Building Construction and Structural Systems

Construction materials and temporary support

Burnt clay brick is made by shaping clay and firing it in a kiln, and the firing proceeds through a fixed sequence of thermal stages. Dehydration removes free and combined water at low temperature, oxidation burns out carbonaceous matter at intermediate temperature, and vitrification occurs at peak temperature, when partial melting and sintering bind the particles. The brick is then cooled under controlled conditions, so any sequence that places vitrification before dehydration or cooling before firing is wrong.

Impurities in the clay strongly affect brick quality. Lime below about ten percent of the clay acts as a flux and lowers the fusion point, iron pyrites oxidize and decompose during burning, and magnesia produces a yellowish tint rather than red, which is due to iron oxide. Alkalis present in excess increase the probability of efflorescence, so they must be controlled.

The rebound hammer is a non-destructive test in which a spring-driven mass strikes the concrete surface. The rebound number reflects surface hardness, and calibration relations convert it into an estimate of compressive strength, while permeability, bond stress and tensile strength require other tests.

A paint consists of a pigment, a binder and a vehicle. The vehicle is the liquid part of the paint that contains the volatile solvent, which evaporates during drying and leaves the film. Metal oxides such as titanium dioxide, zinc oxide and iron oxides are common pigments and form the base of many paints. Gloss is high when the pigment volume concentration is low, because the binder fills the voids and forms a smooth continuous film. Volatile organic compounds are harmful to health and the environment, so low VOC content is preferred.

Surface treatment modifies the outer layer of a metal to improve corrosion resistance or appearance, and it is different from joining or forming processes such as soldering, riveting and extrusion. Thermoplating, also called sherardizing, heats steel in zinc powder below the melting point of zinc so that zinc diffuses into the surface and forms a protective coating.

Anodising forms a hard protective oxide layer on aluminium, so the equivalent treatments for glazing are a hard or low-emissivity coating applied to the glass surface and tempering, which is a heat treatment that increases the strength of glass. Facade finishes such as plastering and vertical greening are not treatments of the glazing itself.

In colour theory, a split complementary scheme uses a base hue together with the two hues adjacent to its complement. Analogous hues lie next to each other on the colour wheel, a triad consists of three hues spaced 120^\circ apart, and a tetrad consists of two complementary pairs. CMYK is a subtractive system because inks absorb wavelengths from white light, whereas RGB is additive.

Sand bulks when moisture is added. Free water pushes the grains apart through surface tension, so the apparent volume rises to a maximum at a moderate moisture content and then falls as the water fills the voids and the sand approaches saturation. The bulking curve therefore rises to a peak and declines, which distinguishes it from the monotonic relations found in workability and strength tests.

Temporary structures used during construction include centering, which supports formwork for arches, domes and slabs until the concrete or masonry can carry its own load, and scaffolding, which provides working platforms for masons and other workers. Shoring is a temporary propping system for an unsafe or damaged structure and has no role in dampness control, while underpinning extends or strengthens an existing foundation.

A dead shore is a temporary prop system that carries the load of a load bearing wall on vertical struts while its lower part is removed and rebuilt, so the wall stays supported throughout the work. Pit underpinning is a permanent foundation method, a flying shore is an inclined strut spanning between two buildings across an open space, and needle scaffolding is an access arrangement, so none of these is the correct choice for rebuilding the lower wall.

Structural systems and seismic behaviour

A soft storey is a storey whose lateral stiffness is much smaller than that of the storeys above or below it, often because the ground floor is open with no infill walls. An abrupt change in stiffness concentrates inter-storey drift and inelastic deformation in that one storey, which produces the characteristic collapse mechanism. The short column effect is a different phenomenon, in which restraint from partial infill shortens the free column length and causes shear failure, and the weak column-strong beam condition is the reverse of the intended strong column-weak beam design principle.

Tall buildings resist lateral wind and seismic loads through several structural systems. A shear truss uses diagonal bracing in the walls, a trussed tube uses exterior diagonal bracing that forms a tube, an outrigger frame connects the central core to perimeter columns through outrigger trusses, and a bundled tube groups several framed tubes into one structure. Each system is associated with particular landmark buildings, and the correct identification depends on matching the building to its lateral system.

The bundled-tube system combines several framed tubes into a single structure, which gives high lateral stiffness and reduces shear lag. The classic example is the tower in Chicago built with nine interconnected framed tubes. Buildings that rely on other lateral systems are not bundled-tube structures.

Seismic effects include secondary phenomena. Liquefaction occurs when saturated loose soil loses its shear strength under shaking, and a tsunami is generated when seafloor displacement during a large undersea earthquake displaces the overlying water. Heatwaves and tornadoes are meteorological phenomena and have no seismic origin.

The point inside the Earth where rupture begins and seismic energy is first released is called the hypocenter or focus. The epicenter is the point on the ground surface directly above the hypocenter, so it is not the place of origin.

Statically determinate beam and truss analysis

In a truss with weightless members, zero-force members can be identified before any calculation is done. At a joint with only two non-collinear members and no external load, both members carry zero force. At a joint with three members, where two are collinear and the third is not, and no external load acts, the non-collinear member carries zero force. Applying these rules to the joints adjacent to a member often shows that its force is zero.

