CAMBER DEFINITION BASIC INFORMATION AND TUTORIALS

What Are Cambers?

Camber is a curvature built into a member or structure so that when it is loaded, it deflects to a desired shape. Camber, when required, might be for dead load only, dead load and partial live load, or dead load and full live load. The decision to camber and how much to camber is one made by the designer.

Rolled beams are generally cambered cold in a machine designed for the purpose, in a large press, known as a bulldozer or gag press, through the use of heat, or a combination of mechanically applied stress and heat.

In a cambering machine, the beam is run through a multiple set of hydraulically controlled rollers and the curvature is induced in a continuous operation. In a gag press, the beam is inched along and given an incremental bend at many points.

There are a variety of specific techniques used to heat-camber beams but in all of them, the side to be shortened is heated with an oxygen-fed torch.

As the part is heated, it tries to elongate. But because it is restrained by unheated material, the heated part with reduced yield stress is forced to upset (increase inelastically in thickness) to relieve its compressive stress.

Since the increase in thickness is inelastic, the part will not return to its original thickness on cooling. When the part is allowed to cool, therefore, it must shorten to return to its original volume. The heated flange therefore experiences a net shortening that produces the camber.

Heat cambering is generally slow and expensive and is typically used in sections larger than the capacity of available equipment. Heat can also be used to straighten or eliminate warping from parts. Some of these procedures are quite complex and intuitive, demanding experience on the part of the operator.

Experience has shown that the residual stresses remaining in a beam after cambering are little different from those due to differential cooling rates of the elements of the shape after it has been produced by hot rolling. Note that allowable design stresses are based to some extent on the fact that residual stresses virtually always exist.

Plate girders usually are cambered by cutting the web plate to the cambered shape before the flanges are attached.

Large bridge and roof trusses are cambered by fabricating the members to lengths that will yield the desired camber when the trusses are assembled. For example, each compression member is fabricated to its geometric (loaded) length plus the calculated axial deformation under load. Similarly, each tension member is fabricated to its geometric length minus the axial deformation.

SETTING OUT VERTICALITY, TUNNELS AND PIPELINES IN CIVIL ENGINEERING PROJECTS

As a building rises the vertical alignment must also be controlled. This can be done by extending building centre lines at right angles to each other out to fixed points clear of the structure.

These lines can then be projected up the building and marked, allowing accurate measurements from these marks at each floor. Alternatively an optical plumb can be used to project a fixed point up through openings in the floors of the building so as to provide a set of reference points at each level.

The standard of setting out for tunnels must be high using carefully calibrated equipment, precise application and double checking everything. An accurate tunnel baseline is first set out on the surface using the methods described above. Transference of this below ground can be done by direct sighting down a shaft if the shaft is sufficiently large to allow this without distortion of sight-lines on the theodolite.

With smaller shafts, plumbing down may be used. A frame is needed either side of the shaft to hold the top ends of the plumb-lines and to allow adjustment to bring them exactly on the baseline. The plumb-line used should be of stainless steel wire, straight and unkinked, and the bob of a special type is held in a bath of oil to damp out any motion.

By this means the tunnel line is reproduced at the bottom of the shaft and can be rechecked as the tunnel proceeds. Many tunnels are nowadays controlled by lasers, the laser gun being set up on a known line parallel to the centre line for the tunnel and aimed at a target.

Where a tunnelling machine is used, the operator can adjust the direction of movement of the machine to keep it on target so that the tunnel is driven in the right direction. For other methods of tunnelling, target marks can be set on the soffit of rings, the tunnel direction being kept on line by adjusting the excavation and packing out any tunnel rings to keep on the proper line.

Lasers are also used in many other situations, usually for controlling construction rather than for original setting out since their accuracy for this may not be good enough. The laser beam gives a straight line at whatever slope or level is required, and so can be used for aligning forms for road pavements or even laying large pipes to a given gradient. For the latter, the laser is positioned at the start of a line of pipes and focused on the required base line.

As each new pipe is fitted into the pipeline a target is placed in the invert of the open end of the pipe, using a spirit level to find the bottom point, and the pipe is adjusted in line and level until the target falls on the laser beam. Bedding and surround to the pipe are then placed to fix the pipe in position.

Rotating lasers are also widely used and once set up give a constant reference plane at a known level. Use of a staff fitted with a reflector allows spot levels to be obtained anywhere in the area covered by the laser. Earthmoving equipment fitted with appropriate sensors can also be operated to control the level of excavation or filling with minimum input other than by the machine operator.

SUSPENSION BRIDGE TYPES BASIC INFORMATION AND TUTORIALS

What Are The Types Of Suspension Bridges?

Several arrangements of suspension bridges are illustrated in Fig. 1. The main cable is continuous, over saddles at the pylons, or towers, from anchorage to anchorage.  

FIGURE 15.9 Suspension-bridge arrangements. (a) One suspended span, with pin-ended stiffening truss. (b) Three suspended spans, with pin-ended stiffening trusses. (c) Three suspended spans, with continuous stiffening truss. (d ) Multispan bridge, with pin-ended stiffening trusses. (e) Self-anchored suspension bridge.

When the main cable in the side spans does not support the bridge deck (side spans independently supported by piers), that portion of the cable from the saddle to the anchorage is virtually straight and is referred to as a straight backstay.

This is also true in the case shown in Fig. 1a where there are no side spans. Figure 1d represents a multispan bridge. This type is not considered efficient, because its flexibility distributes an undesirable portion of the load onto the stiffening trusses and may make horizontal ties necessary at the tops of the pylons.

Ties were used on several French multispan suspension bridges of the nineteenth century. However, it is doubtful whether tied towers would be esthetically acceptable to the general public. Another approach to multispan suspension bridges is that used for the San Francisco–Oakland Bay Bridge (Fig. 2). It is essentially composed of two three-span suspension bridges placed end to end.

