element of structure

7/28/2019 Element of Structure

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FT 14FIRE PHASE

ELEMENT OF STRUCTURE

OBJECTIVE

1. To explain the purpose, function of element of structure in building construction andfactors affecting behavior of material in fire.

REFERENCE

2. Manual of Firemanship Book 8 Building construction and Structural fire protection.

CONTENTS

3. The function of each of the elements of structure in the majority of buildings is tocarry the loads placed upon then. These loads:

a. The Dead Load. Which is the weight of all the parts of the building itselfwhich is imposed on the elements. These are constant.

b. Imposed Load. Which consists of people, furniture, machinery and materialsexpected to be in a building when it is occupied? These loads are variable.

c. Wind Load. Which means all loads due to the effects of wind pressure orsuction?

4. A factor of safety (arrived at by complex calculation) is incorporated to ensure that

materials and elements of adequate strength and stability are used.

a. COLUMNS - Definition: A vertical load bearing member.

- Function : To carry part of the weight of the buildingwhere an internal wall would interfere with the use ofthe building.

b. BEAMS - Definition: Horizontal load bearing member.

- Function: To support an applied load. Simple beam,continuous beam lintel, bresumer.

c. When load is applied, beam bends slightly, upper section being depressed,

lower section put under tensile stress. Material:(1) Timber and laminated timber.(2) Stone.(3) Reinforced concrete.(4) Prestressed concrete.(5) Cast/Open web steel.(6) Steel/open web steel.


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(2) Cavity Brick: External Walls. Objective to prevent rain penetrating

to inside face or wall. Usual type 50mm gap. Half brick walls held togetherwith metal ties. May have air bricks at top and bottom of outer wall, or cavitymay be filled with inert material to give thermal insulation to building.

(3) Solid Brick Faced With Stone:

(a) Cheap substitute for solid stone wall can either be load bearing

or panel wall in framed building.(b) The stone being built up at the same time as the brickwork andbonded to it.

(c) Some inside walls have thin slabs of stone fixed by metalclamps, or even (for internal decoration) by plaster.

Sketches showing the Solid Brick

Sketches showing the Cavity Brick


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(4) Solid Stone. A stone wall is normally thicker than a brick wall (rarelyless than 305mm thick). Many different bodings, local tradition, size and

workability of the stones. Inside, where appearance is unimportant, consistsof a mixture of medium and large stones.

(5) Timber Framed:

(a) Many variations of timber framed, load bearing wall, developedthrough the centuries.

(b) The half-timbered housed is probably the bent example. The

skeleton of the building is heavy timber, the spaces between thetimber being filled with brickwork. Brickwork filling is termednagging.

(c) A brick nagged partition consists of wooden vertical studs,wooden horizontal spaces known as noggin pieces, brick infilling.Normally 114mm thick or 76mm if laid on edge.

(d) Early walls had a layer of reeds fixed on either side by laths tothe studs. The reeds being plastered over with mud or lime plaster togive a wattle-and-daub wall. The space between the two faces wasoften filled with mud or rough plaster. May still be found in oldbuildings particularly half-timbered type. In due coarse the reeds were

replaced by wood laths mailed to the studs.

Sketches showing the Solid Brick Facedwith Stone

Sketches showing the Timber Frame


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(e) Fire resistance of timber-framed walls can be achieved bylining both faces with fire-resisting material (asbestos boarding forexample).

(6) Behavior or Load Bearing Walls in a Fire.

(a) The stability of a brick or stone wall depends, amongst otherthing, upon its:

i. Thickness in relation to its height.

ii. On proper bonding (in particular on sufficient headertying the wall together)

iii. To some extent on its age.

iv. On any horizontal pushing or levering effect exertedupon it.

(b) Stone walls also will be affected by the proportion of smallstones used and the skill with have been laid. The fewer the numberof joints and the thinner they are the greater the strength of a stone

wall.(c) A brick of stone wall, thought capable of supportingconsiderable vertical loads, can only withstand comparatively smallsideways or lateral pressure. For stability, the loading of the wall mustbe centered within the middle third

(d) In general, the collapse of walls which has occurred at fireshas been due to:

i. The burning away of the floors and cross walls, leavinga high wall with no side support.

ii. Expansion of beams into the wall, pushing it, outwards

and so throwing it out of equilibrium.

iii. Disintegration of the joints. Lime and cement jointsmay be so weakened by fire that a jet may be sufficient tothrow the wall off balance and bring it down, or to wash outmortar from joints and destroy stability.

iv. The collapse of support at the base of the wall such asan arch or a heavy steel beam. Provided there is no otherdamage to the wall, the bricks or stones may fall in such a wayas to leave a natural arch over quite a large span (say 1.5 to2m), and thus prevent total collapse. If this remaining on eachside of the gap to support the load above and resist spreading

of the natural arch.

v. Heating and consequent expansion of the inside face ofthe wall throwing the wall outwards.

vi. The levering action of collapsing joints which are intothe wall.

e. FLOORS

(1) Timber Floors (In General).

