Floors

a lecture delivered to the 3th year students

byRadu Pescaru, lecturertel: +40-232-278680/1419pescarur@ce.tuiasi.ro

INTRODUCTION

Floors are horizontal or sometimes slopingbuilding elements. They are structuralelements of the building and they dividethe building into several storeys and (onthe other hand) take over the dead andlive loads acting on their surface andtransmit them to the columns orstructural walls.

The general structure of a floor is divided into two parts:

- the loadbearing structure which can be a rigid slabor a less rigid slab resting on a system of joists orbeams placed in one or two directions (a system ofparallel or cross joists).

- filling in or finishing (materials) - made ofelements and materials placed between the supportingmembers, or on the floor slab, for heat or soundinsulating requirements or as floor finishes.

b. According to the position inside the building there are :- ground floors and;- upper floors which can also be: - on the top of the underground spaces, -intermediary floor or - under pitch or plate (terrace) roofs.

b. Considering the materials used for the structural elements, there are floors:- on timber joists;- with brickwork vaults;- on reinforced concrete beams or plates;- on metallic joists.

c. With respect to the fire resistance floors are divided into the followingcategories:- high fire resistance;- low fire resistance;- combustible;- inflammable.

Floor classification

Functional requirements

a. strength,

b. stability,

c. durability,

d. thermal protection,

e. sound absorption and resistance to sound transmission,

f. fire resistance,

g. economical requirements,

h. other requirements.

Strength –

The floor structure must be strong enough to safetysupport the dead loads of the floor and its finishes,partitions and services and also the anticipatedimposed loads in their most unfavourable combinationwhich can accrue during the lifespan of the building.

The strength of a floor depends on thecharacteristics of the materials, used for thestructure of the floor and the constructive solution.

Stability -

A floor is designed and constructed to serve as ahorizontal surface to support people and theirfurniture, equipment, or machinery. The floor shouldhave adequate stiffness to remain reasonably stableand plain under the vertical loads.

The floor should have enough stiffness to providestability against the horizontal actions (wind,earthquake) and to make all the structural elementssupport together these actions.

Durability -

Floors should be durable for the expected life of thewhole building and require little maintenance or repairalong this time.

Thermal protection –

A floor should provide resistance to the transfer ofthe heat where there is normally a significant airtemperature difference between the opposite sidesof the floor. For this reason the ground floor and theroof slab require a layer of thermal insulatingmaterial to prevent the heat loss through the floor.

Sound absorption and resistance to soundtransmission –

The upper floor that separates dwellings, or separatesnoisy from quiet activities, should act as a barrier tothe transmission of the airborne sound. The reductionof impact sound is best effected by a floor coveringsuch as a carpet and the airborne sound can be reducedby different absorbent panel finishes.

UPPER FLOOR CONSTRUCTION

1. Timber floors

Many buildings made earlier or at the beginning of20th century are constructed with timber floors.Nowadays such floors are used for dwellings,temporary buildings or buildings for tourism ontimber structure, built in mountain or ruralregions where the wood is available, cheaper andother materials are difficult to transport

The floor is framed with timber joists of fir or oak timber. Joistsections are between 10x19 and 15x25 cm. They span over theshorter side of the room and they are fixed over the long side,on loadbearing walls, at a spacing of 60 to 120 cm. This type isnamed single joisted floor and the maximum span of the joist is4.9 m, because over this limit the depth required for timber joistmakes them uneconomic.

Benefits: the timber floors are easy to build.They are lightweight and require little energyconsumption and work to fabricate.

Disadvantages: they are less durable have lowfire and moisture (dampness) resistance. Inaddition, timber floor is less rigid and presentlower bearing capacity than one made ofreinforced concrete or steel.

When the span is larger, cross-beams are introduced tocarry the ends of the joists. The beams can be of steel,timber, or reinforced concrete and are disposed parallel tothe short side of the room disposed at 3 to 5 m with thejoists (as secondary beams) across it.

When the span is larger, cross-beams are introduced to carry theends of the joists. The beams can be of steel, timber, or reinforcedconcrete and are disposed parallel to the short side of the roomdisposed at 3 to 5 m with the joists (as secondary beams) across it.

For stability, the end of the floor joists must have adequate supportsat the wall or beams. Timber floor joists that are built into wallsshould rest at least 20 cm on a wall plate of timber or metal. As aprecaution against the possibility of decay due to unforeseenmoisture penetration, it is wise to create a ventilated space of 3…5cm between the wall and the joist.

Floors in timber: a., b., c.- with visible joists, e.-with hallow (clay…) fill in

elements, f.- with plaster boards fill in elements, g.- with double joists;

1 (1’)-timber joists, 2-flooring timber board, 3-plastering and skin coat for

ceiling, 4-fill in material, 5-hollow lightweight element, 6- plaster board,

7-parquet, 8- batten, 9- sound insulation material.