This system has the disadvantage of requiring three piers in the central portion of the structure where water depths are likely to be a maximum. Suspension bridges may also be classified by type of cable anchorage, external or internal. Most suspension bridges are externally anchored (earth-anchored) to a massive external anchorage (Fig. 1a to d).

In some bridges, however, the ends of the main cables of a suspension bridge are attached to the stiffening trusses, as a result of which the structure becomes self-anchored (Fig. 1e). It does not require external anchorages.

The stiffening trusses of a self-anchored bridge must be designed to take the compression induced by the cables. The cables are attached to the stiffening trusses over a support that resists the vertical component of cable tension. The vertical upward component may relieve or even exceed the dead-load reaction at the end support. If a net uplift occurs, a pendulum link tie-down should be provided at the end support.

Self-anchored suspension bridges are suitable for short to moderate spans (400 to 1,000 ft) where foundation conditions do not permit external anchorages. Such conditions include poor foundation bearing strata and loss of weight due to anchorage submergence. Typical examples of self-anchored suspension bridges are the Paseo Bridge at Kansas City, with a main span of 616 ft, and the former Cologne-Mu¨lheim Bridge (1929) with a 1,033-ft span.

Another type of suspension bridge is referred to as a bridle-chord bridge. Called by Germans Zu¨gelgurtbru¨cke, these structures are typified by the bridge over the Rhine River at Ruhrort-Homberg (Fig. 15.11), erected in 1953, and the one at Krefeld-Urdingen, erected in 1950.  

It is a special class of bridge, intermediate between the suspension and cable-stayed types and having some of the characteristics of both. The main cables are curved but not continuous between towers. Each cable extends from the tower to a span, as in a cable stayed bridge. The span, however, also is suspended from the cables at relatively short intervals over the length of the cables, as in suspension bridges.

A distinction to be made between some early suspension bridges and modern suspension bridges involves the position of the main cables in profile at midspan with respect to the stiffening trusses. In early suspension bridges, the bottom of the main cables at maximum sag penetrated the top chord of the stiffening trusses and continued down to the bottom chord.

Because of the design theory available at the time, the depth of the stiffening trusses was relatively large, as much as 1⁄40 of the span. Inasmuch as the height of the pylons is determined by the sag of the cables and clearance required under the stiffening trusses, moving the midspan location of the cables from the bottom chord to the top chord increases the pylon height by the depth of the stiffening trusses.

In modern suspension bridges, stiffening trusses are much shallower than those used in earlier bridges and the increase in pylon height due to midspan location of the cables is not substantial (as compared with the effect in the Williamsburg Bridge in New York City where the depth of the stiffening trusses is 25% of the main-cable sag).

Although most suspension bridges employ vertical suspender cables to support the stiffening trusses or the deck structural framing directly, a few suspension bridges, for example, the Severn Bridge in England and the Bosporus Bridge in Turkey, have inclined or diagonal suspenders.

In the vertical-suspender system, the main cables are incapable of resisting shears resulting from external loading. Instead, the shears are resisted by the stiffening girders or by displacement of the main cables. In bridges with inclined suspenders, however, a truss action is developed, enabling the suspenders to resist shear.

(Since the cables can support loads only in tension, design of such bridges should ensure that there always is a residual tension in the suspenders; that is, the magnitude of the compression generated by live-load shears should be less than the dead-load tension.) A further advantage of the inclined suspenders is the damping properties of the system with respect to aerodynamic oscillations.

ELASTIC STRENGTH OF STRUCTURAL MATERIALS BASIC INFORMATION

What is Elastic Strength?

To the user and the designer of machines or structures, one significant value to be determined is a limiting stress below which the permanent distortion of the material is so small that the structural damage is negligible and above which it is not negligible. The amount of plastic distortion which may be regarded as negligible varies widely for different materials and for different structural or machine parts.

In connection with this limiting stress for elastic action, a number of technical terms are in use; some of them are

1. Elastic Limit. The greatest stress which a material is capable of withstanding without a permanent deformation remaining on release of stress. Determination of the elastic limit involves repeated application and release of a series of increasing loads until a set is observed upon release of load.

Since the elastic limit of many materials is fairly close to the proportional limit, the latter is sometimes accepted as equivalent to the elastic limit for certain materials. There is, however, no fundamental relation between elastic limit and proportional limit. Obviously, the value of the elastic limit determined will be affected by the sensitivity of apparatus used.

2. Proportional Limit. The greatest stress which a material is capable of withstanding without a deviation from proportionality of stress to strain. The statement that the stresses are proportional to strains below the proportional limit is known as Hooke’s Law. The numerical values of the proportional limit are influenced by methods and instruments used in testing and the scales used for plotting diagrams.

3. Yield Point. The lowest stress at which marked increase in strain of the material occurs without increase in load. If the stress-strain curve shows no abrupt or sudden yielding of this nature, then there is no yield point. Iron and low-carbon steels have yield points, but most metals do not, including iron and low-carbon steels immediately after they have been plastically deformed at ordinary temperatures.

4. Yield Strength. The stress at which a material exhibits a specified limiting permanent set. Its determination involves the selection of an amount of permanent set that is considered the maximum amount of plastic yielding which the material can exhibit, in the particular service condition for which the material is intended, without appreciable structural damage.

A set of 0.2% has been used for several ductile metals, and values of yield strength for various metals are for 0.2% set unless otherwise stated. The yield strength is generally used to determine the elastic strength for materials whose stress-strain curve in the region pr is a smooth curve of gradual curvature.

STRESS AND STRAIN OF STRUCTURAL MATERIALS DEFINITION AND BASIC INFORMATION

Stress.

Stress is the intensity at a point in a body of the internal forces or components of force that act on a given plane through the point. Stress is expressed in force per unit of area (pounds per square inch, kilograms per square millimeter, etc.).

There are three kinds of stress: tensile, compressive, and shearing.

Flexure involves a combination of tensile and compressive stress. Torsion involves shearing stress. It is customary to compute stress on the basis of the original dimensions of the cross section of the body, though “true stress” in tension or compression is sometimes calculated from the area of the time a given stress exists rather than from the original area.

