Floor systems

The floor system is the primary horizontal building structure which must withstand both live loads and dead loads, (BCA Part B1.2) (AS 1170.1). Any flooring system consists of linear beams and joists to distribute floor loads evenly throughout floor surface, transferring horizontal loads down to vertical structures such as beams, columns or load bearing walls.

Floor systems must safely support moving loads. A floor system should be relatively stiff while maintaining its elasticity. If there is too much deflection and vibration, it can badly affect both the floor and ceiling finishes, as well as affecting the amenity of people who live in the space. The important control factor is therefore deflection.

A number of factors determine the depth of a floor system. The basic relationship is between the depth of the floor and the size of the structural bays it is going to span, together with the strength of the materials. However, a deeper floor can house mechanical or electric lines, and can help to insulate against sound travelling between floors in a multi-storey building.

The structural integrity of a floor will depend in part on the size of any cantilevers and openings in the floor. In turn, the structural integrity of the building will be affected by the way the floor is connected to foundations, walls and the like.

Types of floor systems

• Wood joist system

Relatively small joist members closely spaced; joists are supported by either beams or walls.

• Wood plank and beam system

Larger beams spaced further apart and spanned with structural planking or decking and beams supported by girders, posts, or walls

• Steel joist system

Light gauge or open web joists closely spaced and supported by beams and walls.

• Steel beam and decking

Heavier beams spaced further apart and spanned with steel decking or precast concrete planks and beams supported by girders, columns or walls

• Concrete

Precast or cast in place

Concrete floor systems are classified according to the types of span and the resulting form.

Wood flooring is commonly used in single or two storey house construction as heavier or more expensive materials are not required to span the typically small spaces between walls.

Steel flooring systems require prefabrication and delivery which must also be taken in to consideration.

This report will focus on reinforcement concrete and steel frame flooring systems since they are commonly used in high rise constructions. Our selected building has used a steel frame and in situ slab on steel decking.

Concrete flooring is flexible in span and loads depend on design of structure and commonly used in wide variety of low to high rise constructions. Construction can be precast off site or onsite cast in to almost any shape. However, it needs more on-site labour to create form work).

Different types of reinforced concrete floor slab

One-way slab

Slab is supported by structure on either end of slab and used for relatively short spans

One way joist slab

This is used for longer spans and heavier loads. It is not ideal for large concentrated loads.

Two way waffle slab

Ideal for long spans and heavy loads

Two way flat slab

Supported by columns and no beams needed. Ideal for limited height clearance spaces like car parks.

Two way flat plate

Maximum height clearances can be achieved with moderate loads capacity.

Two way slab with Beams

Advantageous where long spans and heavy loads are required; it is usually produced without beams.

Thus the choice of flooring system will depend on such factors as the load it has to bear, the dimensions of the structural bays, and the floor depth. In addition, the process of reinforcing and laying concrete requires that it be supported by formwork until the concrete cures and can support itself.

Process of concrete cast

Form work
Concrete casting requires form work to create the desired shape. It is often designed as a separate structural system to allow for the weight of wet concrete.
All form work surfaces needs to be coated with a parting compound to aid easy removal after the concrete is cured. Sharp corners are usually bevelled or rounded to avoid chipping and ragged edges.

Metal beam slab formwork can be very useful. Similar to the traditional method, stringers and joist are replaced with aluminium or steel beams with metal prop supports. This system is light, completely reusable and provides sharp crisp finish where traditional form work panels often have broken corner or edges.
Another system called Table or flying form systems (United States Patent 4036466) uses "tables" of form work that are reused on storey after storey on multiple level constructions. They are carried (flown) by crane and greatly reduce the time of labor and manual handling.
Reinforcements

Hot rolled steel reinforcements have ribs for better bonding to concrete and providing enough localized resistance to limit cracking and resist stresses while welded wire mesh typically provide temperature reinforcement for slabs resistance to expanding and shrink of slabs and distribute load to wider area.

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Reinforced concrete beams (Must comply with AS 1302, AS 1303 and AS 1304)

Reinforced concrete beams require continuity to reduce the bending moments. There is no separation between columns and beams, and are counted as one modular unit. This flooring system is widely used in todays’ high rise building construction.

Precast concrete slabs

Precast concrete slabs are cast and cured in a plant offsite and transported to site for erection. Design and sizes are limited due to the transportation requirement, but they can greatly reduce onsite labour because precast panels do not require form work on site. Additional concrete toppings help tie panels together as well as cover the uneven surface of panels if additional floor covering is applied. Toppings also provides extra fire resistance rating (AS1480, AS1481), accommodate under floor electrical cabling and extra lateral load rating.

Advantages of precast panels are that they can be laid between concrete beams of steel beam which provides a bit more flexibility in design than reinforced concrete floor.

Since this system uses modular units, it requires fixing system to join two precast panels together.

It is very important to design the precast panels to reduce cracks while transporting, handling and erect on site. Most panels should incorporated lift points and anchoring points to withstand the handling and erection stress.

