Concept of Design of steel structures
Design problems are seldom amenable to solution by exact mathematical formulae. There is a considerable scope for exercising engineering judgement. Hence, there is no “correct solution” to a design problem, as there could be several so-called “correct solutions” to the same problem. This is because the designs are invariably subject to individual interpretation of Standards and Codes, the solutions are also subject to differing ideas about what is or what is NOT required from an engineering and environmental stand point, and the individual designers have ingrained ideas from their past experience, which may be valid to-day only to a limited extent, or may not be valid at all.
Thus the design problems are referred to as “open ended” problems. Nevertheless the Designer has the responsibility for ensuring that the goal of the project is achieved (i) safely, without taking any undue risks to lives and materials and without causing a liability, (ii) within time and (iii) within the (budgeted) cost. Hence, “Engineering Design” may be defined as a creative activity of building a new artefact which provides an optimum solution to satisfy a defined requirement or need, without endangering the environment.
Traditionally the professional Structural Engineer had invariably played a vital role in the design of constructed facilities, often, in close association with other professionals like Architects and others in related disciplines. As a designer, he is responsible for the complete process from the conceptual stages to the finished structure. Increasingly, the Society expects him to assume responsibility for the durability of the product. In other words, the responsibility of a professional Structural Engineer in the 21st century will not be confined merely to the immediate economic and environmental impact of his design decisions; society expects him to make rational and responsible choices by considering the life cycle costs and the long-term environmental effects on the community In the following pages, we will highlight the enhanced role of the Professional Engineer in the 21st century and explore how the two design criteria are interlinked.
The Construction Industry, with all its imperfections and limitations, is rightly perceived as the provider of the Nation’s infrastructure. Clearly, it is of paramount importance to train and educate those who create and manage it, in order to ensure the economic and environmental survival of the world. While the world has witnessed some fantastic advances in Science and Technology in recent years, many of these achievements have been made at an outrageous price, plunging the world into a number of crises, which have impacted directly on the construction industry. The global effect of these dramatic changes in the world in the last 50 years can be collectively termed the “infrastructure crisis”, which has to be encountered and managed by the construction industry.
Issues of durability have always been subjects of debates among Engineers. Is it better to spend (say) 40% more initially, in order that the life of a structure could be doubled? What is better value to the client? Spend less initially or opt for a longer life? Total neglect of durability considerations in all the infrastructure projects undertaken so far combined with primitive construction practices still prevailing in India have resulted in what can only be termed a “durability crisis”. It is now well established that degradation of all structures has become very common in almost all the cities in India and this is particularly true of buildings and structures made of reinforced/prestressed concrete. The great tragedy is that there have been no efforts to address this issue by the present generation of Developers, Engineers, Architects and other design professionals. As a consequence, major problems have been allowed to accumulate for future generations of owners and taxpayers to face.
When a constructed facility is completed, it will be put to use immediately and this results in a return on the capital employed. Delays in the completion of a project would therefore represent a delay in the return on capital invested, besides the loss of interest, which that sum would have earned otherwise. This essential relationship between time and money is well understood in the present context.
Unfortunately for the Indian client, many architects and designers seldom consider the use of alternative materials of construction and the designs are invariably limited to “concrete-intensive” structures. Often the best optimal design solution is obtained by a sensible combination of reinforced and/or prestressed concrete elements with structural steel elements. Even when a “steel-intensive” solution is selected; it is very rare for limiting the selection of materials of construction to steel only.
Although India has an installed capacity to produce 35 million tonnes of steel/year, we manage to produce only 24 million tonnes/year of which the use in the construction sector accounts for around 25% – 30%. By way of comparison, China produced 120 million tonnes of steel during 1999 – 2000 and Japan, 95 million tonnes. The total per capita consumption of steel in all its forms in India is one of the lowest in the world, being 24 kg/annum, compared with 500 kg/annum in the USA and 700 kg/annum in Japan. According to the recent research by the Steel Construction Institute, there is a direct link between the gross national product per capita and the per capita consumption of steel.
Indeed, structural steel has inherently superior characteristics to a very significant extent, when compared with competing materials. For example, to replace one unit area of steel in tension, (with a yield stress of 450 MPa), we would need to use an equivalent plain concrete area of about 200 units. For concrete to be able to compete with Structural Steel in construction, we need to put Reinforcing Steel into it! Even then, there is no way to prevent the cracking of concrete in tension, which often encourages corrosion of reinforcement. In compression (or squash loading), one unit area of steel is the equivalent of 15-20 units of M20 concrete. A comparison of strength/weight ratio will reveal that steel is at least 3.5 times more efficient than concrete. For a given compressive loading, concrete would have 8 times the shortening of steel. Again we need reinforcing steel to prop up the plain concrete.