Strain.

Strain is a measure of the change, due to a force, in the size or shape of a body referred to its original size or shape. Strain is a nondimensional quantity but is frequently expressed in inches per inch, etc.

Under tensile or compressive stress, strain is measured along the dimension under consideration. Shear strain is defined as the tangent of the angular change between two lines originally perpendicular to each other.

Stress-Strain Diagram.

A stress-strain diagram is a diagram plotted with values of stress as ordinates and values of strain as abscissas. Diagrams plotted with values of applied load, moment, or torque as ordinates and with values of deformation, deflection, or angle of twist as abscissas are sometimes referred to as stress-strain diagrams but are more correctly called load-deformation diagrams.

The stress-strain diagram for some materials is affected by the rate of application of the load, by cycles of previous loading, and again by the time during which the load is held constant at specified values; for precise testing, these conditions should be stated definitely in order that the complete significance of any particular diagram may be clearly understood.

Modulus of Elasticity.

The modulus of elasticity is the ratio of stress to corresponding strain below the proportional limit. For many materials, the stress-strain diagram is approximately a straight line below a more or less well-defined stress known as the proportional limit.

Since there are three kinds of stress, there are three moduli of elasticity for a material, that is, the modulus in tension, the modulus in compression, and the modulus in shear.

The value in tension is practically the same, for most ductile metals, as the modulus in compression; the modulus in shear is only about 0.36 to 0.42 of the modulus in tension.

The modulus is expressed in pounds per square inch (or kilograms per square millimeter) and measures the elastic stiffness (the ability to resist elastic deformation under stress) of the material.

COLUMN AISC STANDARDS IN STRUCTURES BASIC INFORMATION

Built-up columns shall satisfy the requirements of AISC Specification Section E6 except as modified in this Section. Transfer of all internal forces and stresses between elements of the built-up column shall be through welds.

1. I-Shaped Welded Columns

The elements of built-up I-shaped columns shall conform to the requirements of the AISC Seismic Provisions. Within a zone extending from 12 in. (300 mm) above the upper beam flange to 12 in. (300 mm) below the lower beam flange, unless specifically indicated in this Standard, the column webs and flanges shall be connected using CJP groove welds with a pair of reinforcing fillet welds. The minimum size of fillet welds shall be the lesser of 5/16 in. (8 mm) or the thickness of the column web.

2. Boxed Wide-Flange Columns

The wide-flange shape of a boxed wide-flange column shall conform to the requirements of the AISC Seismic Provisions. The width-to-thickness ratio (b/t) of plates used as flanges shall not exceed 0.6 SQRT(Es /Fy), where b shall be taken as not less than the clear distance between plates.

The width-to-thickness ratio (h/tw) of plates used only as webs shall conform to the provisions of Table I–8–1 of the AISC Seismic Provisions. Within a zone extending from 12 in. (300 mm) above the upper beam flange to 12 in. (300 mm) below the lower beam flange, flange and web plates of boxed wide-flange columns shall be joined by CJP groove welds. Outside this zone, plate elements shall be continuously connected by fillet or groove welds.

3. Built-up Box Columns

The width-to-thickness ratio (b/t) of plates used as flanges shall not exceed 0.6#Es /Fy #, where b shall be taken as not less than the clear distance between web plates.

The width-to-thickness ratio (h/tw) of plates used only as webs shall conform to the requirements of the AISC Seismic Provisions. Within a zone extending from 12 in. (300 mm) above the upper beam flange to 12 in. (300 mm) below the lower beam flange, flange and web plates of box columns shall be joined by CJP groove welds. Outside this zone, box column web and flange plates shall be continuously connected by fillet welds or groove welds.

4. Flanged Cruciform Columns

The elements of flanged cruciform columns, whether fabricated from rolled shapes or built up from plates, shall meet the requirements of the AISC Seismic Provisions.

User Note: For flanged cruciform columns, the provisions of AISC Specification Section E6 must be considered. Within a zone extending from 12 in. (300 mm) above the upper beam flange to 12 in. (300 mm) below the lower beam flange, the web of the tee-shaped sections shall be welded to the web of the continuous I-shaped section with CJP groove welds with a pair of reinforcing fillet welds.

The minimum size of fillet welds shall be the lesser of 5/16 in. (300 mm) or the thickness of the column web. Continuity plates shall conform to the requirements for wide-flange columns.

EARTHQUAKE LOAD CRITERIA SELECTION TUTORIALS

In IBC 2000, the following basic information is required to determine the seismic loads:

1. Seismic Use Group According to the nature of Building Occupancy, each structure is assigned a Seismic Use Group (I, II, or III) and a corresponding Occupancy Importance (I) factor (I = 1.0, 1.25, or 1.5).

Seismic Use Group I structures are those not assigned to either Seismic Use Group II or III. Seismic Use Group II are structures whose failure would result in a substantial public hazard due to occupancy or use.

Seismic Use Group III is assigned to structures for which failure would result in loss of essential facilities required for post-earthquake recovery and those containing substantial quantities of hazardous substances.

2. Site Class Based on the soil properties, the site of building is classified as A, B, C, D, E, or F to reflect the soil-structure interaction. Refer to IBC 2000 for Site Class definition.

3. Spectral Response Accelerations SS and S1 The spectral response seismic design maps reflect seismic hazards on the basis of contours. They provide the maximum considered earthquake spectral response acceleration at short period SS and at 1-second period S1. They are for Site Class B, with 5% of critical damping. Refer to the maps in IBC 2000.

4. Basic Seismic-Force-Resisting System Different types of structural system have different energy-absorbing characteristic. A response modification coefficient R is used to account for these characteristics.

Systems with higher ductility have higher R values. With the above basic parameters available, the following design and analysis criteria can be determined.

