construction systems 1. topic 11: timber subfloor - advanced

CONSTRUCTION SYSTEMS 1.

Topic 11: Timber Subfloor - Advanced

>>FACULTY OF TECHNOLOGY AND BUILT ENVIRONMENT >>DEPARTMENT TITLE EDIT IN HEADER & FOOTER

Learning outcomes

Students will be able to demonstrate their knowledge of this topic by:

• Understanding the construction of timber foundations and sub-floor framing and applying the requirements of NZS 3604:2011

• Explaining common terminology such as piles, footings, bearers, joists, boundary joists, trimming joists, curtailed joists

• Describing acceptable fixing methods for structural elements in a foundation/sub-floor

• Identifying the requirements for floor insulation, sub-floor ventilation and sub-floor cladding

• Describing the requirements of lateral bracing

Unit Standard 9671:Element 2: Determine and describe the foundations and sub-structure

INTRODUCTION

Lecture 6 covered the basic principles of suspended timber subfloors

This lecture covers more advanced aspects and principles Complexity comes from design and other factors such as varying heights above ground or added requirements such as bracing

Subfloor - Advanced

© Unitec New Zealand

Suspended timber floors are made up of the common elements as shown below, but there may be substitutions of different elements to suit specific situations, provide more efficient solutions and achieve greater cost effectiveness.

Subfloor - Advanced

Floor JoistsTypically (as sized in NZS 3604:2011) floor joists are solid structural timber members 45mm in width and of a depth (height) varying with the span requirement or joist spacing. The distance joists are set apart (joist spacing) is determined by the spanning ability of the flooring product. Most can only span 450mm max.

Subfloor - Advanced

Floor JoistsWhen attempting to span floor joists across distances greater than allowed by NZS3604, consider using an engineered product.

Other factors:Span large spaces with limited floor depth Stiffer floor Run large diameter services in floor space.

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Extensive range of alternative floor joist systems available

All come under the heading of engineered wood products

Sell as proprietary systems

Hyjoist

LVL

Posi-strut

Twinaplate etc

http://www.lumberworx.co.nz/

http://www.youtube.com/watch?v=0q1bxoZHUAI

Subfloor - Advanced

http://www.lumberworx.co.nz/

http://www.youtube.com/watch?v=0q1bxoZHUAI


Alternative Floor Joists

Subfloor - Advanced


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HyJOIST

Conventional Joist240x45 spans – 4.4m

240mm Dp. HyJOIST spans – 5.2m single span &6.1m continuous span

Load-bearing Walls Double joists are required Where there loadbearing wall runs at right angles to the line of the joists, the wall must be located not more than 200m centre-to-centre from a bearer or sub-floor loadbearing wall.

Floor Joist Under Walls

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Refer to NZS 3604:2011 Figure 6.1 for the use of a bearer and pile below a loadbearing or braced wall.

A bearer or pile must be within 200mm in these situations.

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Subfloor support to loadbearing wall parallel to joists


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Subfloor support to loadbearing walls at right angles to joist


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Non load-bearing Walls If walls contain bracing and run parallel to floor joists

Either:Over a joistOrSupported by 100 x 50 solid blocking

Solid blocking is required at:

Either end of the wall above

Each side of door openingsMax 1.2m crs


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Bearers


Lecture 6 dealt with describing and designing the layout and use of bearers for simple situations where the bearers are set out in rows parallel to one particular side of the building.

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However, buildings often have more complex plan shapes.Layout of bearers may also be influenced by other factors:

• Location of an internal load-bearing wall

• Internal point loads (such as a load-bearing post)

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Stringers


Horizontal members fixed to the side of a wall or to the side of a timber sub-floor (such as a bearer or floor joist) to support the ends of joists or rafters.

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Stringer Table 6.5 in NZS 3604:2011 also gives the provision for the fixing of the stringer. Fixings need to be also stated when a stringer is sized.

The fixings are M12 bolts set not less than 100mm into concrete or completely through a timber element.

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Stringers for deck attaching details from NZBC E2/AS1 document shows fixings to timber bearers through the cladding.

Refer also to BRANZ House Building Guide for useful detail diagrams.

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Pile or Isolated Foundations

Piles support the bearer above and are set in poured concrete footings.Tops of piles may be at different heights.NZS 3604:2011 imposes limitations on maximum and minimum pile heights under 6.4.1.

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The different loads imposed on the different bearer lines does mean that the piles supporting a bearer may be at different spacings or the bearer size might vary from row to row.

This needs to be considered as it may be considerably easier for the builder if all the pile spacings and bearer sizes are kept the same using the worst case scenario.

Which of the bearers on the right takes the biggest load and why ??

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Pile footings

Except for driven timber piles, each pile shall be provided with a concrete footing.

Sizing of such footings has 2 elements to consider, pile embedment (depth into the ground), and pile plan size.

NZS3604: 2011 sets minimum pile depths, but these are typically superseded when the soil conditions dictate greater depth is needed eg. With Aucklands expansive clays, the footing depth is considerably more than the minimum in the code.