Floor finishing
One of the problems with concrete floor slabs is that they are very sensitive to temperature. Concrete constantly expands and shrinks depends on temperature. This problem can be resolved with proper planning though. Insulate a concrete slab floor with additional top covering (like carpeting or a protective coating) to stop the heat from escaping (like reflective insulation) then the heat loss will be minimized and it will also reduce cracks. Any material can cover over the concrete slab, from floating timber flooring to carpet finish.

Another major disadvantage of the concrete floor is the colour which only comes in grey, giving a cold and solid appearance. However, there are some options for coloring and decorating a concrete slab floor. Acid stain is more common in coloring a concrete surface and when properly applied, will change the colour of the concrete and create interesting shades and patterns on the slabs. However, it can be expensive and time consuming. Concrete polishing is usually the cheapest option to change the appearance of a concrete slab. Concrete polishing brings out the natural look and gives shine and needs less maintenance.

Steel floor framing systems

Structural steel framing system is similar to traditional wood post and beam construction and flexible and rigid enough to build high rise buildings.

Normally, the process of construction is pre fabricated off site. This process is relatively fast and accuracy can be achieved to architectural detail. As with precast slabs, steel framing system design is limited by the need for road transportation. Fire resistant assemblies or coatings are required. Exposed conditions also require corrosion resistance coatings, which can be very expensive, such as the millions of dollars spent every year to maintain the Sydney Harbour Bridge to protect against corrosion.

There are many different types of steel beams available depends on design and loads. I beams and C channels are widely used. They are normally bolted or welded to join two steel pieces together. Welding joins have a neat look but they are expensive to fabricate on site.

The strength of structure connection is depends on the sizes of connecting members such as tees, angles or plates as well as configuration of the bolts and welding joints.

To maximize span between two bearing walls or columns, open web steel joist floor can be used. This flooring system can be found in large open spaces like work- shops or warehouse type of buildings. However open web steel joist is limited to standard depths and manufactured length.

Floor decking consists of concrete over metal decking; precast concrete, plywood or wood planking can be used.

Different types of metal floor decking

Metal decking is corrugated to increase its stiffness and span capability. Metal floor decking can be used as a temporary platform during construction and used as permanent form work for concrete floor.

Form decking

Composite decking

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Cellular decking

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Concrete flooring is one of the most versatile high-rise building constructions. It is comparatively economical, easy to make, offers continuity and solidity, and will bond with other materials. Concrete has a great variety of applications because it meets structural demands and lends itself to architectural treatment. The compressive strength of concrete (meaning its ability to resist compression) is very high, but its tensile strength (ability to resist stretching, bending, or twisting) is relatively low. Consequently, concrete which must resist a good deal of stretching, bending, or twisting—such as concrete in beams, girders, walls, columns, and the like—must be reinforced with steel.

Now that we have discussed numerous styles of flooring system it is time to look at a specific example. The building we will look at is an apartment building over 5 storeys. The building is constructed using a steel frame consisting of steel beams and columns on a regular grid on each floor. Where longer spans are required, secondary floor beams are installed. The floor spans between the main and secondary beams through the use of profiled steel decking. The decking supports the weight of both the wet concrete and the required construction loading. In situ reinforced concrete is placed on top. The image shows the steel decking resting on a structural I beam.

Steel columns are incorporated into separating walls between apartments and contribute to distributing the vertical load to the foundations. Columns are usually in square or rectangular hollow sections and typical sections range from 100x100, 150x150, 200x100 and 200x150 dependent upon the separating wall width.

Separating, internal walls comprise of light steel infill. The C sections are generally 100mm deep and have spacing of 600mm.

Light steel infill also provides a suitable system for external cladding where glazing does not span from column to column. There are many types of cladding that can be attached to steel infill walling including alucobond, timber, foam, cement sheeting and so on.

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As we are using a hypothetical building I will show two typical styles of shear wall construction and two more innovative approaches used in steel frame structures.

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The main function of steel plate shear wall is to act as a lateral load resisting system in order to resist horizontal story shear. In general, steel plate shear wall system consists of a steel plate wall, boundary columns and horizontal floor beams. Together, the steel plate wall and boundary columns act as a vertical plate girder. The columns act as flanges of the vertical plate girder and the steel plate wall acts as its web. The horizontal floor beams act, more-or-less, as transverse stiffeners in a plate girder.

Now it is time to look at fire ratings and fire construction. First we will detail the BCA requirements and then we will discuss whether the building in question complies.

In order to understand the fire requirements of this building we must first classify the resisting construction required. First we must classify the actual building. In this case we refer to the BCA

Class 2: A dwelling containing 2 or more sole occupancy units each being a separate dwelling.

As we are looking at a multi storey residential building containing multiple units we have a class 2 structure.

• Table C1.1 TYPE OF CONSTRUCTION REQUIRED
| Rise in storeys | Class of building |
| | 2, 3, 9 | 5, 6, 7, 8 |
|4 OR MORE |A |A |
|3 |A |B |
|2 |B |C |
|1 |C |C |

Calculation of rise in storeys

(a) The rise in storeys is the sum of the greatest number of storeys at any part of the external walls of the building and any storeys within the roof space—

(i) above the finished ground next to that part; or

(ii) if part of the external wall is on the boundary of the allotment, above the natural ground level at the relevant part of the boundary.