TYPES OF STRUCTURAL LOADS DEFINITION AND TUTORIALS

External loads on a structure may be classified in several different ways. In one classification, they may be considered as static or dynamic.

Static loads are forces that are applied slowly and then remain nearly constant. One example is the weight, or dead load, of a floor or roof system.

Dynamic loads vary with time. They include repeated and impact loads.

Repeated loads are forces that are applied a number of times, causing a variation in the magnitude, and sometimes also in the sense, of the internal forces. A good example is an off-balance motor.

Impact loads are forces that require a structure or its components to absorb energy in a short interval of time. An example is the dropping of a heavy weight on a floor slab, or the shock wave from an explosion striking the walls and roof of a building.

External forces may also be classified as distributed and concentrated.

Uniformly distributed loads are forces that are, or for practical purposes may be considered, constant over a surface area of the supporting member. Dead weight of a rolled-steel I beam is a good example.

Concentrated loads are forces that have such a small contact area as to be negligible compared with the entire surface area of the supporting member. A beam supported on a girder, for example, may be considered, for all practical purposes, a concentrated load on the girder.

Another common classification for external forces labels them axial, eccentric, and torsional.

An axial load is a force whose resultant passes through the centroid of a section under consideration and is perpendicular to the plane of the section.

An eccentric load is a force perpendicular to the plane of the section under consideration but not passing through the centroid of the section, thus bending the supporting member.

Torsional loads are forces that are offset from the shear center of the section under consideration and are inclined to or in the plane of the section, thus twisting the supporting member.

Also, building codes classify loads in accordance with the nature of the source. For example:

Dead loads include materials, equipment, constructions, or other elements of weight supported in, on, or by a building, including its own weight, that are intended to remain permanently in place.

Live loads include all occupants, materials, equipment, constructions, or other elements of weight supported in, on, or by a building and that will or are likely to be moved or relocated during the expected life of the building.

Impact loads are a fraction of the live loads used to account for additional stresses and deflections resulting from movement of the live loads.

Wind loads are maximum forces that may be applied to a building by wind in a mean recurrence interval, or a set of forces that will produce equivalent stresses.

Snow loads are maximum forces that may be applied by snow accumulation in a mean recurrence interval.

Seismic loads are forces that produce maximum stresses or deformations in a building during an earthquake.

STRUCTURAL COLUMN CURVES REFERENCE AND CIVIL ENGINEERING TUTORIALS

STRUCTURAL COLUMN CURVES BASIC REFERENCE
What Are Structural Column Curves?

Curves obtained by plotting the critical stress for various values of the slenderness ratio are called column curves. For axially loaded, initially straight columns, the column curve consists of two parts: (1) the Euler critical values, and (2) the Engesser, or tangent-modulus critical values.

Column curves: (a) stress-strain curve for a material that does not have a sharply defined yield pont: (b) column curve for this material; (c) stress-strain curve for a material with a sharply defined yield point; (d ) column curve for that material.

The latter are greatly affected by the shape of the stress-strain curve for the material of which the column is made, as shown in Fig. 5.44.

The stress-strain curve for a material, such as an aluminum alloy or high-strength steel, which does not have a sharply defined yield point, is shown in Fig. 5.44a.

The corresponding column curve is drawn in Fig. 5.44b.

In contrast, Fig. 5.44c presents the stress strain curve for structural steel, with a sharply defined point, and Fig. 5.44d the related column curve.

This curve becomes horizontal as the critical stress approaches the yield strength of the material and the tangent modulus becomes zero, whereas the column curve in Fig. 5.44b continues to rise with decreasing values of the slenderness ratio.

Examination of Fig. 44d also indicates that slender columns, which fall in the elastic range, where the column curve has a large slope, are very sensitive to variations in the factor k, which represents the effect of end conditions.

On the other hand, in the inelastic range, where the column curve is relatively flat, the critical stress is relatively insensitive to changes in k.

Hence the effect of end conditions on the stability of a column is of much greater significance for long columns than for short columns.

CONTINUOUS BEAMS BASICS AND TUTORIALS

CONTINUOUS BEAMS BASIC INFORMATION
What Are Continuous Beams?


Beam continuity may represent an efficient stactical solution with reference to both load capacity and stiffness. In composite buildings, different kinds of continuity may, in principle, be achieved, as indicated by Puhali et al., between the beams and the columns and, possibly, between adjacent beams.

Furthermore, the degree of continuity can vary significantly in relation to the performance of joints as to both strength and stiffness: joints can be designed to be full or partial strength (strength) and rigid, semi-rigid, or pinned (stiffness).

Despite the growing popularity of semi-rigid partial strength joints, rigid joints may still be considered the solution most used in building frames. Structural solutions for the flooring system were also proposed, which allow an efficient use of beam continuity without the burden of costly joints.

In bridge structures, the use of continuous beams is very advantageous for it enables joints along the beams to be substantially reduced, or even eliminated. This results in a remarkable reduction in design work load, fabrication and construction problems, and structural cost.

From the structural point of view, the main benefits of continuous beams are the following:  at the serviceability limit state: deformability is lower than that of simply supported beams, providing a reduction of deflections and vibrations problems  at the ultimate limit state: moment redistributionmay allow an efficient use of resistance capacity of the sections under positive and negative moment.

However, the continuous beam is subjected to hogging (negative) bending moments at intermediate supports, and its response in these regions is not efficient as under sagging moments, for the slab is in tension and the lower part of the steel section is in compression.

The first practical consequence is the necessity of an adequate reinforcement in the slab. Besides, the following problems arise:  at the serviceability limit state: concrete in tension cracks and the related problems such as control of the cracks width, the need of a minimum reinforcement, etc., have to be accounted for in the design.

Moreover, deformability increases reducing the beneficial effect of the beam continuity  at the ultimate limit state: compression in steel could cause buckling problems either locally (in the bottom flange in compression and/or in the web) or globally (distortional lateral-torsional buckling)

Other problems can arise as well; i.e., in simply supported beams, the shear-moment interaction is usually negligible, while at the intermediate supports of continuous beams both shear and bending can simultaneously attain high values, and shear-moment interaction becomes critical.

