Engineered wood products for modern methods of construction - James Jones and Sons Ltd

This architectural CPD was launched in mid July 2008 and is the first in a series of planned CPD presentations to be offered by James Jones and sons limited (JJ&S).

 

Introduction

James Jones and sons is the largest independent UK sawmiller and engineered timber manufacturer. We have 500 employees with a 2007 turnover in the region of £89 million. Our core products include construction Timbers including carcassing, I-Joists and fencing, as well as pallet wood. In 1999 we became the first and still the only UK owned company to manufacture I-Joist in volume. This was due to reduced carcassing sales enforced by the importation of I-Joists from North America.

 

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Engineered Wood Products

Definition of Engineered Wood Products

I-Joists are manufactured structural members that combine flanges made from softwood or LVL with a thin web generally made from OSB or high grade fibreboard. Open web joist to manufacture and structural members that combine cords made from softwood with metal or timber strutting to form the web. I-Joists and open web joists available in the UK apart of systems from the UK, North America and Scandinavia. Ewps are tested to BBA and trod accumark requirements and in accordance with EN408 many have CE marks though showing the product meets all the essential requirements of the relevant European directive.

Types of Engineered Wood Products

I-Joists are manufactured structural members that combined flanges made from softwood or LVL with a thin web generally made from OSB or high grade fibreboard. I-Joist flanges are manufactured from softwood CK&L grades or composite wood LVL. I-Joist webs are manufactured from OSB 9-15mm and fibreboard 6-9mm. Synthetic glues are used for the web to web and web to flange connections due of their commercial viability. Open web joists are manufactured structural members that combine cords made from softwood with metal or timber strutting to form the web.

 

Trussed rafter system providers produce metal timber web systems. The timber chords are based upon the standard trussed rafter material grade and size is TR26 46 by 72mm, 46 by147mm. Laminated veneer lumber LVL is a structural member manufactured by bonding together thin vertical softwood veneers under heat and pressure.

 

Laminated strand lumber LSL is a structural member made by cutting long thin strands directly from debarked logs. The strands are blended coated with adhesive and formed by steam injection pressing. Parallel strand lumber PSL is a structural member made by cutting long thin strands from timber veneers. The strands are orientated coated with adhesive and fed into a continuous press and microwave cured.

 

Orientated strand board OSB is aboard product manufactured from large timber flakes arranged in a three layer matt and bonded under heat and pressure. OSB is manufactured in various grades to suit their intended use and moisture content.

 

Fibreboard hardboard is aboard product manufactured from timber fibres under heat and pressure. Generally, no adhesive is used adhesion coming from the fibres own inherent adhesive properties. Often referred to as hardboard it can be impregnated with oil or resin and heat cured tempered to enhance strength and moisture resistance.

 

Glue laminated timber (Glulam) is a structural member manufactured by gluing together a number of solid timber laminations with their grain in the longitudinal direction, members could be straight or curved. 

 

I-Joists

 

Definition of I-Joists

I-Joists were introduced to the UK 15 years ago by North American producers. Now in 2008 the UK timber engineering industry has six I-Joist systems on offer.

 

Technical Benefits of I-Joists

I-Joists are precision engineered under strictly controlled conditions in accordance with BBA and TRADA curemark requirements and in accordance with EN 408.

 

There are numerous benefits to using I-Joists on site these include: 

• Half the weight of a solid timber section
• Dressed solid timber flanges
• No splinters or splitting
• Cutting and waste less than 1% compared to 11% for solid timber (this is because the floor packages supplied sized to fit)
• Fewer pieces
• Speedier installation
• Can be easily trimmed using basic hand tools
• No strutting or midspan noggins required
• Has movement
• Shrinkage is negligible
• Greater ease and speed of service installation compared to solid timber
• In some cases, the entire web can be removed

 

There are numerous benefits of using I-Joists within a design, these include:

• Similar design principles to that of steel beams,
• Long lengths enable the ability to multi span joists,
• Freely available beam design software,
• Short supply chain,
• Quality assured.

