ADVANCED CONSTRUCTION TECHNLOGY

INDUSTRIAL AND COMMERCIAL BUILDINGS

Introduction

It is a building designed to house industrial operations and provides the necessary conditions

for workers and the operation of industrial equipment. Examples of industrial buildings

include factory-offices, factory-warehouses, industrial parks or light and heavy

manufacturing buildings.

A commercial building is one that is dedicated to commercial activities. An alternative

definition of a commercial building is a structure that is not used for residential or civic

functions. Various retailers and other businesses lease space in commercial buildings in order

to operate without buying a property. Common examples of commercial buildings include

stores, offices, schools, gymnasiums, libraries, museums, hospitals, clinics.

Application of steel as a structural element

The earliest textile mills and warehouses in UK were constructed with load bearing brick or

stone walls supporting timber floors and roofs. Heating was by open coal fires and lighting by

oil lamps. Following a series of disastrous fires in mills, the first six-storey mill which

consisted of load bearing external walls and internal cast iron columns supporting timber

beams at 3.0 centres was built in 1792 -3. A later development was the use of cast iron beams

in place of timber. The first structural steel framed building to be erected in England was the

Ritz Hotel in London (1904-5) in which the whole of the weight of the masonry walls and

floors and roofs was carried by the steel frame.

The advantages of the structural steel frame are the speed of erection of the readily prepared

steel members and the accuracy of setting out and connections. With the use of sprayed on or

dry lining materials to encase steel members to provide protection against damage by fire, a

structural steel frame may be cheaper than a reinforced concrete structural frame because of

speed of erection and economy in material and construction labour costs.

Functional requirements

The functional requirements of a structural frame are: Strength and stability, Durability and

freedom from maintenance, Fire safety.

Strength and stability – According to Building Regulation codes, buildings be constructed

so that the load bearing elements, foundations, walls, floors and roofs have adequate strength

and stability to support the dead loads of the construction and anticipated imposed loads on

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roofs, floors and walls without such undue deflection or deformation as might adversely

affect the strength and stability of parts or the whole of the building.

The strength of the load bearing elements of the structure is assumed either from knowledge

of the behaviour of similar traditional elements, such as walls and floors under load, or by

calculations of the behaviour of parts or the whole of a structure under load, based on data

from experimental tests, with various factors of safety to make allowance for unforeseen

construction or design errors. The strength of individual elements of a structure may be

reasonably accurately assessed taking account of tests on materials and making allowance for

variations of strength in both natural and manmade materials.

The strength of combinations of elements such as columns and beams depends on the rigidity

of the connection and the consequent interaction of the elements. Here calculations make

assumptions, based on tests, of the likely behavior of the joined elements as a simple

calculation or a more complex calculation of the behaviour of the parts of the whole of the

structure. Various factors of safety are included in calculations to allow for unforeseen

circumstances.

Imposed loads are those loads that it is assumed the building or structure is designed to

support taking account of the expected occupation or use of the building or structure.

Assumptions are made of the likely maximum loads that the floors of a category of building

uses and may be expected to support. The load of the occupants and their furniture on the

floors of residential buildings will generally be less than that of goods stored on a warehouse

floor. The loads imposed on roofs by snow are determined by taking account of expected

snow loads in the geographical location of the building. Loads imposed on walls and roofs by

wind (wind loads) are determined by reference to the situation of the building on a map of the

country on which basic wind speeds have been plotted.

The stability of a building depends initially on a reasonably firm, stable foundation. The

stability of a structure depends on the strength of the materials of the load bearing elements in

supporting, without undue deflection or deformation, both concentric and eccentric loads on

vertical elements and the ability of the structure to resist lateral pressure of wind on walls and

roofs.

The dead weight of skeleton framed multi-storey buildings is not generally, by itself, capable

of resisting lateral wind pressure without undue deflection and deformation without some

form of bracing to enhance stability. Unlike the joints in a reinforced concrete structural

frame, the normal joints between vertical and horizontal members of a structural steel frame

do not provide much stiffness in resisting lateral wind pressure.

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Durability and freedom from maintenance - The members of a structural steel frame are

usually inside the wall fabric of buildings so that in usual circumstances the steel is in a

comparatively dry atmosphere which is unlikely to cause progressive, destructive corrosion

of steel. Structural steel will, therefore, provide reasonable durability for the expected life of

the majority of buildings and require no maintenance. Where the structural steel frame is

partially or wholly built into the enclosing masonry or brick walls the external wall thickness

is generally adequate to prevent such penetration of moisture as is likely to cause corrosion of

steel. Where there is some likelihood of penetration of moisture to the structural steel, it is

practice to provide protection by the application of paint or bitumen coatings or the

application of a damp-proof layer. Where it is anticipated that moisture may cause corrosion

of the steel, either externally or from a moisture laden interior, one of the weathering steels,

that are much less subject to corrosion, is used.

Fire safety - Building Regulations are concerned to ensure a reasonable standard of safety in

case of fire. The application of the Regulations is directed to the safe escape of people from

buildings in case of fire rather than the protection of the building and its contents. Insurance

companies that provide cover against the risks of damage to the building and contents by fire

may require additional fire protection such as sprinklers.

METHODS OF DESIGN

Permissible stress design method - With the introduction of steel as a structural material in

the late nineteenth and early years of the twentieth century, the permissible stress method of

design was accepted as a basis for the calculation of the sizes of structural members. Having

established and agreed a yield stress for mild steel the permissible tensile stress was taken as

the yield stress divided by a factor of safety to allow for unforeseen overloading, defective

workmanship and variations in steel. The yield stress in steel is that stress at which the steel

no longer behaves elastically and suffers irrecoverable elongation as shown in Fig. 1 which is

a typical stress / strain curve for mild steel.