DRAINAGE FOR SUBGRADE STRUCTURES BASICS AND TUTORIALS

DRAINAGE FOR SUBGRADE STRUCTURES BASIC INFORMATION
How To Design Drainage Subgrade Structures?


Subgrade structures located above groundwater level in drained soil may be in contact with water and wet soil for periods of indefinite duration after long continued rains and spring thaws.

Drainage of surface and subsurface water, however, may greatly reduce the time during which the walls and floor of a structure are subjected to water, may prevent leakage through openings resulting from poor workmanship and reduce the capillary penetration of water into the structure.

If subsurface water cannot be removed by drainage, the structure must be made waterproof or highly water-resistant.

Surface water may be diverted by grading the ground surface away from the walls and by carrying the runoff from roofs away from the building. The slope of the ground surface should be at least 1⁄4 in / ft for a minimum distance of 10 ft from the walls.

Runoff from high ground adjacent to the structure should also be diverted. Proper subsurface drainage of ground water away from basement walls and floors requires a drain of adequate size, sloped continuously, and, where necessary, carried around corners of the building without breaking continuity.

The drain should lead to a storm sewer or to a lower elevation that will not be flooded and permit water to back up in the drain.

Drain tile should have a minimum diameter of 6 in and should be laid in gravel or other kind of porous bed at least 6 in below the basement floor. The open joints between the tile should be covered with a wire screen or building paper to prevent clogging of the drain with fine material.

Gravel should be laid above the tile, filling the excavation to an elevation well above the top of the footing. Where considerable water may be expected in heavy soil, the gravel fill should be carried up nearly to the ground surface and should extend from the wall a distance of at least 12 in (Fig. 3.7).

STRUCTURAL STEEL TENSION TEST BASICS AND TUTORIALS

STRUCTURAL STEEL TENSION TEST BASIC INFORMATION
What Is Structural Steel Tension Test?


The tension test (ASTM E8) on steel is performed to determine the yield strength, yield point, ultimate (tensile) strength, elongation, and reduction of area. Typically, the test is performed at temperatures between 10°C and 35°C (50°F to 95°F).

The test specimen can be either full sized or machined into a shape, as prescribed in the product specifications for the material being tested. It is desirable to use a small cross-sectional area at the center portion of the specimen to ensure fracture within the gauge length.

Several cross-sectional shapes are permitted, such as round and rectangular, as shown in Figure 3.15. Plate, sheet, round rod, wire, and tube specimens may be used. A 12.5 (1/2 in.) diameter round specimen is used in many cases. The gauge length over which the elongation is measured typically is four times the diameter for most round-rod specimens.

Various types of gripping devices may be used to hold the specimen, depending on its shape. In all cases, the axis of the test specimen should be placed at the center of the testing machine head to ensure axial tensile stresses within the gauge length without bending.

An extensometer with a dial gauge or an LVDT is used to measure the deformation of the entire gauge length. The test is performed by applying an axial load to the specimen at a specified rate.

Mild steel has a unique stress–strain relation. As the stress is increased beyond the proportion limit, the steel will yield, at which time the strain will increase without an increase in stress (actually the stress will slightly decrease). As tension increases past the yield point, strain increases following a nonlinear relation up to the point of failure.

Note that the decrease in stress after the peak does not mean a decrease in strength. In fact, the actual stress continues to increase until failure. The reason for the apparent decrease is that a neck is formed in the steel specimen, causing an appreciable decrease in the cross-sectional area.

The traditional, or engineering, way of calculating the stress and strain uses the original cross-sectional area and gauge length. If the stress and stains are calculated based on the instantaneous cross-sectional area and gauge length, a true stress–strain curve is obtained, which is different than the engineering stress–strain curve.

The true stress is larger than the engineering stress, because of the reduced cross-sectional area at the neck. Also, the true strain is larger than the engineering strain, since the increase in length at the vicinity of the neck is much larger than the increase in length outside of the neck.

The specimen experiences the largest deformation (contraction of the cross-sectional area and increase in length) at the regions closest to the neck, due to the nonuniform distribution of the deformation. The large increase in length at the neck increases the true strain to a large extent because the definition of true strain utilizes a ratio of the change in length in an infinitesimal gauge length.

By decreasing the gauge length toward an infinitesimal size and increasing the length due to localization in the neck, the numerator of an expression is increased while the denominator stays small, resulting in a significant increase in the ratio of the two numbers.

Note that when calculating the true strain, a small gauge length should be used at the neck, since the properties of the material (such as the cross section) at the neck represent the true material properties. For various practical applications, however, the engineering stresses and strains are used, rather than the true stresses and strains.

Different carbon-content steels have different stress–strain relations. Increasing the carbon content in the steel increases the yield stress and reduces the ductility. Below shows the tension stress–strain diagram for hot-rolled steel bars containing carbons from 0.19% to 0.90%.

Increasing the carbon content from 0.19% to 0.90% increases the yield stress from 280 MPa to 620 MPa (40 ksi to 90 ksi). Also, this increase in carbon content decreases the fracture strain from about 0.27 m/m to 0.09 m/m. Note that the increase in carbon content does not change the modulus of elasticity.


Steel is generally assumed to be a homogeneous and isotropic material. However, in the production of structural members, the final shape may be obtained by cold rolling.

This essentially causes the steel to undergo plastic deformations, with the degree of deformation varying throughout the member. Plastic deformation causes an increase in yield strength and a reduction in ductility.

This figure demonstrates that the measured properties vary, depending on the orientation of the sample relative to the axis of rolling (Hassett, 2003). Thus, it is necessary to specify how the sample is collected when evaluating the mechanical properties of steel.

FOUNDATION CLASSIFICATIONS AND SELECT DEFINITION BASICS AND TUTORIALS

FOUNDATION CLASSIFICATIONS AND SELECT DEFINITION BASIC INFORMATION
What Are Structure Foundations?


