Saturday, May 31, 2014

How to join Concrete

To watch the video go to ‘How to join Concrete’.

When we place concrete there are 3 different types of joint that are commonly used, the Expansion Joint, the Control Joint and the Construction Joint.

Although they all look the same on the surface of the slab they are used for different reasons and to achieve different results.

Expansion Joint
The Expansion Joint is used where large amounts of concrete are poured together such as a large slab or footpath or where the concrete is to be placed against a different material that will restrict the movement of the concrete.

To form the joint a piece of compressible material such a purpose made foam or bitumen impregnated fibre is placed in between the two sections or the slab and other material.

The result of insufficient expansion joints can quite often be seen in footpaths where sections pushing against each other lift up causing an uneven surface.

Control Joint

The Control Joint is used to control where the slab will crack. It is almost certain that a concrete slab will crack at some stage so by using Control joints we can control where it will crack and improve the look of the finished product.

The Control Joint is traditionally formed by using a jointing tool to create a groove in the slab. The groove creates a weak point in the slab while hiding the resulting crack. More recently the Control Joints are being cut into the slab after it has been poured, generally within a few days, using a diamond blade.

The placement of these joints is not an exact science and comes from experience however the common places that a slab will crack are at high stress points such as internal corners and along the line where the reinforcement sheets join.

Construction Joint.

The Construction Joint is used where we want to do a pour a slab in sections but we still want the slab to look and act as if it one slab.

If we were to just pour the two slab sections against each other without joining them together there is a chance that the movement of the ground could lift one side of the join and not the other again creating an uneven surface and a trip point. 

There are two common methods of joining the sections, The Key Joint and the Dowel Joint.

The Key Joint
The Key Joint uses a profiled metal extrusion used as part of the formwork.

The extrusion creates a groove in the first section, when the second section is poured against it it fills the groove. 

The groove will allow the two sections of the slab to move up and down together while still allowing it to separate. The key joint can also incorporate a expansion joint.

The Dowel Joint
The Dowel Joint involves drilling a series of holes into the poured section.

These hole are lubricated and then a dowel, usually reinforcement rod, is inserted into the holes.

When the second section is poured it will lock onto the dowels tying the two section together while still allowing for expansion and contraction. The Dowel Joint can be used with the Key Joint and can also incorporate a expansion joint.

Friday, May 30, 2014

Why use reinforcement in Concrete

To watch the video go to 'Why use reinforcement in Concrete'

 The mix of concrete and steel in reinforced concrete is a common and highly efficient building material used in the building industry on a daily basis, but how does it work.

Concrete is strong in Compression

and weak in Tension

Steel is strong in Tension

and weak in Compression.

The combination of the components obviously complement each other but how does a concrete slab become affected by compression and tension forces.

Let say that a concrete slab is used as the floor slab in a multi-storey building. It will be supported at both end by the walls of the building.

The self weight of the slab as well as the dead and live loads that are placed on the floor will try to make the slab bow in the middle.

When this occurs the top of the slab is placed into compression and the bottom of the slab in tension

So it stand to reason that the reinforcement is placed in the bottom section of the slab in this case.

What would happen if the support was moved in from the edge of the slab similar to what would happen in the case of a cantilevered balcony.

Now the weight of the overhang  will cause the top of the slab to be in tension over the support while the bottom of the slab is still in tension between the supports.

While it is good to have a basic understanding of the relationship between concrete and steel it is the job of the structural engineer to work out the size of the concrete slab or beam and placement and size of the reinforcement.

Saturday, May 24, 2014

Metre, Square Metre and Cubic Metre.

to view the video on this topic got to Metre, Square Metre and Cubic Metre.

In the construction industry here in Australia we use the metric system of measurement focusing mainly on millimetres and metres. Millimetres are obviously used where smaller dimensions are needed as when they are used for larger dimensions the number become pretty large and confusing.

Metres (m), sometime called lineal metres (lin.m) is used to measure the length or perimeter of something, a piece of timber, the cornice in a room, the architrave around a door or window.

Square Meters
Square Meters (m2) is used when we need to know the area of something. I always think that an area calculation needs 2 measurements (L x W) so the answer should be in Square Metres (m2).

Area can be used when we need to know how much reinforcement we need in a concrete slab, floor coverings in a room or paint needed on a wall.

Cubic Metres
Cubic Metres (m3) is used when we need to know the volume of something. Again I alway think that an volume calculation needs 3 measurements (L x W x D) so the answer should be in Cubic Metres (m3).

Volume can be used when we need to know how much concrete is in a slab, how much spoil will come out of an excavation or how much fill will be required.

I sometimes think that people visualize the as the 1m x 1m x 1m block as above and can’t make the connection that if the object is not at least 1m in all dimensions the it cant be a cubic metre however as you can see from the pictures opposite this is not the case. These objects are 1m x 1m x 0.100m thick and there are 10 in each picture this is equal to a cubic metre.

Standard Timber Lengths

To watch the video on this go to 'Standard Timber Lengths'

So you need a length or several lengths of timber from the hardware store. You know what length you need but what length does the store stock?

