In this article we will discuss about:- 1. Steel – A Construction Material 2. Production of Steel 3. Properties 4. Fire Protection 5. Fatigue Effect 6. Brittle Fracture 7. Corrosion Protection 8. Structural Steel Products 9. Sectional Properties 10. Structural Idealization 11. Structural Elements 12. Structural Design.
Steel – A Construction Material:
Steel is the most commonly used structural metal, due to its various properties like great strength, good ductility and high strength, allowing easy fabrication. Due to its high strength members of light sections can be used to carry a heavy load, which means there is considerable reduction in dead load.
It is due to this reason, steel becomes an affordable material for making very long span structures like auditoriums, sport halls etc. Due to their stiffness, steel members deflect to very small extents often not needing special consideration. Steel allows itself to be worked easily in the fabricating shop in various ways like, drilling, sawing, flame cutting etc.
It allows joining easily by welding. Steel is also comparatively cheaper to other metals. For instance, Aluminium may cost 3 to 4 times more than the usual grades of steel. Steel allows easy erections and may not need form work. Considerable part of the steel structure can be prefabricated accurately in the workshop, away from the construction site.
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Due to its relatively lower weight, steel allows making large span structures. It is worthy to note that construction is fast with steel. For example, constructing a bridge over a busy road or railway line can be done in the shortest duration of time, considerably minimizing the period of obstruction. Steel also allows any later modifications like extensions easily.
Production of Steel:
The main constituent of structural steel is iron. Iron is available on the earth’s surface in combination with other elements, in the form of ores. The deposits of iron ore are distinguished by the quantity of metallic iron in combination with other elements contained. The commonly occurring ores are oxides of iron combined with earthy materials and chemically adulterated with sulphur, phosphorous etc.
There are three commercial forms of iron products, viz., wrought iron, steel and cast iron based on the amount of carbon contained (table 1.1).
Steel in modern times is made from the raw material on iron produced by a blast furnace. Iron ore is fed into the blast furnace with coke and limestone. The temperature is raised by a powerful blast of air so as to melt the iron which is run off. The iron obtained at this stage has high carbon content.
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Steel is obtained by removing most of the carbon content by blowing oxygen through molten iron. The steel so made is subjected to a process of rolling to bring it to the required shape. The reheating along with the mechanical working it is subjected to, in the process of rolling; the tensile strength of steel gets increased.
In the generally adopted process, the steel is squeezed between a pair of rotating cylinders called rolls. By this process, the original ingots are changed to the required shapes like plates, bars, wires or structural sections.
Properties of Structural Steel:
Certain properties are very relevant and are of great consideration for a structural designer. It is first worthy to study the behaviour of steel under the influence of a load by examining the stress-strain relation for a low carbon mild steel specimen shown in Fig. 1.1.
Fig. 1.2 shows a portion of the stress-stain diagram to a magnified scale. Studying these diagrams it is worthy to note certain important characteristics of mild steel.
Steel exhibits the property of elasticity till it reaches a well-defined yield point. If the loading is removed while the stress is within the yield point, the deformation vanishes completely and the material reverts to its original unstressed dimensions.
In reality a linear relation between stress and strain is exhibited at stresses below a stress called the proportional limit which is difficult to determine as the deviation from this state to the yield state is within a very close range. In the range of linear relation between the stress and strain, the slope of the stress-strain diagram is the modulus of elasticity of the material. Its value is taken as 2.05 × 105 N/mm2 irrespective of the type of steel.
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Tensile Strength:
The stress applied to cause failure of the material is considerably greater than the yield stress. This can be seen in Fig. 1.2.
Ductility:
Steel has this important property to undergo substantial deformation without fracture. At the stage of failure, the strain may be 0.25 for mild steel, whereas this strain at failure will be less in the case of high carbon steels. It may also be noted that the range of elastic strain is just a small part of the total strain occurring as the material reaches the fracture stage.
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To study the behaviour of steel stressed beyond the elastic unit, it is convenient to simplify the stress- strain curve to an elastic-plastic stress-strain diagram as shown in Fig. 1.3. Since this simplification discards the region of strain hardening the modified stress-strain diagram is a conservative approximation to the actual strength of the material.
In addition to the mechanical properties mentioned above it is also necessary to be aware of the susceptibility of steel to certain other effects.