For a statically determinate beam, the bending moment at a section equals the algebraic sum of the moments of all forces on one side of the section, taken about that section. Under the usual convention, sagging moments that produce tension at the bottom fibre are positive and hogging moments that produce tension at the top fibre are negative, so the sign must be checked carefully when the section is in an overhang or near a support.

At a support, the hogging moment produced by a load on an overhang equals the load multiplied by its distance from the support. Support reactions are found from moment equilibrium about one support, and the maximum hogging moment in a beam with overhangs is usually at a support where the moment is largest in magnitude.

For a simply supported beam under a uniformly distributed load, the mid-span deflection is \delta = \dfrac{5wL^4}{384EI}, and for a rectangular section I = \dfrac{bD^3}{12}. Hence \delta \propto \dfrac{L^4}{bD^3}. If both the length and the depth are doubled with other parameters unchanged, the deflection changes by the factor \dfrac{2^4}{2^3} = 2, so it doubles.

For an overhang beam under a uniformly distributed load, the reactions are first found by taking moments about a support. The bending moment at a distance x from the support within the span is then written as a function of x, for example M(x) = R_A x - \dfrac{wx^2}{2} for a span measured from the left support. Setting this expression to zero and choosing the root that lies inside the span gives the point of contraflexure.

Brick masonry bond quantity estimation

The number of bricks in a masonry wall is estimated by dividing the wall volume or face area by the volume or face area occupied by one brick including its mortar joints. The effective size of a brick in estimation is therefore the nominal dimension plus the joint thickness in each direction, and the same method applies to walls in different bonds.

In Flemish bond each course alternates headers and stretchers. One repeating unit along the face contains the lengths of one header and one stretcher with their joints, and the number of bricks in that unit, including backing bricks behind the stretcher, gives the count per unit length. Bricks per square metre of wall follow from the count per course and the number of courses per metre of height, and the bond type therefore changes the quantity required.

The brickwork in a semi-circular arch is the area of the semi-annulus between the inner and outer radii multiplied by the breadth of the wall. The inner radius is half the clear span, and the outer radius is the inner radius plus the thickness of the arch.

A dry wall is built without mortar joints, so the number of units is simply the wall volume divided by the volume of one unit. Mortar joints are ignored in this calculation, which is why the count is lower than for a wall built with mortar of the same dimensions.

Suspension bridge cable structural form

A flexible cable under a uniform load per unit horizontal length takes a parabolic profile. Taking equilibrium of a segment of cable gives y = \dfrac{wx^2}{2H}, where w is the load per unit horizontal length and H is the horizontal tension. Hangers that are equidistant and carry equal loads approximate this uniform loading, so the parabola is the best approximation for a weightless main cable. The catenary is the shape taken by a cable carrying its own weight uniformly distributed along its length, which is a different case.

In a suspension bridge the primary suspended main cable carries the deck through vertical hangers. The pylon, or tower, supports the main cable at the top, and the anchorage block at the end of the deck resists the cable tension. The stiffening deck carries traffic and distributes concentrated loads, and inclined stay cables run from the tower to the deck in cable-stayed or hybrid arrangements.

Structural joining systems and connectors

Each joining system is associated with the material for which it is commonly used. Welding is a fusion joint used mainly for structural steel members. Mortar is the binder that holds masonry units such as bricks together. A mortise and tenon is a traditional carpentry joint for timber members. A spider connector is a point fitting that holds glass panes in structural glazing. Matching the joint to the material is the basis of this type of question.

Hardware and fastening components are also matched to their functions. A mortise is a recess cut into a member to receive a lock. A garnet is a hinge-type fitting that allows a door or leaf to swing. An aldrop is a bolt-type door fastening that secures a leaf against movement. A gusset is a flat plate that connects structural members at a joint.

Steel wire elongation under axial load

The axial elongation of a bar carrying a constant axial force is \delta = \dfrac{PL}{AE}, where P is the axial force, L is the length, A is the cross-sectional area and E is the elastic modulus. For a hanging load the force equals the hung mass multiplied by gravitational acceleration, and for a wire the area is A = \dfrac{\pi d^2}{4}. Units must be kept consistent, using N and mm for stress and modulus in N/mm^2.

A bar hanging under its own weight carries an axial force that increases linearly from zero at the free end to the total weight at the support. The maximum tensile stress and strain therefore occur at the top support, where the stress is \sigma = P_{\max}/A and the strain is \varepsilon = \sigma/E. Self-weight must be included whenever the member is long and heavy relative to the applied load.

Construction and services terminology

In project scheduling, free float is the amount by which an activity can be delayed without delaying the start of any succeeding activity, and it is distinct from total float, which is the delay that does not affect the overall completion date. These terms are often asked together with the other site and services items.

A floating floor is laid on a resilient layer that isolates it from the structural slab, so it acts as an acoustical buffer against impact and airborne sound. A float valve is a device in a water tank that closes the inlet when the water reaches the set level, so it controls overflow. A metal float is a hand tool used by plasterers to smooth and finish plaster surfaces.