Structural Design -3-Structural Systems

Structural Design 1 Structural Systems

The purpose of this set of notes is to give a brief overview of common construction systems for domestic and commercial applications, typically used in the Irish construction industry

Residential Construction

Houses:

Traditionally constructed (in Ireland) using Masonry walls Timber Decking on one way spanning timber joists. (Precast concrete flooring such as T beam

and block are also regularly used, mainly for sound-proofing benefit) Duo pitch prefabricated timber roof trusses. (For dormer construction and complicated one off

houses, cut roofs with structural steel purlins are commonly used). In recent years, sustainability issues, improved thermal performance and speed of erection have increased the popularity of timber frame construction.

Traditional Decking on Timber Joists Eco Joist Construction

T Beam & Block Construction


LVL Joist LVL Joist used as rafter in roof

Non reinforced masonry is adequate for structures up to 4 stories high. For 5 stories high and above, robustness must be addressed, so masonry (which has no tensile strength) is inadequate. Timber Joists Sizes are adequate for spans up to 6.0m. LVL joists and eco joists are available in deeper sizes than cut timber and hence can span further up to 8.0m see load tables on web sites below;

http://www.dontim-eng.co.uk/products_timber.php4

http://www.gangnail.co.uk/index.php/products/ecojoist

Apartment Blocks

Generally constructed using reinforced masonry walls or insitu/precast concrete walls. Floors are constructed using precast prestressed systems such as, hollowcore or wideslab. Prestressing allows shallower construction than normally reinforced concrete. As floor plans are usually identical for multiple floors of apartment buildings, this type of construction is particularly suitable for precast construction techniques. Heavy construction, such as the systems above, have in-built fire proofing and sound proofing qualities.

Some timber frame apartment buildings have been constructed in the UK, currently up to a maximum of 6 stories tall. Fire proofing and Sound proofing are issues which must be overcome in this type of construction.


Hollowcore Units

Wideslab Units

See websites below for details of precast units

http://www.concast.ie

http://www.floodflooring.com/Home.htm

http://www.oreillyconcrete.com/

http://www.bancrete.com/

Structural Design 1 Structural Systems Commercial Buildings Typical Column Spacings

Commercial clients like to have maximum flexibility to arrange their business they would prefer to have no columns at all if possible. This can be achieved in single storey buildings, but can prove impractical for multi-storey structures

For office space over basement carparks, grids in the region of 7.5m X 7.5m are typical.

In a retail application, this arrangement may not be desirable to the client. Larger anchor tenants, such as Tesco, Dunnes etc may require minimum grids in the order of 12m X 12m or a rectangular arrangement, say 8m X 16m over a basement carpark.

For light industrial/warehousing/manufacturing units, single storey buildings with minimum internal columns are generally employed. Clear spans in excess of 50m are achievable.

Construction systems include;

Insitu Concrete Slabs, beams and columns Precast concrete slabs on precast concrete frames (beams and columns) Precast concrete slabs on steel frames (beams and columns) Composite deck/composite beams and steel columns Steel/Concrete/Glulam Portal Frames Steel lattice girders/space frames Steel/timber gridshells

Insitu Concrete Options

Two way spanning slabs on downstand beams Flat Slab Construction

Solid slab construction is commonly used for spans of up to 9m. For spans greater than this, the self weight of the slab makes these systems uneconomical.

Ribbed (one way spanning) slab Waffle (two way spanning) slabs

Ribbed and waffle slabs are commonly used for aesthetics and also to reduce the slab self weight for spans in excess of 9m.

Post Tensioned Construction

Post tensioning is used for long span construction, > 10m. It allows a reduction in slab depth (and weight) of about 25% when compared with conventional reinforced concrete slabs.


Two way spanning slabs on downstand beams

Engineering G.A. Plan of 2 Way Spanning Slab with Downstand Beams

Expensive to construct extra formwork and complicated steel fixing Relatively deep solution Allows good flexibility in terms of possible ope sizes and locations Suitable for heavily loaded areas, such as plant and storage

Maximum 2.0 L

1.0 L


Flat Slab Construction

Flat Slab Construction (with Column Heads)

Engineering G.A. Plan of Flat Slab


Economic to construct Relatively shallow solution Poor flexibility in terms of possible ope sizes and locations May have issues with shear around column locations, when slab subject to heavy loading

Structural Design 1 Structural Systems Ribbed and Waffle Slab Construction

Very expensive to construct Use of bespoke fibreglass formwork Lightweight solution suitable for longer spans than solid reinforced concrete slabs Good flexibility in terms of possible ope sizes and locations Good finish can be achieved no need for false ceiling


Table below by IStructE: Manual for the design of concrete building structures to Eurocode 2 applies to reinforced concrete. It does not apply to prestressed or post tension concrete design


Precast Concrete Frame Construction Precast prestressed slabs on precast (sometimes prestressed) beams/walls supported by precast columns;

Flooring Unit Types

T beam (pre-stressed) and block has maximum span of up to 6m Wideslab (pre-stressed) has a maximum span of approx. 8m Hollowcore (pre-stressed) and Double Tees (pre-stressed) can span up to 18m Katzenberger / Omnia (reinforced) slabs can span up to 6m

Structural Screeds of 75mm minimum are usually added to the top of precast flooring units. For robustness, horizontal and peripheral ties are generally contained within the screed.

Precast hollowcore flooring units on precast beams and columns


Precast double tee flooring units on precast frame

Fast construction time Precast units provide working surface for follow on trades Very economic for multi storey buildings with identical floor plans Reduces trades on site very little shuttering/carpentry required Building design must be finalised prior to fabrication of units little flexibility for change, once

production has begun

Structural Design 1 Structural Systems Steel Frame Construction Steel Frames can be very similar to precast concrete frames that is precast flooring units spanning onto steel beams, supported by steel columns.

Precast Hollowcore slabs on downstand steel beams and columns

Precast Hollowcore slabs on slimflor steel beams and columns


Engineering G.A. Plan of Steel Frame Construction

Structural Design 1 Structural Systems Composite Construction

Alternatively, the steel beams and the concrete screed may be designed as a composite element, if; All beams are designed as simply supported There is sufficient depth and width of screed over the entire length of the beam. Shear Studs are installed to provide the shear connection between the steel and the concrete.

When simply supported and subject to vertical load, the top of a beam will be in compression, while the bottom is in tension. Concrete is good in compression, but poor in tension, while steel is very good in tension. Composite design therefore makes efficient use of the material properties of both concrete and steel to produce an economic solution.

Shear Studs welded to top of Composite Steel Beams

Slabs can also be designed compositely, by combining a permanent steel shutter (Comflor etc) to resist tension, while the concrete screed resists the compressive stresses.

Build Up of Composite Metal Deck Slab


Engineering G.A. Plan of Composite Deck Floor

Fast construction time Very economic for multi storey buildings with identical floor plans Reduces trades on site very little shuttering/carpentry required Decking units are easily cut on site for complex or non-square layouts

Reference Websites for Steel Frame Construction below;

http://www.tata.com

http://www.amse.ie/index.htm

http://www.siac.co.uk/index.jsp?b=0-93&pID=99&nID=133 http://www.duggansteel.ie/

http://www.asdwestok.co.uk/

http://www.kingspanpanels.ie/common/structural/htm/main_index.htm

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Structural Design -3-Structural Systems