Fire Protection for Steel:
In the earlier days, there was a belief that uncased iron was an absolutely fire proof element. Later it was found by actual observations that though iron is inflammable, it gets severely distorted and becomes weak and may become unfit.
Fig. 1.4 shows how strength is affected by temperature. One of the methods of fire protection of steel is by providing the steel encased in concrete or brick work. Though concrete may be taken to protect against fire its use in this way has a limitation. A great disadvantage is the considerable increase in dead load by about 10% and the additional labour of placing the concrete.
It may also result in needing heavier foundation. More over the concrete casing can be considered for beams and columns and cannot be provided for roof trusses, lattice girders and space frames. As an improvement light weight encasing materials are being used these days. In this way besides reducing the dead load, these encasing materials are easy and quick to be applied without needing form work. These are available in the form of dry sheets.
Vermiculite, Gypsum and Perlite are the main lightweight materials used. Either they are available in the form of sheets or they are available in planter form. A modern technic in fire protection is to provide hollow columns through which cold water can be circulated. Yet another method is to use a special paint which produces froth on getting heated forming a protective layer, protecting the steel inside. While planning, the columns may be located far from the sources of heat.
Fatigue Effect in Steel Design:
Pulsating loads at high frequency cause extreme variations in stresses. They may also cause stress reversals. Such sudden long range load variations and stress reversals reduce the strength of steel members over a period of time or over a number of cycles.
A failure due to this cause is called a fatigue failure. This is of great importance at places of stress concentrations. Observations have revealed that fatigue failure usually gets initiated from a very small crack produced as a consequence of very high stress at that point.
Fatigue effect is an important consideration in the design of tension members of bridge trusses. Connection of members at joints should be made with HSFG bolts. However in the normal building constructions fatigue effect will not be of serious concern.
Brittle Fracture in Steel:
Steel being a ductile material can be subjected to extension by about one fifth of its original length to reach a failure state. But in some exceptional cases, it can fail suddenly almost with no extension in a brittle fashion.
The causes of such brittle fracture could be the following:
(i) Steel is ductile at a higher temperature but brittle below a critical temperature.
(ii) A zone of stress concentration is likely to develop a brittle fracture.
Corrosion Protection of Steel:
Unprotected steel readily rusts or corrodes. Smoke, soot, sea water, acidic or alkaline vapours and aggressive environments hasten the process of corrosion. The usual protective treatment against corrosion is by covering the exposed steel with paint or a metallic coating or with a sheating with plastic coat.
Another technic is by providing a metal coating like galvanising or zinc spraying. Electroplating can be done for small items like fasteners. Metal spraying using aluminium or zinc may be done. Whatever treatment is given against corrosion, it is very important that the surface to be treated must be cleaned thoroughly. For structural steel work, good surface cleaning may be achieved by blast cleaning. In this process, very small abrasive iron particles are directed to the surface using compressed air.
Structural Steel Products:
The hot steel ingots can be shaped to various standard shapes to be used as structural components, by passing them through rolling mills. Fig.1.6 shows the various structural sections that are practically used.
(i) l-Sections:
I-Sections are used as beams and columns. The shape of the I-Section is best suited to resist bending moment and shearing force. In an I-Section about 80% of the bending moment is resisted by the flanges and the rest of the bending moment is resisted by the web.
Similarly, about 95% of the shear force is resisted by the web and the rest of the shear force is resisted by the flanges. Sometimes l-Sections with cover-plates are used to resist large bending moments.
The standard I-Sections are classified as follows:
(a) Indian Standard Joist Beams (ISJB).
(b) Indian Standard Light Beams (ISLB).
(c) Indian Standard Medium Beams (ISMB).
(d) Indian Standard Wide Beams (ISWB).
(e) Indian Standard Heavy Beams (ISHB).
I-Sections in the categories, ISJB, ISLB and ISMB are generally used as beams. ISJB sections are meant to resist bending about x-x axis only. ISLB and ISMB sections are meant to resist bending about x-x axis and to some extent about y-y axis. Two I-Sections in combination may be used for a column. ISWB and ISHB sections are used for columns.
(ii) Channels:
Channels are used as beams and columns. Because of its shape, a channel member affords connection of an angle to its web. Built-up channels are very convenient for columns. Double channel members are often used for bridge truss members.