Foundations may be classified based on where the load is carried by the ground, producing:

Shallow foundations—termed bases, footings, spread footings, or mats. The depth is generally D/B < 1 but may be somewhat more. Refer to Fig. 1-la.

Deep foundations—piles, drilled piers, or drilled caissons. Lp/B > 4+ with a pile illustrated
in Fig. l-\b.

Figure 1-1 illustrates general cases of the three basic foundation types considered in this text and provides some definitions commonly used in this type of work. Because all the definitions and symbols shown will be used throughout the text, the reader should give this figure careful study.

The superstructure brings loads to the soil interface using column-type members. The loadcarrying columns are usually of steel or concrete with allowable design compressive stresses on the order of 14O+ MPa (steel) to 1O+ MPa (concrete) and therefore are of relatively small cross-sectional area. The supporting capacity of the soil, from either strength or deformation considerations, is seldom over 1000 kPa but more often on the order of 200 to 250 kPa.

This means the foundation is interfacing two materials with a strength ratio on the order of several hundred. As a consequence the loads must be "spread" to the soil in a manner such that its limiting strength is not exceeded and resulting deformations are tolerable. Shallow foundations accomplish this by spreading the loads laterally, hence the term spread footing.

Where a spread footing (or simply footing) supports a single column, a mat is a special footing used to support several randomly spaced columns or to support several rows of parallel columns and may underlie a portion of or the entire building. The mat may also be supported, in turn, by piles or drilled piers.

Foundations supporting machinery and such are sometimes termed bases. Machinery and the like can produce a substantial load intensity over a small area, so the base is used as a load-spreading device similar to the footing.

Deep foundations are analogous to spread footings but distribute the load vertically rather than horizontally. A qualitative load distribution over depth for a pile is shown in Fig. 1-1 b. The terms drilled pier and drilled caisson are for the pile type member that is constructed by drilling a 0.76+-m diameter hole in the soil, adding reinforcing as necessary, and backfilling the cavity with concrete.

A major consideration for both spread footings (and mats) and piles is the distribution of stresses in the stress influence zone beneath the foundation [footing or pile tip (or point)].

The theoretical distribution of vertical stress beneath a square footing on the ground surface is shown in Fig. IAa. It is evident that below a critical depth of about 5B the soil has a negligible increase in stress (about 0.02qo) from the footing load.

This influence depth depends on B, however. For example, if B = 0.3 m, the critical stress zone is 5 X 0.3 = 1.5 m, and if B = 3 m, the zone is 15 m for a zonal influence depth ratio of 1 : 10. Because these B values are in a possible range beneath a large building, any poor soils below a depth of 2 m would have a considerable influence on the design of the wider footings.

Any structure used to retain soil or other material (see Fig. 1-lc) in a geometric shape other than that naturally occurring under the influence of gravity is a retaining structure.

Retaining structures may be constructed of a large number of materials including geotextiles, wood and metal sheeting, plain or reinforced concrete, reinforced earth, precast concrete elements, closely spaced pilings, interlocking wood or metal elements (crib walls), and so on. Sometimes the retaining structure is permanent and in other cases it is removed when it is no longer needed.

The foundations selected for study in this text are so numerous that their specialized study is appropriate. Every building in existence rests on a foundation whether formally designed or not. Every basement wall in any building is a retaining structure, whether formally designed or not.

Major buildings in areas underlain with thick cohesive soil deposits nearly always use piles or drilled caissons to carry the loads vertically to more competent strata, primarily to control settlement. Note that nearly every major city is underlain by clay or has zones where clay is present and requires piles or caissons.

Numerous bridges have retaining structures at the abutments and spread foundations carrying the intermediate spans. Usually the abutment end reactions are carried into the ground by piles. Harbor and offshore structures (used primarily for oil production) use piles extensively and for both vertical and lateral loads.

HANDBOOK OF STRUCTURAL STEEL CONNECTION DESIGN AND DETAILS FREE EBOOK DOWNLOAD LINK

HANDBOOK OF STRUCTURAL STEEL CONNECTION DESIGN AND DETAILS FREE EBOOK
Free E-Book Download Link: Handbook of Structural Steel Connection Design and Details

Handbook of Structural Steel Connection Design and Details Editorial Reviews


This book not not only gives you the best and latest methods in connection design, it supplies fabricated examples on the CD-ROM that you can use for instant application and configuration of your own designs.

Featuring a broad range of design methods and details, the Handbook demonstrates the newest techniques and materials in welded joint design and production...seismically resistant connnections...partially restrained connections...steel decks...inspection and quality control...and more.

You get the newest connection designs based on load and resistance factor AISC design methods; special methods for seismic connection design; new material on fracture and fatigue design; improved methods of connection force analysis for various structures; 400 illustrations that show you how to do the job right; and much more.

Book Description
Publication Date: April 15, 1999 | ISBN-10: 0070614970 | ISBN-13: 978-0070614970 | Edition: 1

About the Author
Akbar R. Tamboli is a senior project engineer with CUH2A in Princeton, New Jersey. He was previously vice president and project manager with Irwin G. Cantor, P.E., Consulting Engineers in New York City. A Fellow of the American Society of Civil Engineers, Mr. Tamboli is the editor of Steel Design Handbook: LRFD Method, published by McGraw-Hill.

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BEST CIVIL AND STRUCTURAL ENGINEERING SCHOOL/ UNIVERSITIES IN THE WORLD IN 2011

CIVIL AND STRUCTURAL ENGINEERING SCHOOL/ UNIVERSITIES BEST IN THE WORLD
QS World University Ranking Best Civil and Structural Engineering School In The World For 2011

MIT tops the first ever QS World University Ranking® for Civil and Structural Engineering, which also sees a top-five performance from Imperial College London and two Asian universities in the top ten.

A diverse top 20 features nine universities from the US, three from the UK, two from Singapore, and one apiece from Japan, Switzerland, Australia, the Netherlands, China and Canada.