 

I-Joists Fire Resistance Benefits

I-Joist fire resistance is provided by the ceiling lining, this is reflected in the fire certificates. 30 minutes fire resistance requires a 15mm plasterboard ceiling, alternatives are available. 60 minutes fire resistance requires a 19mm + 12.5mm plasterboard ceiling, alternatives are available. Ceiling downlighters are also permitted within this certification. I-Joists are building regulations Part B fire safety compliant for use in roofs, floors and walls provided correct detailing is used. I-Joists can be hung or built into walls using approved metal work systems from Cullen and Simpson. When calculating the effective fire on LVL, LSL, PSL and glulam, use a charring rate of 0.66mm per minute. Protection can be easily applied to these members by encasing in plaster boards and or surface treating.

 

I-Joists Acoustic Performance Benefits

Provided the correct detailing is used, I-Joists can be readily specified for use in intermediate floors to achieve the sound requirement of 40 decibels as defined in the building regulations part E resistance to the passage of sound. The 40 DB requirements can be achieved using the following minimum material specifications; 18mm chipboard, increased the thickness density for a better performance, 220mm I-Joist increased the depth of the joist for a better performance, 400mm design spacing increasing the spacing for a better performance, 15mm plasterboard increase the thickness density for a better performance.

 

Other factors to consider:

• insulation,
• resilient bars,
• dynamic, battens.

 

Provided the correct detailing is used, I-Joists can be readily specified for use in separating floors to achieve the sound requirement of 45 DB airborne sound and 62DB impact sound, as defined in the building regulations part E resistance to the passage of sound.

 

Use of robust details are DEFT-1 for I-Joists and EFT-3 for open web joists, will achieve these requirements within a timber frame structure. Specifications of an I-Joist separating floor within a masonry structure will require a pre-completion test ECT.

 

I-Joists Thermal Efficiency Benefits

The inherent shape of the I-Joist enables more insulation to be introduced into the joist dead zone. Thus, incredibly the timber becomes the weaker element within the panel with regard to thermal efficiency. U values the measure of how well a building components e.g. A wall, roof or window keeps heat inside a building, can be significantly improved by using an I-Joist. For example a 245mm I-Joist with standard mineral wool insulation provides a U value of 0.2 watts per metre squared kelvin. Within renovation projects the use of I-Joists can greatly enhance the energy efficiency of the structure.

 

Environmental Benefits of I-Joists

I-Joists have good chain of custody certification which may include forest stewardship council FSC or the program for the endorsement of forest certification PE FC. The eco profiling system developed by BRE building research establishment, provides a rigorous and consistent approach to assessing the environmental impacts of construction materials. The BRE auditing procedure independently assesses the environmental impact of all parts of the production process over a 12 month period, delivering a complete cradle to grave assessment. Some I-Joists meet the coveted green guide to housing A rating.

 

Structural Benefits of I-Joists

I-Joists are manufactured in lengths of up to 20 metres. This provides scope for larger clear spans and enables the designer to reap the benefits of multi spam beam design within structures. The majority of I-Joist manufacturers however limit their production to 12-15 meters due to health and safety and logistic issues. The most common depth of I-Joist used are 240 and 245mm. This depth accounts for almost half of all UK sales. The shallower joist 145 and 195mm tend to be used for projects where there is restricted height, while the 220mm depth is very popular in traditional brick and block residential structured floors. 300mm deep I-Joists provide significant architectural design scope with regard to room sizes and the deeper joists 350 and 400 and 450 millimetre tend to be used more commonly in non-residential floor and roof applications.

 

I-Joist flange depth range from 28-45mm smaller depth reflect LVL. I-Joist flange width range from a 38 to 97mm smaller width reflect LVL softwood. Flanges provide the builder with more scope for fixing those nails screws and bolts can be driven both vertically and horizontally. The shallow depths used in the flanges eliminate the shrinkage factor usually associated with in solid timber sections, as such nail screw popping in ceiling linings and twisting of timber joists are not problems experienced using, I-Joists.