2. Masonry arched and vaulted floors

Arches or vaults were employedin Mediaeval and Renaissancebuildings whenever it wasnecessary to provide a floorstronger and more solid thancould be obtained by the use oftimber. The advent of steel andreinforced concrete, materials inwhich tensile strength can bedeveloped, has today almosteliminated the arch as a form offlooring.

When arches are applied in flooring, they give a curved orvaulted ceiling to the room beneath. The floor itself must beformed on a level above the highest point of the arch. A bulkyinfilling is thus required above the springing.

3. Metallic floors

Metallic floors especially forrooms with large spans andimportant loads e.g. in industrialbuildings or multi-storied civilbuildings. The floors are made ofmetallic structural elements andinfilling materials or elements.The structural elements arebeams of hot rolled structuralsteel with joist section (I), Tsection or channel section (U),placed after one or two directionsaccording to the value of theloads and the nature of theinfilling elements.

Prefabricated plates fixed on

metal joists

The metal joists can be imbedded in

the concrete slab that has

to support the floor finishing

in filling materials.

4. Reinforced concrete floors

The reinforced concrete floor is often used for offices,large blocks of flats, factories and large public buildingsbecause it can safety support the imposed loads, it has agood resistance to damage by fire and a great rigidityand durability.

The disadvantages of these materials used for floors arethe great self-weight and the reduced power of thermaland sound insulation. In addition, the monolithic concretefloors need temporary shuttering to support theconcrete while it is still wet and plastic.

4.1. Monolithic reinforced concrete floor

A monolithic reinforced concrete floor is one unbrokensolid mass, between 80 and 300 mm thick, cast in site,and reinforced with mild steel reinforcing bars.

To give the required form to the element while concreteit is still wet and plastic temporary shuttering isnecessary. In addition, the shuttering offer thenecessary support of the element for minimum 7 to 10days after concrete has been cast.

Solid slab floors are used in buildings with brickwork or concretestructural walls with a maximum span of 4…5 m. The monolithic reinforcedconcrete floor slabs are usually between 80 and 100 mm thick. They liewith each end on the supporting walls on a 150…200 mm reinforcedconcrete straps or flanges (continuous contour beam-type members).

4.1.1 Solid concrete floor slabs

4.1.2 Floor plate with beams along one direction

If the floor surface isgreater than 25 m2 or thespan is greater than 5 to 6 m,or it has to support heavyloads, it may be found that asimple slab floor becomesuneconomic (too thick andheavy). A lighter constructioncan be obtained by formingintermediate beams; toreduce the span over whichthe slab has to act.The beams are made of thesame concrete as the floorslab, with rectangle section,and the distance betweenbeams is 3 to 5m.

4.1.3 Floors with main and secondary cross beams

Where the floorslab has to span overlarge surfaces, it isdesigned with mainbeams parallel tothe short side ofthe floor at 3 to 6 mintervals. Across themain beams aredisposed secondarybeams to sustain thefloor plate. Usuallythe space betweenthe secondary beamsis 1.5 to 2.5m but itcan rise to 4m.

4.1.4 Floors with close ribs and hollow blocks

Hollow blocks or pots are laid on the stuttering end to end in parallelrows, spaced apart at about 7.5 to 10 cm. The reinforcement isplaced in these spaces created between the block rows. Thenconcrete is poured to fill these spaces and cover the blocks and toform a series of tee-beams. The resulting floor consists of a closemesh of reinforced concrete tee-beams with strips of hollowedconcrete or clay blocks fixed between them. The blocks are 30 cmlong and 25 or 30 cm wide and 7.5 to 20 cm high.

4.1.5 Grid reinforced concrete floor

This is a development of a two-way spanning slab in which the two sets of reinforcement are concentrated in ribs. In situ ribbed floor is cast on channel steel moulds supported by beams and props.

This type of floor is particularly suitable for heavily loaded floors and is economic over relatively wide spans where the weight of the solid slab would be excessive.

4.1.6 Flat slab floors

At this floor, the slab has uniform thickness without projecting beams and is known colloquially ‘mushrooms construction’ because of the expanded or flared columns heads capes.

The column cape can be square or circular depending upon the shape of the column.

The slab is reinforced on both directions and on the line of columns grid the reinforcement bars are more closely spaced forming ‘column bands’.

4.2. Precast concrete floors

Precast concrete floors are designed primarily to eliminateshuttering and wet concrete pouring processes that make theconstruction of an in situ cast reinforced floor a slowprocedure.

The advantage of the completely precast floor is that it can beconstructed rapidly and readily it provides a platform fromwhich further work on the building can immediately go on.

A disadvantage is that it is less easy to provide a rigidconnection between the precast unit and the supporting beamsor wall. The precast beam or the precast rib and filler blockconstruction must act together as a single unit to horizontalactions (wind, earthquake).

