portions of the structure should consider the possibility of low cycle fatigue-structures responding to earthquakes in which case rarely go through more than 20 cycles of response. Fatigue failure can initiate at notches and cracks which run at right angles to the direction of stress. Inadequate fabrications procedures can drastically reduce ductility. Welding should follow the best standards of quality and inspection. The weld metal should be able to closely match the properties of the parent plates. Bolt holes should be drilled and not punched or reamed. Cutting of the sections may be affected by sawing only. The component parts should be assembled and aligned in such a manner that they are neither twisted nor otherwise damaged.

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CONCLUDING REMARKS

Steel is a versatile material with enormous ductility, an important characteristic required for the absorption of substantial amount of energy imparted by earthquake loading. Therefore, it is best suited for design of structures subjected to seismic forces. Multi-storey buildings are generally constructed in steel as framed structures. A ductile frame can undergo important inelastic deformations, localized in the neighborhood of sections with maximum bending moment. Considerable care is also needed in the design of steel structures to check failures due to instability and brittle fracture to ensure the development of full ductility and energy dissipation capacity under earthquake loading. Some of the failure modes of steel structures are: local buckling, flexural buckling, lateral-torsional buckling, etc. Load and Resistant Factor Design (LRFD) is a design practice globally adopted to take care of the inelastic behavior of steel. Steel used should have adequate ductility and consistency of mechanical properties to satisfy seismic requirements, besides satisfying four general requirements, such as adequate notch ductility, free from lamination, resistance to lamellar tearing, and good workability. Good workmanship is also essentially required in the construction of steel structures which has to resist severe earthquake forces.

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Bhavikatti, S.S., (2012). Design of Steel Structures, Second Edition, New Delhi, India, 402. Duggal.S.K, (2007). Earthquake Resistant Design of Structures, Oxford University Press, New Delhi, India, 448. IS 800 (2007). Indian Standard General Construction in Steel-code of practice, Bureau of Indian Standards, New Delhi, India IS 801 (1975). Indian Standard Code of practice for use of Cold-formed light gauge steel Structural members in general Building construction (first revision), Seventh reprint December 1998, Bureau of Indian Standards, New Delhi, India. Jara, M., Hernandez, C., Garcia R., & Robles, F., S (1989). Earthquake Spectra: Seismic Retrofitting of RCC Building by Steel Bracing and Infill Walls, (5), 1. Hjelmstad, K.D., Foutch, D.A,Valle, E. D. & Downs, R.E. (1988). Forced Vibration Studies of an RC Building Retrofit with Steel Bracing, In: Proceedings of Ninth World Conference on Earthquake Engineering, Tokyo-Kyoto, Japan, (7). Modani, P.O. & Godbole, R. P. (2012). Effect Of Different Exposure Conditions On The Rate Of Corrosion Of Steel Bars In: Proc, Proceedings Of The National Conference On Innovative Trends For Technology Developments (Technocon-2012), on Jan 6-7, 2012, (Org.) Datta, Meghe Institute of Engineering, Technology and Research, Sawangi (Meghe), Wardha, Maharashtra, India, 88. Makesh, A.P., Dasarathy, A.K., Moses Aranganathan, S., & Saileysh Sivaraja, S. (2012). Building Application Of Cold Form Steel Structures, In: Proc, Proceedings of International Conference on Emerging Engineering Trends (ICMEET-2K12), April 12-13, Magna College of Engineering, Chennai, India, 154-158. Senthil, R., Punitha Kumar, A., & Satheesh Kumar, R. (2012). Axial Strength Of CFRP Strengthened Circular Hollow Steel Sections”, In: Proc, International conference on Earthquake Resistant Construction Practices (ICEQRCP-2012), July 27th and 28th, Organised by the Department in Civil Engineering, Dr MGR Educational & Research Institute University, Chennai, Tamil Nadu, India, 227-238. Thakare N. U. & Nandurkar, B.P. (2012). Analytical Study of Transmission Line Towers of Different Configurations for Economical Design, In: Proceedings of the National Conference on Innovative Trends for Technology Developments (Technocon-2012), on Jan 6-7, 2012, Organised by DattaMeghe Institute of Engineering, Technology & Research, Sawangi (Meghe), Wardha, Maharashtra, India, 91.

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