In structures built of Structural Steel, occasional human errors (like accidental overloading) do not usually cause any great havoc, as there is a considerable reserve strength and ductility. Steel may thus be regarded as a forgiving material whereas concrete structures under accidental overload may well suffer catastrophic collapse of the whole structure. Repair and retrofit of steel members and their strengthening at a future date (for example, to take account of enhanced loading) is a lot simpler than that of reinforced concrete members. The quality of steel-intensive construction is invariably superior, when compared with all other competing systems (including concrete structures) thus ensuring enhanced durability. This is especially true in India, where quality control in construction at site is poor.
Structural Steel is recyclable and environment-friendly. Over 400 million tonnes of steel infrastructure and technology for the recycling of steel is very well established. Steel is the world’s most versatile material to recycle. But once recycled, steel can hop from one product to another without losing its quality. Steel from cans, for instance, can as easily turn up in precision blades for turbines or super strong suspension cables. Recycling of steel saves energy and primary resources and reduces waste. A characteristic of steel buildings is that they can readily be designed to facilitate disassembly or deconstruction at the end of their useful lives. This has many environmental and economic advantages; it can mean that steel components can be re-used in future buildings without the need for recycling, and the consequent avoidance of the energy used and CO2 emitted from the steel production processes.
Steel-intensive construction causes the least disturbance to the community in which the structure is located. Fast-track construction techniques developed in recent years using steel-intensive solutions, have been demonstrated to cause the least disruption to traffic and minimise financial losses to the community and business. Even though “the initial cost” of a concrete intensive structure may sometimes appear to be cheaper, compared to the equivalent steel-intensive structure, it has been proved time and again that its total lifetime cost is significantly higher. Thus the popular perception of the concrete-intensive structure being cheaper is NOT based on verifiable facts! There is therefore no real cost advantage either.
Except in a few special structures like tower cranes and transmission towers, it is rare to build a structure entirely in steel. Frequently the optimal solution is obtained by employing concrete elements compositely with structural steel, especially in multi-storeyed buildings and bridges. These methods ensure significant cost benefits to the developers (or owners of property) as well as to the community. Composite structural forms have been extensively developed in the western world to maximise the respective benefits of using structural steel and concrete in combination, but this technology is largely ignored in India, despite its obvious benefits. The sizes of composite beams and columns will be appreciably smaller and lighter than that of the corresponding reinforced or prestressed sections for resisting the same load. A direct economy in the tonnage of steel and indirect economies due to a decrease in construction depths of the floors and reduced foundation costs will, therefore, be achieved. Generally, improvements in strengths of the order of 30% can be expected by mobilising the composite action. An independent study carried out by the Central Building Research Institute (CBRI) Roorkee demonstrated that there are substantial cost savings to be achieved by the use of Composite Construction
A structural engineer’s responsibility is to design the structural systems of buildings, bridges, dams, offshore platforms etc . A system is an assemblage of components with specific objectives and goals and subject to certain constraints or restrictions. System
components are required to co-exist and function in harmony, with each component meeting a specific performance. Systems design is the application of a scientific method to the selection and assembly of components to form the optimum system, to achieve the specified goals and objectives, while satisfying the given constraints or restrictions.
In practice, any constructed facility can be considered as a “System”. The Structural System is one of its major subsystems and is indeed its backbone. Some of the other coexisting subsystems are those connected with the mechanical, electrical, plumbing and lighting facilities. Structural components have to meet the design requirements of adequate strength under extreme loads and required stiffness under day-today service loads, while satisfying the criteria of economy, buildability and durability..
Examples of civil engineering systems include buildings, bridges, airports, railroads, tunnels, water supply network etc. For example, a building system is an assemblage constructed to provide shelter for human activities or enclosure for stored materials. It is subject to restrictions by building specifications on height, floor area etc. Constraints include ability to withstand loads from human activities and from natural forces like wind and earthquakes. As pointed above, a system consists of many subsystems, i.e. components of the system. For example, in a building, major subsystems are structural framing, foundations, cladding, non-structural walls and plumbing. Each of these subsystems consists of several interrelated components. In the case of structural framing, the components include columns, beams, bracing, connections etc. The richness and variety of structural systems can be appreciated by the available building structural types that range from massive building blocks to shell structures, from structures above or below ground or in water, to structures in outer space. Examples of a few steel-framed structures are shown in Fig. 2.1.
Before starting the design of a system, the designer should establish the goals for the system. These specify what the system is to accomplish and how it will affect the environment and other systems or vice versa. Goals are generally made in statements of specific design objectives such as purpose, time and cost limitation, environmental constraints etc., which would enable the generation of initial and alternative designs. The goals for a system design applied to a subsystem serve the same purpose as for a system. They indicate the required function of the subsystem and how it affects and is affected by other subsystems.