(1) Factors in the performance in fire of timber floors:

(a) Whether the flooring is plain-edge (butted) or tongued grooved,chipboard or plywood.

(b) The thickness of the flooring.


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(c) The load-bearing capacity of the joints.

(d) The contribution made to the fire resistance by the ceiling.

(2) Types of Timber Floor:

(a) Timber Joisted Floor. Generally used for upper floors ofhoused of all periods. Butt -jointed or T & G (chipboard/plywood in

more recent buildings) boarding between 16 32 mm thick is used.Laid on wooden joists usually not lese than 50mm thick and varying indepths from 127 to 180mm according to the distance spanned.

(b) The joist may be prevented from twisting by strutting (solidboard or two cross herringbone struts). One the underside of thejoints is the ceiling, usually of lath and plaster, or, in modern work ofbuilding board finished with a thin coat of plaster. Spaces create a

hazard in fire situations.

(c) Scottish Method of Securing Ceilings. Battens (Branders)secured to underside of joist, laths or plasterboard nailed to branderscreating more space. Branders to some job as struts.

(d) Purged Joist Type. Battens nailed half way down the joistsand boarding fixed to them. Space then filled with ashes or otherpacking (may be flammable) finished off with a mixture of line andsand. Increases fire resistance.

(4) Joists. The way in which the joists are supported on the walls isimportant.

(a) To old work, the joists are simply built into the wall, incollapses; they would cause a levering action.

(b) Another method is to fasten the ends of the joists into awooden wall plate built into the walls this tends to weaken the wall.

(c) A built-in wrought steel wall plate is sometimes encountered.Unless joist pockets are big enough the joists can cause leveringwhen the collapse.

(d) Better methods include: Wall plats secured on wrought ironbrackets built into wall.

(e) Corbelled brickwork forming a ledge for wooden wall plate.

A typical timber-joisted floor as use in domestic house


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A brick arched floor carried on cast-iron beams and columns

(5) Reduction in wall thickness by 114mm at each floor and rest the wallplate on the resulting ledge.

(6) Brick Arches on Cast Iron Beams. Cast iron (sometimes steel)beams carried on cast iron columns space between beams filled in withshallow brick arches. Parts near beam built of bricks on end, the centre beingbricks on edge. Uneven floor filled with weak concrete into which woodenbattens are laid on which the flooring is fastened (could be battens are laid onwhich the flooring is fastened could be stone flagged or tiles) old building

pre 1890.

(7) Steel Filler Joints and Mass Concrete. Area divided by steel

joists, sufficiently small gape to be spanned by the mass concreteunreinforced concrete. First type has good fire record, especially ifsupported on substantial brick walls. Second type has thin section of floor (76

to 101mm) danger of slabs cracking away from steel. Both types shouldhave protection for the steel joist which sometimes is exposed to fire onunderside.

Sketches showing various arrangements for supporting floor joist.

(1) Joist with square end in pocket;(2) Joist carried on wooden wall plate;(3) Joist with splayed end in pocket;(4) Joist carried on woodwn wall plate carried on bracket;(5) Joist carried on wooden wall plate on corbelled brickwork;(6) Joist carried on wooden wall plate on a ledge formed by reducing

the thickness of the wall.


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(8) Solid Reinforced Concrete. The steel rods which take the tensionare situated in the lower part of the slab, protected with a thin layer of

concrete rarely less than 127mm thick. Thin floor, 76 101mm, may be usedfor very small spaces; may have poor fire.

(9) Reinforced Concrete Rib. Series of reinforced concrete beams 450t0 610mm apart, spaces between being spanned with reinforced concreteslabs, structurally continuous with the beams. Thinnest part may be as littleas 50 76mm. Fire resistance inferior to heavier reinforced concrete floor.

Mass concrete and steel filler joist floor


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(10) Pre Stressed Concrete:

(a) Pre-stressed solid concrete planks laid side by and thencovered with concrete which bonds them together. In effect, a solidslab floor with reinforcement by pre-stressed concrete plankinginstead of wooden shuttering.

(b) Pre-stressed hollow concrete planks are also used. Ranging

in depth from 150 400mm, made up to 14m in length. Similar tosolid type but with addition of the bars.

(11) Hollow Tile:

(a) Two types: Hollow clay blocks set in concrete covered on topwith a layer of concrete. Hollow precise concrete beams laid closetogether with ends resting on steel joints. Joints between beams filledwith thin concrete.

(b) Both type liable to lose lower face in a fire but, as bottom faceis not structurally essential, the floor as a whole usually resistspassage of fire. (Difficult to determine what type of floor, appears justthe same as solid concrete).

Solid pre-stressed plank type of concrete floor

Pre-stressed hollow plank type of concrete floor


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f. ROOFS:

(1) Flat Roofs. Simplest type of roof. Similar in construction to a floor;

may consist of wooden joists or concrete, hollow tile or, in fact any type offloor construction. Above is fixed some kind of waterproof layer such asasphalt bitumen pregnated roofing felt, and metal sheeting.