Channels are classified into the following categories:
(a) Indian Standard Joist Channels (ISJC).
(b) Indian Standard Light Channels (ISLC).
(c) Indian Standard Medium Channels (ISMC).
(iii) Angles:
Angles are available as equal angles and unequal angles. The legs of an equal angle are equal in length. Angles have great applications in the fabrications. Angles may be used as connecting elements to connect structural elements.
They are also used as tension and compression members of trusses. They are used in combination with plates to form a plate girder. When unequal angles are used in combination for a compression or a tension member, the outstanding legs are the shorter legs generally.
The area of a leg of an angle = [Length of the leg – t/2] t, where t = Thickness of the angle.
(iv) T-Sections:
T-Sections have great applications in furniture fabrication. These sections are good to resist biaxial bending. They are also used as purtins.
These are classified into the following categories:
(a) Indian Standard Normal T-bars (ISNT).
(b) Indian Standard Long legged T-bars (ISST).
(c) Indian Standard Wide T-bars (ISWT).
(d) Indian Standard Light T-bars (ISLT).
(v) Plates:
Plates and strips can be made into hollow sections like, square, rectangle and circle by hot rolling (Fig. 1.7). Thin plates and strips can be made into a wide range of cold rolled sections (Fig. 1.8).
(vi) Castellate Beams:
A technic in fabrication is a method of increasing the depth of steel beams by castellating (Fig. 1.9). A line in zig-zag fashion is cut along the web of an I-Section using an automatic flame cutting machine. The two halves formed are rearranged so that the teeth of the parts match up and are welded.
A further expansion can be achieved by interesting plates between the teeth (Fig. 1.10).
(vii) Compound Sections:
Compound sections are made in many ways mentioned below:
(a) A rolled steel section can be strengthened by welding on its cover plates (Fig. 1.11).
(b) Two different rolled sections can be combined (Example: Crane girder) the two components resist loads in separate directions (Fig. 1.12).
(c) Two steel sections can be connected with patterns or lacing plates to form a strong member acting as a single unit (Fig. 1.13).
(d) Built-up sections made by welding plates. Built-up sections forming I, H and box members can be made by welding plates. These members are called plate girders (Fig. 1.14). These members are meant to carry heavy loads and for long spans.
(viii) Structural Steel Tubes:
Structural Steel tubes are used for making trusses, domes and as scaffolds. These tubes are available in sizes ranging from 15 mm internal diameter to 150 mm internal diameter. For the same internal diameter three different sizes with different thicknesses are available. Large steel tubes can be used as columns.
(ix) Other Steel Sections:
Steel bars of square sections are made in various sizes ranging from 5 mm side up to 200 mm side. These are called ISSQ bars.
Solid circular bars (ISRO bars) are available of varying sizes ranging from 5 mm to 200 mm. These round bars are used as reinforcements in R.C.C. structures, in making trusses and sheds.
Steel of high strength called high tensile steel rods are used in prestressed concrete members.
Steel rails are used in railways and as crane rails.
Corrugated G.I. sheets are used as roofs of workshops, godowns and sheds.
Sectional Properties of Structural Steel Sections:
The Sectional Properties of a Structural Steel Sections are the Following:
(i) The exact dimensions of the section.
(ii) Position of the centroid for a section asymmetrical about one or both axes.
(iii) Area of cross section.
(iv) Moments of inertia about various axes.
(v) Radii of gyration about various axes.
(vi) Elastic and Plastic moduli of the section about principal axes.
The sectional properties of rolled steel standard sections are given in ISI structural Engineers’ Hand Book by Bureau of Indian Standards.
The properties of compound and built-up sections should be calculated from first principles.
The properties of the symmetrical built-up section shown in Fig. 1.15 are as follows:
Area, A = 2 BT + dt
For unsymmetrical sections such as those shown in Fig. 1.16, the elastic and plastic properties should be calculated from first principles.
Structural Idealization of Steel:
After a decision has been taken to construct a steel structure it is necessary to select a suitable structural system. There are some factors which influence in the choice of such system.
These are briefly given below:
(i) Magnitudes of Spans Involved:
In the case of long spans or the need for large clear floor area special considerations are necessary.
(ii) Vertical Loads Liable to Act:
Due consideration must be given to presence of heavy concentrated loads on floors or the need to accommodate cranes.