Below is the Top 20 Universities in the World for Civil and Structural Engineering:

1. Massachusetts Institute of Technology (MIT) United States
2. Stanford University United States
3. University of Cambridge United Kingdom
4. University of California, Berkeley (UCB) United States
5. Imperial College London United Kingdom
6. University of Oxford United Kingdom
7. National University of Singapore (NUS) Singapore
8. The University of Tokyo Japan
9. California Institute of Technology (Caltech) United States
10. ETH Zurich (Swiss Federal Institute of Technology) Switzerland
11. The University of Melbourne Australia
12. University of Illinois at Urbana-Champaign United States
13. Delft University of Technology Netherlands
14. University of California, Los Angeles (UCLA) United States
15. University of Texas at Austin United States
16. Cornell University United States
17. Tsinghua University China
18. Nanyang Technological University (NTU) Singapore
19. University of Michigan United States
20. University of Toronto Canada

Metrics for the selection are its contribution to the Academe, Rate of Employment, and its Citations received.
For the Complete List, read this site.

FIVE TYPES OF PORTLAND CEMENT BASIC AND TUTORIALS

PORTLAND CEMENT TYPES BASIC INFORMATION
What Are The Five Types of Portland Cement?


Portland cement has become the most widely used cement in the world. Portland cement got its name because the cured concrete it produced was the same color as a gray stone quarried in nearby Portland, England.

There are five types of portland cement, each with different characteristics.

■ Type I is a general-purpose cement and is by far the most commonly used, especially in residential work. Type I portland cement is suitable whenever the special characteristics of other types are not required.

■ Type II cement has moderate resistance to sulfates, which are found in some soil and groundwater, and generates less heat during hydration than Type I. This reduced curing temperature can be particularly helpful in large structures such as piers and heavy retaining walls, especially when the concrete is placed in warm weather.

■ Type III is a “high early strength” cement. High early strength does not mean higher strength—only that strength develops at a faster rate. This can be an advantage during winter construction because it reduces the time during which fresh concrete must be protected from the cold. Early strength gain can also permit removal of forms and shoring more quickly.

■ Type IV cement produces less heat during hydration than Type I or Type II and is used only in massive civil engineering structures such as dams, large highway pilings, or heavy bridge abutments. Its strength development and curing rates, though, are much slower than Type I.

■ Type V cement is used in concrete exposed to soil or groundwater that has high sulfate concentrations. This type of cement is usually available only in areas where it is likely to be needed. In the United States, Type V cement is common only in the southwestern states.

Types I, II, and III portland cement can also be made with a foaming agent that produces millions of evenly distributed microscopic air bubbles in the concrete mix. When manufactured in this way, the cements are said to be air entrained, and are designated as Types IA, IIA, and IIIA. Air-entrained cements require mechanical mixing.

Finely ground cement increases the workability of harsh mixes, making them more cohesive and reducing tendencies toward segregation. Coarsely ground cement reduces stickiness. Cement packages that are marked ASTM A150 meet industry standards for both physical and chemical requirements.

Portland cement comes in three colors—grey, white, and buff. The white and buff are more expensive and typically used in commercial rather than residential projects to achieve special color effects.

 Liquid or powder pigments can be added to a concrete mix, and liquid stains can be used to color the surface of cured concrete, but both will add to the cost. For most applications, ordinary gray concrete made with gray cement is suitable. Colored concrete should be reserved for special areas like a front entrance, a patio, or a pool deck

STEEL CABLE FOR STRUCTURAL APPLICATIONS BASIC AND TUTORIALS

STEEL CABLE FOR STRUCTURAL APPLICATIONS BASIC INFORMATION
What Are Steel Cables For Structural Works?

Steel cables have been used for many years in bridge construction and are occasionally used in building construction for the support of roofs and floors. The types of cables used for these applications are referred to as bridge strand or bridge rope.

In this use, bridge is a generic term that denotes a specific type of high-quality strand or rope.

A strand is an arrangement of wires laid helically about a center wire to produce a symmetrical section.

A rope is a group of strands laid helically around a core composed of either a strand or another wire rope.

The term cable is often used indiscriminately in referring to wires, strands, or ropes. Strand may be specified under ASTM A586; wire rope, under A603.

During manufacture, the individual wires in bridge strand and rope are generally galvanized to provide resistance to corrosion. Also, the finished cable is prestretched.

In this process, the strand or rope is subjected to a predetermined load of not more than 55% of the breaking strength for a sufficient length of time to remove the ‘‘structural stretch’’ caused primarily by radial and axial adjustment of the wires or strands to the load.

Thus, under normal design loadings, the elongation that occurs is essentially elastic and may be calculated from the elastic-modulus values given in Table 1.8.

Strands and ropes are manufactured from cold-drawn wire and do not have a definite yield point. Therefore, a working load or design load is determined by dividing the specified minimum breaking strength for a specific size by a suitable safety factor.

The breaking strengths for selected sizes of bridge strand and rope are listed in Table 1.8.

CYCLONE RESISTANT BUILDING BASICS AND TUTORIALS

BUILDINGS THAT ARE CYCLONE RESISTANT
How To Make Cyclone Resistant Building?


A cyclone is a storm accompanied by high speed whistling and howling winds. It brings torrential rains. A cyclone storm develops over tropical ocean and blows at speed as high as 200–240 km/hour.

It is usually accompanied by lightning, thunder and continuous downpour of rain. Cyclones extend from 150 km to 1200 km in lateral directions with forced winds spiralling around a central low pressure area.

The central region of light winds and low pressure, known as the ‘eye’ of cyclone has an average diameter of 20 to 30 km. This central eye is surrounded by a ring of very strong winds extending up to 40 to 50 km beyond centre.

This region is called ‘wall cloud’. In this region strongest winds and torrential rains occur. Beyond this region winds spiralling extend outwards to large distances, which goes on reducing with the distance from the centre of the cyclone.