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Pile Embedment

Leave a min. 100 mm of conc. below bottom of pile Pile shall be embedded in its footing sufficiently, or temporarily braced, to provide stability during construction.

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Footing Depths

Bottom of pile footing should be at least the depth of footing but not less than 200 mm.

Different pile type requirements:

Ordinary piles: 200 mm

Braced piles: 450 mm

Anchor piles: 900 mm


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Footing Plan Size

NZS 3604:2011 footing depth requirement is a minimum onlyThe altered footing depths described above do not alter the use of the footing sizing table for the plan size of the footing. If the soil does not provide “good ground” for bearing, then the footing plan size will require to be calculated by an engineer.Typically, in Auckland’s expansive clays’, the NZS 3604:2011 are acceptable for plan footing sizing, but not minimum depth.

Subfloor - Advanced


PILE TYPES Piles are vertical members, generally made of timber, that support bearers and transfer loads into the footings.

They come in a variety of different types depending on what function they perform and the distance between the bearer and the ground.

The most cost effective and common is the ordinary pile. It is used wherever possible and supports vertical loads only.

However, suspended timber subfloors are also subjected to potential horizontal loading from wind and earthquake, making it necessary to also use pile systems that can withstand these loads.

The type of system selected depends on a variety of conditions such as the distance between ground and bearer (pile length), soil conditions, cost effectiveness etc. See the following slides.

Subfloor - Advanced


Ordinary Piles

Most common type of pile used everywhere except where a pile needs to be braced. Determining what piles need to be braced and by what system, is calculated separately as part of subfloor bracing, then applied to the pile layout.

Min. 125mm sq. or 140mm diam.

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Reinforced concrete pile foundations awaiting concrete

Standard square timber pile foundations awaiting concrete

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Anchor Piles

Anchor piles provide resistance to uplift and horizontal loads and the number and location as determined by bracing calculation

Min. 900mm min. deep

Evenly spaced throughout foundation

350mm sq. or 400mm diam. min.

600mm max. height

Fixed w/ M12 bolt or 6kN fixing


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Braced Piles

Connect at top end to eitherpile, bearer or floor joist(see diagrams in NZS 3604:2011) Generally used where clearance is greater than 600mm between ground and bearer

Refer NZS 3604 for pile criteria

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Braced Piles:

Two adjacent piles connected to each other with a diagonal timber braceInstalled deeper than ordinary pilesWork as pair to provide resistance to lateral (horizontal) loads.

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Pile/bearer connections for an anchor pile system

Pile\bearer connection for a braced pile system

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Foundation Walls

Foundation walls are continuous concrete or masonry walls on strip footings that support a building (generally the perimeter).

The internal part of the subfloor is still made up of piles. Foundation walls can be low height walls (Figure 6.11) or as high as 2600.

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Foundation Walls can use subfloor jack framing to reduce height of foundation wall

Often used on steep sites

Up to 2.0m high

Reinforced concrete masonry or poured in-situ reinforced concrete.

Detailed under NZS 3604:2011

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Lateral support for foundation walls

Where the top of the foundation wall is not connected to the floor framing, it shall be one of the following two systems:

(a)A cantilever foundation wall

(b) A stepped footing, see above

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Standard Foundation WallsFoundation walls can be made without a cantilevered footing.

In-situ poured concrete or concrete masonry can be used to support timber floors, with a 2.0m max height (not a full floor)

For full Reinf. Conc. Masonry buildings you can use NZS 4229.

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Cantilevered Foundation Walls

NZS 3604:2011 sets out criteria

Foundation walls can be used as subfloor bracing unless:

Openings are over 600mm wide

Wall length is shorter than 1500

Either poured insitu concrete or concrete masonry construction.

Can be up to 2.0m high and reinforced as shown inNZS 3604:2011 Fig. 6.13

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38© Unitec New Zealand

Conc. or Conc. Masonry Fdn. walls can be designed as per Section 6.11 FDN. only (restricted to 2m min. height)

A full storey can be designed in Conc. Masonry only using NZS 4229. A full storey a fully reinf. conc. floor is SED.

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What is the alternate uplift rating for an anchor pile?

What is the maximum height of an anchor pile?

How close to the ground can the bottom of a diagonal timber brace be placed?

What is the maximum height of a concrete foundation wall?

What is the min. width of the footings of a laterally un-reinforced foundation wall?

Content Review


What are the implications of this lecture topic?

All students from different diplomas will need to understand the principles of timber subfloors whether they be technicians, quantifiers, or managers.

There is a quiz based partially on this topic that forms part of the assessment criteria for this course. This is a non-marked, formative quiz but needs to be undertaken and passed to allow the student to sit the test. Students can attempt this quiz as many times as needed to pass.

There is a POE exercise based on this lecture topic and it is suggested that it is undertaken as quickly as possible while the material is fresh, and before the next one is given out.

If you have struggled with answering the lecture review questions or not understood parts of this lecture, then further reading is recommended – see the notes on moodle and related publishings.

Subfloor - Advanced


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construction systems 1. topic 11: timber subfloor - advanced

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