In Australia timber is available in standard lengths, each length is 0.300m, 300mm, greater that the last. It will depend on the Hardware store that you go to what their shortest length is however they will generally start at 1.8m or 2.1m and then increase by 0.300m. This gives you standard lengths of 1.8, 2.1, 2.4, 2.7, 3.0, 3.3, 3.6, 3.9, 4.2, 4.5, 4.8, 5.1, 5.4, 5.7, 6.0.

If you are buying MGP (Mechanically Graded Pine) it is only available in lengths of 0.600m therefore you can only get 2.4, 3.0, 3.6, 4.2, 4.8, 5.4 and 6.0.

Let say you need a length of 2.295m you will need to get a 2.400m length and cut it to size on site, getting the store to cut it is possible but they usually charge for it. If you need multiple lengths at 2.295 you could get multiple 2.4m lengths or you could order half the number of 4.8m lengths.

Roof Styles

To watch the Video go to 'Roof Styles'

Monoslope: Also known as a ‘Monopitch Roof or a ‘Flat Roof, it is any roof with a continuous slope, which has no ridge. Skillion and lean-to roofs are monoslope roofs.

Troughed: Basically two monoslope roof that meet at the lower edge of both roofs to form a box gutter ot trough. Also called a "Valley Roof" or a "Butterfly roof".

Sawtooth: Basically a series of monoslope roofs connected by a Box Gutter. This creates a saw tooth shape. Similar to a Clerestory or Monitor roof the vertical faces are usually windows that let in light. Commonly used for commercial and industrial work.

Clerestory: This roof is two monoslope roofs at two different levels separated by a row of windows. The which provide light and/or ventilation to the rooms below. It gets its name from the upper part of a church nave, which is the main source of light.

Gable: This is a pitched roof and vertical ends. It is used as stand alone roof or as an add-on to a main roof to form gables over entries or simply decorating the main roof surface in the form of a dummy gable.

A-frame: This is a steeply pitched roof, shaped similar an ‘A’. Commonly used in snow areas to allow the snow to slide off easily, rather than have it add excessive load to the roof frame.

Hip or Hipped: This is a roof with four sloping sides on a rectangular base. The member at the corners are called hips hence the name.

Half-pitch : This refers to a roof, which has a pitch angle of 45° angle. Or the rise of the roof is equal to the half span.

Pyramid: This is a roof with square or other regular polygon shaped base, with all hips being equal  and meeting at an apex which has no ridge.

Hip & valley: This is basically a ‘T’ or ‘L’ shaped  hip roof. The member at the internal corner is called the valley hence the name.  To have a true Hip and Valley roof the width of both arms of the ‘T’ or ‘L’ must be the same.

Broken Hip & valley: Similar to the hip & valley type except the ridges are not at the same height as the main roof. This occurs when the arms of the ‘T’ or ‘L’ ar not the same width. The Hip that goes from the Ridge of the Main Roof to the Ridges of the Minor Roof is called the Broken Hip.

Dutch gable: This is a  combination of a gable and a hip type roof. It has small gables or gablets at either end of the ridge. It may also called a ‘half-hipped roof’ or a ‘Gambrel’.

Gambrel roof: This roof is similar to the Dutch gable however the gabets are generally bigger.

Jerkin head: This is combination of a Hipped and Gable roof, it is hipped from the end of the ridge half way down to the fascia, and gabled the rest of the way. Sometimes called a ‘Hipped gable’ or a ‘Clipped gable’.

Bellcast: This is a roof, where the pitch changes to a lower pitch or angle near the eaves. It is commonly used where the main roof pitch meets the lower pitch of a covered balcony or verandah.

Station: Similar to the Bellcast however the roof typically cantilevers past supports on both sides to provide shelter. Commonly used for train and bus stations.

Deck: This basically a roof with a flat top like the top has been cut off the top of the roof to form a deck. With the addition of a handrail the deck can be used for entertainments or as an observation platform.

Mansard: Similar to a hipped roof except all sides have a two pitches. The sides are steeper while the top section flattens. It was named after the French architect Francois Mansart, who died in 1666. Also known as a ‘Curb roof’ or a ‘French roof’.

Monitor: A Monitor roof has a portion of a roof, which is raised up above the main roof usually with continuous vertical glazing around the perimeter for natural lighting and ventilation. Mainly used for industrial buildings to let in light and save on lighting costs.

Tudor: A steeply pitched roof, greater than 45o, usually with gable ends.

Dormer Windows: Dormer windows are used to let in light and increase the ‘headspace’ in the room.

Helm : This is a pyramid style roof, it has a square base, with four gables connected at the line of the fascia. The remaining roof surfaces are diamond-shaped. This type of roof is commonly used for spires on square towers.

Hyperbolic paraboloid: A form of shell roof construction, created by raising the diagonally opposite corners on a square base. This style of roof is becoming more popular with the introduction of fabric based roofing materials commonly used on stadiums and for shade.