The loads to be carried by a structural steel frame are dead, imposed and wind loads. Dead

loads comprise the weight of the structure including walls, floors, roof and all permanent

fixtures. Imposed loads include all moveable items that are stored on or usually supported by

floors, such as goods, people, furniture and moveable equipment. Wind loads are those

applied by wind pressure or suction on the building. Dead loads can be accurately calculated.

Imposed loads are assumed from the usual use of the building to give reasonable maximum

loads that are likely to occur. Wind loads are derived from the maximum wind speeds.

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Having determined the combination of loads that are likely to cause the worst working

conditions the structure is to support, the forces acting on the structural members are

calculated by the elastic method of analysis to predict the maximum elastic working stresses

in the members of the structural frame. Beam sections are then selected so that the maximum

predicted stress does not exceed the permissible stress.

In this calculation a factor of safety is

applied to the stress in the material of the structural frame. The permissible compressive

stress depends on whether a column fails due to buckling or yielding and is determined from

the slenderness ratio of the column, Young's modulus and the yield stress divided by a factor

of safety. The permissible stress method of design provides a safe and reasonably economic

method of design for simply connected frames and is the most commonly used method of

design for structural steel frames.

A simply connected frame is a frame in which the beams are assumed to be simply supported

by columns to the extent that whilst the columns support beam ends, the beam is not fixed to

the column and in consequence when the beam bends (deflects) under load, bending is not

restrained by the column. Where a beam bears on a shelf angle fixed to a column and the top

of the beam is fixed to the column by means of a small top cleat designed to maintain the

beam in a vertical position, as illustrated later, it is reasonable to assume that the beam is

simply supported and will largely behave as if it had a pin jointed connection to the column.

Collapse or load factor method of design - Where beams are rigidly fixed to columns and

where the horizontal or near horizontal members of a frame, such as the portal frame, are

rigidly fixed to posts or columns then beams do not suffer the same bending under load that

they would if simply supported by columns or posts. The effect of the rigid connection of

beam ends to columns is to restrain simple bending, as illustrated in Fig. 2.

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The fixed and beam bends in two directions,

upwards near fixed ends and downwards at the centre. The upward bending is termed

negative bending and the downward positive bending. It will be seen that bending at the ends

of the beam is prevented by the rigid connections that take some of the stress due to loading

and transfer it to the supporting columns. Just as the rigid connection of beam to column

causes negative or upward bending of the beam at ends so a comparable, but smaller,

deformation of the column will occur. Using the elastic method of analysis to determine

working stress in a fixed end beam to select a beam section adequate for the permissible

stress design method, produces a section greater than is needed to provide a reasonable factor

of safety against collapse, because in practice the permissible stress is not reached and in

consequence the beam could safely support a greater load.

The collapse or load factor method of design seeks to provide a safety factor, against collapse

applied to particular types of structural frame for economy in the use of materials by using

the load factor which is applied to the loads instead of stress in materials. The load factor

method was developed principally for use in the design of reinforced concrete and welded

connection steel frames with rigid connections as an alternative to the permissible stress

method, as a means to economy in the selection of structural sections. In the use of the load

factor method of design, plastic analysis is used. In this method of analysis of the forces

acting in members it is presumed that extreme fibre stress will reach or exceed yield stress

and the fibres behave plastically. This is a valid assumption as in practice the fibers of the

whole section play a part in sustaining stress and under working loads extreme fibre stress

would not reach yield point.

Limit state method of design - The purpose of structural analysis is to predict the conditions

applicable to a structure that would cause it to become either unserviceable in use or unable

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to support loads to the extent that members might fail. In the permissible stress method a

limit is set on the predicted working stress in the members of the frame by the use of a factor

of safety applied to the predicted yield stress of the materials used. In the load factor method

of design a limit is set on the working loads to ensure that they do not exceed a limit

determined by the application of a factor of safety to the loads that would cause collapse of

the structure.

The limit state method of design seeks to determine the limiting states of both materials and

loads that would cause a particular structure to become unserviceable in use or unsafe due to

excessive load. The limiting conditions that are considered are serviceability during the

useful life of the building and the ultimate limit state of strength. Serviceability limit states

set limits to the behavior of the structure to limit excessive deflection, excessive vibration and

irreparable damage due to material fatigue or corrosion that would otherwise make the

building unserviceable in use. Ultimate limit states of strength set limits to strength in

resisting yielding, rupture, buckling and transformation into a mechanism and stability

against overturning and fracture due to fatigue or low temperature brittleness.

In use the limit state method of design sets characteristic loads and characteristic strengths

which are those loads and strengths that have an acceptable chance of not being exceeded

during the life of the building. To take account of the variability of loads and strength of

materials in actual use a number of partial safety factors may be applied to the characteristic

loads and strengths, to determine safe working loads and strengths.

The limit state method of design has not been accepted wholeheartedly by structural

engineers because, they say, it is academic, highly mathematical, increases design time and

does not lead to economic structures. The opponents of limit state design prefer and use

permissible stress design for simplicity in execution and the knowledge that the use of the

more complex limit state design method may be rewarded with little significant reduction in

frame sections. Structural engineers profess to predict the likely behaviour of a structure from

an acceptance of working loads and yield stresses in materials so as to design a structure that

will be both safe and serviceable during the life of a building. There is often little reward in

employing other than the permissible stress method of design for the majority of buildings so

that the use of the limit state method is confined in the main to larger and more complex

structures where the additional design time is justified by more adventurous and economic

design.

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