(2) Pitched Roofs. Use to obtain greater spans that with simple beam.

(a) The simplest and most common from is called a close coupledroof.

(b) Mansard roof (two angles of pitch), the objective being toplace a room in the roof space.

(3) Trussed Roofs.

Spanning 10 60m between supports.

(a) Composite Truss : Timber compression members.Wrought iron tension rode.

(b) King Post Truss : Wooden members.

(c) Queen Post Truss : Two vertical ties strutted apart.

(d) Belfast Roof : Used to span large gape betweenSupports.

Northern Light. Various materials can be used in many differentforms to provide a trussed roof, i.e.

Flats roofs

Pitched Roof


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(4) Portal or Rigid Frame Roof. Continuous member conforming tovertical columns. Has the effect of extending roof load to rest of structure.Used mainly in single-story buildings.

Sketch showing how a timber truss for a Belfast roof is made up


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(5) Shell Roofs. A members curved in one or more directions andcovered with felt or asphalt and bitumen. Can be constructed of a number ofmaterials, timber aluminum or sheet steel. Majority are made of reinforcedconcrete.

(a) Roofing Materials.

i. Slates, tiles and shingles: Usually hooked on tobattens and remain in position by their own weight. (Battertype of work: nailed to battens). Sometimes tiles are bedded inmortar (torching). Shingles made of wood (not normally met inUK).

ii. Sheeting. Corrugated iron, aluminum and asbestoscement sheet fixed to purlines.

iii. Decking. Asbestos cement, timber wood wool,strawboard, aluminum and steel units. Need a waterprooflayer of asphalt or roofing felt. Decking more usually applied to

flat roofs.

iv. Bitumens Felt. Roof surfaces of bituminous felt mustbe laid on boarding or flat steel sheet. This steel sheet isreinforced with ribs and is supported on purline in come way ascorrugated iron.

g. STAIRWAYS. Four features of a well designed stairway.

(1) The staircase itself be built of fire-resisting materials to preventcollapse in fire conditions.

(2) It should have fire-resisting walls round it to prevent a fire breakingthrough onto it, or, should the fire do so by some means, to prevent the firespreading to another floor.

(3) The head of the stairs should be ventilated to prevent mushrooming.

(4) All doorways leading onto the stairways should be fitted with self-closing, fire-resisting doors to prevent fire reaching it and to reduce aircurrents which fan the fire.

Fireman, although not required to be involved with requirements of actualconstruction of stairways, will need to be cover sent with the more common termsused and with various types of stairway met.

h. DOORS. Seven principle types:(1) Hinged doors :

(a) Flush


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(b) Paneled(c) Ledged(d) Metal

FLUSH PANELLED

LEDGED


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(2) Swing doors

(3) Revolving doors

(4) Cylinder doors

Self-closing type of double swing door


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(5) Folding doors (Usually light construction and similar design hingeddoors.

(6) Cantilever doors

(7) Roller Shutters

i. WINDOWS

(1) Most common wave of opening:

(a) Side hinging


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(b) Pivoting

(c) Sliding sash, either vertically or horizontally.

(d) Top or bottom hanging. Windows generally referred to as oftwo types of opening casement or sash.

(2) Double Glazed Windows. Extra insulation, often hermetically sealed.

Can explode when involved in severe fire.

(3) Leaded Windows. Number of small panes held together with striplead. Value lies in the glass.


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(6) French Windows. Not strictly windows.

j. ROOF LIGHTS. A roof light is a form of window in the plane of the roof and isfixed. An opening roof light is referred to as a skylight. Form of Roof Light:

(1) Lantern Light.

(2) Monitor Light

(3) Dome Light

(4) Lens Light (also found in pavements)


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(5) Dead Light (glazing or translucent sheeting fixed in the slope of a roof,or lens lights in a flat or barrel roof, so that they do not open).

k. CEILINGS. Two basic types:

(1) Those which are integral with, or are secured to, the side (soft) of thefloor.

(2) Those which are suspended from the underside of the floor.

(3) Integral ceiling sometimes have fastened to them polystyrene tileswhich create a fire hazard due to the manner of flying. Suspended ceilingsmay be subdivided into three groups:

(a) In which the ceiling is flat.

(b) In which architectural forms, plaster decorated domes,covering, etc, are framed into the ceiling.

(c) In which the ceiling protects the steelwork of the floor above.


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(1) Ventilation in which the vitiated air is extracted from the building byfans, fresh air finding its way out through doors and windows.

(2) Ventilation in which fresh air is forced into the building by fans, vitiatedair finding its way out through doors/windows.

(3) Ventilation in which fans are used both to force fresh air in and to

extract vitiated air.

The ductings of these systems, together with ducting of other services (heating,electricity, gas etc) provide a means of smoke/heat travel.

7. COLLAPSE OF BUILDINGS. Signs:

a. Bulging wall.b. Cracks in wall.c. Dropped arches.d. Displaced or bulging columns.e. Floors sagging.

f. Floors pulling away from walls.

element of structure

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