(b) A storey is not counted if—

(i) it is situated at the top of the building and contains only heating, ventilating or lift equipment, water tanks, or similar service units or equipment; or

(ii) it is situated partly below the finished ground and the underside of the ceiling is not more than 1 m above the average finished level of the ground at the external wall, or if the external wall is more than 12 m long, the average for the 12 m part where the ground is lowest.

This section of the BCA allows us to classify what type of fire construction is required. In our case we have a building with 4 or more storeys that is class 2. Therefore we will be looking at type A.

The BCA regulates materials that are used for floor, wall and ceiling coverings separately in Class 2 to 9 buildings (not single family homes). These are stipulated by "Specification C1.10a". It is important to take note of these requirements while looking at floor structures as separating walls, lift shafts and stairways must be designed to be built in conjunction with floor structures to ensure that they meet the required fire construction rules.

The term that is used is Fire Hazard Properties and the reference clause in the BCA is "C1.10 Fire Hazard Properties".

The BCA requires any material or assembly used for flooring, floor covering and wall and ceiling lining materials to comply with Specification C1.10a. All other materials (i.e. whatever holds up the floor covering or lining) are required to comply with Specification C1.10.

The BCA also specifies specific requirements for open spaces and common areas.

Requirements for open spaces and vehicular access

(a) An open space required by C2.3 must—

(i) be wholly within the allotment except that any road, river, or public place adjoining the allotment, but not the farthest 6 m of it may be included; and

(ii) include vehicular access in accordance with (b); and

(iii) not be used for the storage or processing of materials; and

(iv) not be built upon, except for guard houses and service structures (such as electricity substations and pump houses) which may encroach upon the width of the space if they do not unduly impede fire-fighting at any part of the perimeter of the allotment or unduly add to the risk of spread of fire to any building on an adjoining allotment.

(b) Vehicular access required by this Part—

(i) must be capable of providing continuous access for emergency vehicles to enable travel in a forward direction from a public road around the entire building; and

(ii) must have a minimum unobstructed width of 6 m with no part of its furthest boundary more than 18 m from the building and in no part of the 6 m width be built upon or used for any purpose other than vehicular or pedestrian movement; and

(iii) must provide reasonable pedestrian access from the vehicular access to the building; and

(iv) must have a load bearing capacity and unobstructed height to permit the operation and passage of fire brigade vehicles; and

(v) must be wholly within the allotment except that a public road complying with (i), (ii), (iii) and (iv) may serve as the vehicular access or part thereof.

Due to the size of our building and the need for stairs and lift shafts

Separation of lift shafts

(a) Any lift connecting more than 2 storeys, or more than 3 storeys if the building is sprinklered, (other than lifts which are wholly within an atrium) must be separated from the remainder of the building by enclosure in a shaft in which— (i) in a building required to be of Type A construction—the walls have the relevant FRL prescribed by Specification C1.1; and (ii) in a building required to be of Type B construction — the walls— A) if load bearing, have the relevant FRL prescribed by Table 4 of Specification C1.1; or (B) if non-load bearing, be of non-combustible construction.
(c) An emergency lift must be contained within a fire-resisting shaft having an FRL of not less than 120/120/120.
(d) Openings for lift landing doors and services must be protected in accordance with the Deemed-to-Satisfy Provisions of Part C3.
Stairways and lifts in one shaft
A stairway and lift must not be in the same shaft if either the stairway or the lift is required to be in a fire-resisting shaft

Additional requirements for lift shafts
(a) In addition to the requirements of Clauses 3.1 and 3.2, a wall system for use in a lift shaft that is required to be fire-resisting must be subjected to dynamic test by the imposition of—
(i) where the lift car speed is 7 m/s or less — 106 cycles of a uniformly distributed load between 0 and 0.2 kPa (or its equivalent); or
(ii) where the lift car speed is greater than 7 m/s — 106 cycles of a uniformly distributed load between 0 and 0.35 kPa (or its equivalent) in accordance with Clause 5(e) and must fulfil the damage criteria of Clause 6(b).
(b) The wall system must be subjected to the static test in accordance with Clause 3.2(b) after the successful conclusion of the dynamic test specified in (a).

The following table is from the BCA and sates requirements for fire rating building materials. As we are looking at floor structures the most important part is that for floors. Class 2 buildings require floors to have a 90/90/90 – structural adequacy/integrity/insulation rating.

Minimum FRLs in a Class 2 building

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In conclusion, there are a number of different multi storey construction methods that are chosen based on site conditions, council conditions and the systems cost and speed of construction. The building discussed uses a standard construction system of steel framing, steel decking and in situ concrete. The steel frame allows for numerous types of cladding to enhance appearance while ensuring that vertical and horizontal loads are transferred to the foundations. Steel decking increases the span on the floors and allow more flexibility with the placement of columns to ensure that they can be located in a relative grid pattern within separating walls.

Due to the building being class 2 and is over 4 storeys its construction is governed by type A fire safety construction. We have looked at fire safety in relative detail and the building has complied with all of the requirements.

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