(iii) Lateral or Horizontal Loads:
A planning has to be done to decide the way the horizontal loads, say wind loads will be resisted. This can be done by providing a rigid frame with rigid joints or by providing suitable bracings which act with the framing, or by providing a separate independent bracing system like shear walls. This planning has its relevance in tall structures.
(iv) Various Services to Be Provided:
Services such as water electricity gas etc. are important services to be provided. These are generally accommodated under the floors. When such services are required to a large extent as in hospitals, it is usual to provide special type of flooring allowing easy installation of pipe work, ducting etc.
(v) Site Conditions:
Ground conditions at the site influence the type of foundation to be provided, like ordinary or raft or piled etc.
Besides the above it is also necessary to decide the manner in which the structure is to be erected. It is also necessary to decide whether the steel work should be kept visible as in the case of exhibition halls. The way the designer has to decide to satisfy the various requirements like those mentioned above (some of which are likely to conflict with each other) becomes a tough and may sometimes be an ignored aspect of structural design.
Observation of past satisfactory schemes, good structural judgement, discussions with others in the profession as well as with the clients will go a long way towards a satisfactory structural arrangement.
Structures we mostly handle come in any of the categories listed in the table 1.2 below:
Of these the first category viz.; bearing wall construction involves steel beams forming the roofs and floors bearing directly on masonry walls. This is limited to low-rise lightly loaded buildings.
Fig. 1.19 shows a steel frame work of beams and columns which is commonly seen nowadays. This type of construction can be done for small simple low rise as well as highly complicated huge multi-storey structures. Based on the manner, the beam-column joints are made these systems may be simple constructions or continuous constructions.
In the case of simple constructions the beams are assumed to be free to rotate relative to the columns and may therefore be considered as simply supported. Any moment transmitted to the column due to the eccentricity of the beam column reaction may be considered.
These simple connections can be satisfactorily done at the site by bolting. Continuous constructions or also called rigid frames possess rigidity in the beam-column connection so that the angle between the beam and column is maintained undisturbed, when the structure is loaded. These rigid connections involve additional fabrications which may involve higher cost of erection, but due to the advantage of rigidity attained, the member sizes required may be minimized.
In the case of long span constructions the beams for roofs and floors are of very long spans and as such the normal rolled sections will not be sufficient and instead deep built up beams or plate girders have to be provided. To cover very large areas space frames and arches are also used.
In the case of tall structures say structures of 20 storeys or more, the wind load effect becomes an important dominant consideration. Some effective arrangement has to be provided for adequate stiffness for the structure. Fig. 1.20 shows two convenient mechanisms to provide the stiffnesses required. The frame can be braced with diagonal members or by in built walls or by rigid frame constructions.
Structural Elements of Steel Framed Building:
A steel frame building consists of a skeletal frame which supports all the loads to which the building is subjected.
The various elements of a steel framed building are the following:
(i) Beams and Girders:
These support vertical loads and are therefore subjected to bending moments and shear forces.
(ii) Ties:
These are members subjected to axial tension.
(iii) Struts, Columns and Stanchions:
These are members subjected to compressive loads. They may in addition be subjected to bending moments also.
(iv) Trusses and Lattice Girders:
These are framed members consisting of compression and tension members. These framed units carry lateral loads.
(v) Purlins:
These are members meant to support roof sheeting.
(vi) Bracings:
These are diagonal ties and struts connected to columns and roof trusses. They are meant to support wind loads. They provide stability to the building.
(vii) Joints:
Joints are provided to connect members.
Example:
Truss joints, Joints connecting beams and columns.
(viii) Bases:
Bases are meant to transmit loads from column to foundation.
Structural Design of Steel Framed Building:
Steel structures may be designed based on the following three theories:
A building design is arrived at with the participation of a multi-disciplined team of which the architect and the structural engineer have their major role. It is the architect who makes the plans for the building to suit the needs of the client, while it is the structural engineer’s role in examining a number of alternative structural arrangements and to work out preliminary designs so that an economical and satisfactory arrangement is selected.
Once a structural arrangement is selected, the process of structural design takes the following course:
(i) Load Analysis.
(ii) Analysis of frames, trusses, girders, columns, floor system, connections.
(iii) Design of all the structural elements from the above data.
(iv) Arriving at the actual structural arrangement with detailed structural drawings.