Logbook week 4

LOGBOOK WEEK 4

SPAN AND SPACING  

BEAMS AND CANTILEVERS

Span is the distance between two structural supports.

Spacing is the distance between like members, such as columns or beams, measured centre to centre.

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Beam is a horizontal structural member, which carries loads to vertical members, columns.

A cantilever is a beam supported at only one end, which carries loads the length of the member to the support.

FLOOR AND FRAMING SYSTEMS       

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In situ concrete needs to be allowed to set before elements placed on top, so it is strong enough.

Dead and live loads carried horizontally to beams and then to columns of foundations.

Metal formwork permanent part of building.

If joists are further apart, then, regardless of material, the flooring must be stronger to cover the further span.

Slab depth is found by span divided by 30. Eg 6m spans slab depth approx. 200mm.

Choice of type of system depends on loads, cost and efficiency, function of structure.

Steel framing systems take various forms as some have heavy structural steel and others have light gauge steel framing. Can also combine with concrete slabs to provide tensile strength. Fire rating affects this as concrete copes better with fires for longer periods.

Timber systems combine bearers (primary beams) with joists (secondary beams). Span of bearers determines spacing of piers or stumps and spacing of bearers equals span of joists.

INSITU CONCRETE        

Any concrete element that has been poured into framework and cured on site.

Process includes fabrication and assembly of formwork, placing reinforcement, pouring, vibrating and curing (setting) of concrete.

Once poured, limited time before it hardens and is unworkable, so work has to progress quickly for it to be properly positioned, air bubbles removed and finish applied.

Generally used for structural purposes such as footings, slabs, retaining walls and bespoke structural elements.

Can be sprayed into place using pressure hose (SHOTCRETE), this is useful for basement wall or overhead surfaces.

Construction joints divide construction into smaller, manageable sections of work.

Control joints absorb the expansions and contractions of concrete due to thermal changes.

Both joints are potential weak points and must be detailed properly for water and moisture control.

PRE CAST CONCRETE         

Concrete that has been fabricated in a controlled environment and then transported onto site to be installed.

More standardised, higher quality, easier to quality control, allows work to progress quicker.

Panels usually lifted by crane into positon.

Commonly used as retaining walls and columns.

Construction joints – panel/enamel nature of pre cast concrete means that joints occur naturally when elements meet each other.

Structural joints – type and performance of joints between elements and to other parts of building are viral to overall structure.

Pre cast members have plates at joints.

Joints depend on preferred aesthetic finish.

Members are limited in size due to transport, as panels need to fit onto trucks and have restrictions.

CONCRETE   

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When cement mixes with water in binds the sand and gravel together to make concrete.

1 part cement, 2 parts sand, 4 parts crushed rock, ½ part water.

When cement powder and water mixed chemical reaction occurs, hydration, where crystals are formed that interlock and bind sand, crushed rock and cement/water paste.

Too much water gives weak mixture, not enough water gives unworkable concrete.

Concrete is fluid and shapeless before it hardens, allowing it to be set into numerous shapes.

Formwork is the temporary support or moulds used to hold the liquid in place before it sets. It can be in situ or pre cast out of timber, metal, plastic, formply etc.

Spreaders keep formwork apart, ties hold together, bracing is needed to keep in position.

During curing process of concrete formwork needs to be supported by bracing and props. Formwork that remains in place is called sacrificial formwork.

Multiple finished available; raked, sand blasted, exposed aggregate, etc.

Concrete is strong in compression but weak in tension, to improve tensile strength steel mesh or bars are used, this is known as reinforced concrete when steel is added.

Reinforcement quantity higher at base.

Properties: highly rigid, low fragility, very low ductility, low flexibility, medium permeability, medium – high density, poor conductivity heat and electricity, highly durable, medium – low recyclability, high embodied energy, non-renewable but long lasting, generally cost effective depending on labour.

Not completely waterproof so problems can occur is steel rods too close to surface and can rust and cause degredation of concrete and therefore building as a whole.

PANTHEON            

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Roman empire, 1st century AD. Important as dedicated to all gods not just one as was usual.

Circular room behind a temple portico with series of niches and indentations.

Three main elements, portico, drum and hemispherical dome.

Portico like traditional temple (rectangular), obscures drum and dome behind it.

Drum made of brick face concrete, 6.5 metres think to counteract lateral forces of dome.

Brick face concrete common.

Dome 43.2 metres in diameter, largest spanned concrete shell dome in existence.

Dome constructed, stemmed from arch technologies which is transferring forces to floor using only compressive forces.

Dome is spinning arch concept.

Romans created concrete as large aggregate rather than small, packed together with mortar base.

Romans also discovered volcanic residue used to set cement, making it more powerful.

Dome of pantheon made of concrete, with range of aggregate types (limestone at base, higher up travertine, higher up further brick aggregate, and in dome shell, volcanic pumice for lightness) Oculus 9m in diameter is giant opening allowing light inside. Also means don’t have to build at centre of dome where most vulnerable. To resist dome pushing back into oculus, a compression ring, horizontal arch was positioned at the top.

FLOOR SYSTEMS                  

Horizontal planes that support live loads and dead loads.

Floor transfers loads horizontally across space to beams or columns or loadbearing walls.

Floors can be a series of linear beams and joists with sheathing or decking, or a slab or reinforced concrete.

Depth is directly related to size and proportion of spans and materials used.

Concrete: in situ slabs are classified by span and form, precast planks may be supported by beams or loadbearing walls.

Steel: steel beams support steel decking or concrete slabs and stiffener plates are sometimes added for strength.

Beams may be supported by girders, columns or loadbearing walls.

Beam framing is typically an essential part of steel framing.

Steel decking or wood planks have short spans.

Light gauge or open web joists supported by beams or loadbearing walls.

Joists have limited overhang potential.

Timber: Beams support structural planking or decking.

Beams may be supported by girders, posts or loadbearing walls.

Point loads and openings require additional framing.

Underside of floor structure can be left exposed or ceiling applied.

Small, closely spaced joists may be supported by beams or loadbearing walls.

Joist framing varies in shape and form.

Subflooring, underlayment and applied ceiling finishes have short spans.

SCALE Scale is important in being able to define a set of boundaries for a project and providing a reference of size. This is vital to those viewing drawings and graphics in the construction industry as they are able to easily understand the nature of a project. Scale allows large and small objects to be represented in a practical manner, for example, 30 storey high developments can be reduced to A3 drawings.

Generally a scale of 1:100 is adopted for structural drawings where 1 metre is represented by 10mm.

The difference between the drawings and the site is that the site is much clearer about materials and position of structural elements. The drawings provide more internal information but are at times, hard to visualise into a standing structure.

The drawings provide scale information so compare quite readily to the actual building.

Architectural drawings differ to the structural drawings as the structural drawings focus more on the members, supports and joints of the structure while the architectural drawings have a higher priority towards the overall effect and appearance.

GLOSSARY      

Joist: secondary horizontal structural member in floor or roofing.

Bearer: primary horizontal structural member for floors and/or roofing.

Steel decking: light gauged metal sheets for roofing and/or flooring. (Construction Dictionary, 2014).

Girder: horizontal structural member to support vertical loads, can be either a single piece or multiple joined together. (Merriam-Webster, 2014).

Concrete plank: usually pre cast, hollow or solid beam for floor or roof decking (Construction Definition, 2014).

Spacing: distance between like members.

REFERENCE LIST         

http://www.merriam-webster.com/dictionary/girder http://www.dictionaryofconstruction.com/definition/decking.html m m m m m m