4.2.1 Floors made of beams lying side by side

The simplest form of precast concrete floor consists of a number of units,each in effect a beam, lying side by side, and each spanning more or lessindependently between supports. These floors are fixed simply by layingthe units in position and then are connected together as a slab with a layeror concrete topping. The cross section of the elements is designed toprovide (by apposition) a plane surface only on the upper face (forindustrial buildings) or both upper and bottom faces (of dwelling buildings).

4.2.2 Precast Concrete stripfloors

Hollow beams - The solid units are relatively narrow and heavy, and developments in

form or hollow beam sections of grater unit width, and much the same weight, enable

floor areas to be laid more quickly.

Each beam is reinforced in the bottom corners and in some types in the top corners.

The sides are splayed or shaped to form a narrow space between the beams. This is

filled with grout to assist the units to act together in some measure, the adjacent faces

of the beams being grooved or castellated to provide mechanical bond. Continuity over

supports is obtained by the insertion of reinforcement rods in the joints prior to

grouting. These should be welded to steel beam if they are to serve as the anchors to

the precast units required in buildings over four storeys high.

Channel beams - Precast reinforced concrete channel units are laid with edges

touching slightly, and covered with a layer of concrete toping.

The channel section slab units are reinforced with common reinforcement rods for

short span and prestressed reinforcement foe larger span.

Channel beams are used especially for industrial building floors or roofs giving an

aspect of ribbed ceiling. In this type of buildings channel has standard dimensions:

the simply reinforced units are 1.5 to 3 m wide and 3 to 12 m long and the

prestressed units can reach a length of 12 to 18 m.

4.2.3 Floors with precast ribs and filler

Solid reinforced

pretension beams of

minimum B250,

generally shaped like

an inverted T, are used.

The beams are placed

at 0.4 to 1 m (centres)

with their ends resting

on walls. Hollow blocks

or plates of concrete,

plaster, are then placed

to fill the space

between the beams.

Then the floor is

finished with a layer of

constructional concrete

tapping.

4.2.4 Large precast floor panels

Sometime it is more practical

to use large floor units with

the dimensions of a whole

room (10 to 20 m2) or of half

room.

The hollow beam floor has

been developed in the form

wide slabs, which can be as

wide as 2.70 m, the width of

any particular span being

dependent on the lifting

capacity of the crane

available. The large precast

floor slabs are designed with

toes and projected steel bars

in order to provide a stiff joint

between elements.

The concrete plates are

reinforced on cross directions

with steel roods or networks.

4.2.5 Prestressed reinforced concrete floors

Pre-stressing of the

reinforcement can be applied to

many types of concrete floors.

This is advantageous where

wide spans are imposed

because it reduces the thickness

and dead weight.

It is economic the combine

prestressed precast elements

with in situ concrete.

One type is the rib and filler floor

which uses prestressed planks of

clay or concrete supporting filler

blocks. In situ concrete is cast

between and over the fillers to

form a rib having the prestressed

plank as the tension zone.

Another type uses prestressed concrete planks placed close together side by side

without filler blocks to form permanent slab shuttering. The edges are grooved to

provide a dovetail key for the in situ concrete structural topping in addition to the

natural bond between the rough top of the plank and the toping.

Prestressed precast tee-sections may be used in composite construction either in

rib and filler block combination or placed close together to form a flat soffit and

filled over with solid in situ cast concrete.

5. Ground floors particular characteristics

Almost all the types of upper floor can be used for the construction of

ground floor. However, in the case of ground floor design consideration of

the level at which the floor shall be placed relative to the surrounding

ground is important. A number of factors determine the adoption of the

floor level, including the nature of the site and the form of the floor

construction. According to the level of the floor, the problems of isolation

against heat loss or water penetration through the floor are solved in

different ways.

The upper part of the fill is a bed

of well-consolidated suitable hard

material known as hardcore

which is generally pot down

under the ground floor slab. It

consists of brick or concrete

rubble, broken stone or other

inert, coarsely graded material

such as hard, well-burnt furnace

clinker.

The in fill is lied to a minimum

thickness of 10 cm and is usually

ashes or other fine material

before the concrete is laid.

The concrete slab is not less then

10 cm thick and the top surface is

finished with a power float or is

shaped finished to a screed

according the floor finish to be

applied.

It is also important to incorporate a damp-proof membrane in thefloor structure. Materials that may be used for membranes aremastic asphalt, bituminous felt, hot-applied pitch or bitumen,cold-applied bitumen solution, pitch or bitumen/rubber emulsionand polyethylene sheeting.

To prevent heat loss through solid ground floors especially nearthe edges a layer of thermal insulating materials is required.Materials suitable for floor insulation are dens resin-bondedmineral or glass fibre slabs and polystyrene and cork slabs. Theseshould be placed above the damp-proof membrane and be turnedup at the edges of the floor slab to prevent heat loss though theall. The insulation at the edges is necessary to brake thepotential cold bridge and avoid condensation at the floor edges.The horizontal insulation is always necessary in conjunction withfloor heating.