Having set down the goals, the designer defines the system objectives. These objectives are similar to goals but explain in detail the requirements that the system must satisfy to attain the goals. Some of the essential objectives of any project relate to health, safety and welfare requirements of the occupants, which are generally defined in local building codes or building regulations. Other special objectives include minimisation of initial costs, life-cycle costs, construction time etc. At least one criterion (e.g. Fire resistance) must be associated with each objective. A criterion is a range of values within which the performance of the system must lie (e.g. Two hours fire rating is needed). The criterion serves as a guide in the evaluation of alternative systems to the project.
Figure 2.1 Examples of steel frame structure
2.4 Constraints and Standards
Constraints are restrictions on the values of design variables, which may or may not be under the control of the designer. For example, an I-beam section of 200 mm depth may be desirable, but not available. There are also various legal and building code requirements. A minimum of one standard must be associated with each constraint.
2.5 Codes and Specifications
A structural engineer is guided in his design efforts by the relevant codes and specifications. Although the word ‘codes’ and ‘specification’ are normally used interchangeably, there is a distinction between them. A detailed set of rules and suggestions prepared by an interested party is called an engineering specification. On the other hand, Codes are frequently formulated by a group of professionals with a view to their adoption by the profession as a whole. These are revised at regular intervals based on new developments in materials, research, construction techniques etc. Though codes offer general guidance to a certain extent, they do not provide answers to all the problems that arise in practice. Mere adherence to codes and specifications will curb all initiatives and innovative designs.
2.6 Design Requirements
The primary structural safety requirement is met by ensuring that the structure has an acceptably low risk of failure during its design life. Another important requirement is that the structure must be sufficiently stiff to ensure that excessive deflection and vibrations do not affect the in-service performance of the structure. The requirement of harmony within the structure is affected by the relationships between the different subsystems of the main system, the architectural subsystem, the mechanical and electrical subsystems, and the functional subsystems required by the use of the structure. Finally, the system should be in harmony with its environment, and should not react unfavourably with either the community or its physical surroundings.
Conceptual design refers to the task of choosing a suitable system. (As an example architect is generally concerned with the building layout, limits and parameters). In modern construction practices, a multidisciplinary team of architect, structural designer and service engineer together evolve the conceptual design. A typical organisational chart for a multidiscipline design team is seen in Fig. 2.2, which shows the inter-relationship between the various design professionals. The structural engineer is charged with the task of ensuring that the structure will resist and transfer the forces and loads acting on it with adequate safety, while supporting other subsystems and making due allowance for the requirements of serviceability, economy, harmony and constructability. The iterative process of achieving such a design is shown in Fig. 2.3. Since several simplifying approximations are made in the preliminary design, it is necessary to re-check the design. The loads are recalculated more precisely and the structure is reanalysed. The performance of the structure is then re-
evaluated with respect to the structural requirements, and any changes in the member and joint sizes are made as shown in Figure 2.4.
Figure 2.2 Example of multi – discipline project organisation
Figure 2.3 The overall design process (Iterative)
Figure 2.4 The structural design process
Frequently, there is a fundamental confusion among students between a problem on Analysis and one on Design. In an analysis problem, all the parameters are known. (For example, if deflection of a loaded beam is required and the span, loading and the cross sectional properties are all known then a unique solution for the value of deflection can be arrived). The problem encountered in design (as compared to analysis) is that it involves the selection of the span, assessment of the loading, choice of the material of which the beam element is made, definition of its cross section and so on.. As a consequence, no unique solution can be offered for any design problem. It is clear that the designer has to make several decisions, each of which could affect the final result. Considerable engineering discretion of the designer is implicit in every design project.
The aim of the comparison of designs is to enable the designer to ascertain the most acceptable solution that meets the requirements for the given structure. All factors must be taken into consideration. Factors to be taken into account in a typical building project are given below by way of illustration:
Materials to be used
Arrangement and structural system (e.g. flooring system) to be adopted
Fabrication and type of jointing
Proposed method of erection of the framework
Type of construction for floor, walls, cladding and finishes
Installation of ventilating/ heating plant, lifts, water supply, power etc.
Corrosion protection required
Fire protection required
Operating and maintenance costs
Figure 2.6 Beam and column construction
Aesthetic considerations are important in many cases and the choice of design may not always be based on cost alone. The weight saving may be offset by the higher cost of the stronger material or the higher cost of fabrication/construction of complicated systems. Often no one solution for a given structure is prominent or obvious to the exclusion of all other alternatives. As an example, we can illustrate several choices available to the designer for a single bay, single storey structure as shown in Figure 2.5. An example of beam and column system frequently used is illustrated in Figure 2.6. Cable stayed structures are frequently employed in long span bridges and buildings and are shown in Fig. 2.7. In the following chapters, the analysis and design of steel elements are discussed in depth, followed by the analysis and design of selection of structural elements.
Figure 2.7 Cable-stayed structures