The following care should be taken in designing buildings in cyclone prone areas:

1. Foundations should be deeper

2. R.C.C. framed structures are to be preferred over load bearing structures

3. Sloping roofs should be avoided.

4. Cantilever projections should be avoided.

5. Roof and parapet wall should be properly anchored to the columns and walls.

6. Height of the buildings should be restricted.

7. Suitable wind load should be considered in the building design.

8. Openings in the wall should be less.

9. Structure should not rest on loose soil.

CEMENTITIOUS (CONCRETE CEMENT) MATERIALS TYPES BASICS AND TUTORIALS

TYPES OF CONCRETE CEMENT MATERIALS BASIC INFORMATION
What Are The Basic Types Of Concrete Cement?

The goal of the investigation of cementitious materials should be to determine the suitability and availability of the various types of cement, pozzolan, and ground granulated blast-furnace (GGBF) slag for the structures involved and to select necessary options that may be needed with the available aggregates.

In cases where types or quantities of available cementitious materials are unusually limited, it may be necessary to consider altered structural shapes, changing the types of structure, altered construction sequence, imported aggregates, or other means of achieving an economical, serviceable structure.


The following types of cementitious material should be considered when selecting the materials:

(1) Portland cement. Portland cement and airentraining portland cement are described in American
Society for Testing and Materials (ASTM) C 150 (CRD-C 201).

(2) Blended hydraulic cement. The types of blended hydraulic cements are described in ASTM C 595
(CRD-C 203). ASTM Type I (PM) shall not be used; reference paragraph 4-3b(7) of this manual.

(3) Pozzolan. Coal fly ash and natural pozzolan are classified and defined in ASTM C 618 (CRD-C 255).

(4) GGBF slag. GGBF slag is described in ASTM C 989 (CRD-C 205).

(5) Other hydraulic cements.

(a) Expansive hydraulic cement. Expansive hydraulic cements are described in ASTM C 845 (CRD-C 204).

(b) Calcium-aluminate cement. Calcium-aluminate cements (also called high-alumina cement) are characterized by a rapid strength gain, high resistance to sulfate attack, resistance to acid attack, and resistance to high temperatures.

However, strength is lost at mildly elevated temperatures (e.g. >85 °F) in the presence of moisture. This negative feature makes calcium-aluminate cement impractical for most construction. It is used predominantly in the manufacture of refractory materials.

(c) Proprietary high early-strength cements. Cements are available that gain strength very rapidly, sometimes reaching compressive strengths of several thousand pounds per square in. (psi) in a few hours. These cements are marketed under various brand names. They are often not widely available, and the cost is much higher than portland cement. The extremely rapid strength gain makes them particularly suitable for pavement patching.

(6) Silica fume. Silica fume is a pozzolan. It is a byproduct of silicon and ferro-silicon alloy production.
Silica fume usually contains about 90 percent SiO2 in microscopic particles in the range of 0.1 to 0.2 μm. These properties make it an efficient filler as well as a very reactive pozzolan.

Silica fume combined with a high-range * water reducer is used in very high-strength concrete. Silica fume is described in ASTM C1240 (CRD-C270). Detailed information can be found in paragraphs 2-2d(5) and 10-10.*

(7) Air-entraining portland cement. Air-entraining portland cement is only allowed for use on structures covered by the specifications for "Concrete for Minor Structures," CW-03307. Air-entraining admixtures are used on all other Corps civil works structures since this allows the air content to be closely controlled and varied if need be.

TYPES OF TRUSSES IN STRUCTURES BASICS AND TUTORIALS

TRUSSES TYPES USED IN STRUCTURAL ENGINEERING BASIC INFO
What Are the Different Types of Trusses In Construction?


Trusses. When depth limits permit, a more economical way of spanning long distances is with trusses, for both floor and roof construction. Because of their greater depth, trusses usually provide greater stiffness against deflection when compared pound for pound with the corresponding rolled beam or plate girder that
otherwise would be required.

Six general types of trusses frequently used in building frames are shown in Fig. 7.11 together with modifications that can be made to suit particular conditions.

Trusses in Fig. 7.11a to d and k may be used as the principal supporting members in floor and roof framing. Types e to j serve a similar function in the framing of symmetrical roofs having a pronounced pitch. As shown, types a to d have a top chord that is not quite parallel to the bottom chord. Such an arrangement is
used to provide for drainage of flat roofs.

Most of the connections of the roof beams (purlins), which these trusses support, can be identical, which would not be the case if the top chord were dead level and the elevation of the purlins varied. When used in floors, truss types a to d have parallel chords.

Properly proportioned, bow string trusses (Fig. 7.11j) have the unique characteristic that the stress in their web members is relatively small. The top chord, which usually is formed in the arc of a circle, is stressed in compression, and the bottom chord is stressed in tension. In spite of the relatively expensive operation of forming the top chord, this type of truss has proved very popular in roof framing on spans of moderate lengths up to about 100 ft.

The Vierendeel truss (Fig. 7.11k) generally is shop welded to the extent possible to develop full rigidity of connections between the verticals and chords. It is useful where absence of diagonals is desirable to permit passage between the verticals.

Trusses also may be used for long spans, as three-dimensional trusses (space frames) or as grids. In two-way girds, one set of parallel lines of trusses is intersected at 90 by another set of trusses so that the verticals are common to both sets.

Because of the rigid connections at the intersections, loads are distributed nearly equally to all trusses. Reduced truss depth and weight savings are among the apparent advantages of such grids.

Long-span joists are light trusses closely spaced to support floors and flat roofs. They conform to standard specifications (Table 7.1) and to standard loading. Both Pratt and Warren types are used, the shape of chords and webs varying with the fabricator.

Yet, all joists with the same designation have the same guaranteed load supporting capacity. The standard loading tables list allowable loads for joists up to 72 in deep and with clear span up to 144 ft. The joists may have parallel or sloping chords or other configuration.