 

I-Joist web thickness is range from 6-15mm as a general rule the greater the thickness of the web the stronger than sheer performance of the joist. Holes can be easily cut in the web of I-Joist however the size and location of the holes depends upon the forces within the web. Notwithstanding this, a 35mm hole can be drilled anywhere along the joist length thereby allowing easy service installation.

 

As I-Joists are manufactured in long lengths spans of 6 meters are easily designed and sourced. This gives architects the potential to remove internal load bearing walls that may have been required for solid timber designs, for example or 300 millimetre I-Joists in a domestic floor construction can easily clear a span of six metres, a 450 millimetre I-Joist in a flat roof construction and easily span 11 metres.

 

Durability Benefits of I-Joists

I-Joists can only be used in service class one or two environments as defined in BS5268 Part 2. I-Joists cannot be used as external finishes in fully exposed conditions service class 3. BBA certification states that I-Joists are guaranteed for the life of the building in excess of 60 years.

 

Economical Benefits of I-Joists

Bespoke design software allows the designer to undertake a complete appraisal of the most cost effective I-Joist floor and roof solutions. I-Joist floors are twice as fast to install solid timber floors, half the weight of solid timber sections, fewer pieces, no strutting or mid span noggins, no lapping joists, bespoke layout and slide fixing details, no room for error. The long lengths lightweight and excellent thermal properties of I-Joists make them an excellent choice for use within prefabricated systems.

 

One of the major benefits of using an I-Joist system is that the complete package is provided to site. This means that all the elements within the floor or roof structure supplied from a one stop shop. Thus, ensuring that everything is on site to install the system. As all services can easily pass through the web material of the I-Joist the installation time is quick and effortless. 35mm holes can be drilled anywhere along the centre line of the joist with a scope to remove the entire web in certain locations.

 

Freely available I-Joist software will detail holes in the web. I-Joist systems are maintenance free, due to the very stable nature of the engineered products used within the systems, movement associated with poorly performing floors is illuminated, it is this movement that causes squeaks and creaks.

 

A guarantee comes as standard with all I-Joist floors as long as all the correct ancillary items, hangers, blocking, noggins etcetera, are supplied with the system and the system is installed as detailed within the I-Joist manufacturers literature.

 

I-Joist Certificates

I-Joist systems will have some or all of the following certification. BBA British Board of Agrement certified, ETA European technical approval certified, ISO 14001 environmental management certification EMS, ISO 9001 internal quality management. I-Joist systems are fully compliant with the following parts of the building regulations. Part A structure, part B fire safety, part E resistance to the passage of sound. I-Joists are designed in accordance with British standards BS5268 or European standards EC-0,1 and 5. I-Joists are accepted by NHBC and Zurich the uks leading warranty and insurance providers. NHBC chapter 6.4 has specific requirements for I-Joists. NHBC in 2006 reduced the allowable deflection in timber floors to 12mm, 14mm in BS5268. 

 

I-Joists Care and Maintenance

Care must be taken at all stages of the I-Joist installation to limit the effect of moisture ingress. I-Joists will typically arrive on site with the moisture content of between 14 and 16% this will reduce to 10-12% when installed within a service class one environment. Basic measures to limit the risk of moisture ingress are as follows, protect joists from the elements, I-Joists should be wrapped during all stages of transport and storage, store I-Joists clear of the ground using bearers at 3.0 meter spacings, store I-Joists away from vegetation and surface water, store I-Joists in the vertical plane and leave wrapped until ready for use.

 

If I-Joists are supplied to site over length, they can be easily cut using conventional woodworking tools. A simple saw, tape and hammer are all that is needed to install a set of I-Joists. Deck and ceiling lining materials can easily be fixed to I-Joist flanges using screws or nails.

 

Solid timber flanges allow the builder to simply nail all noggin requirements into the side of the flanges. While LVL flanges require the use of zed clips. I-Joists are engineered products thus, care must be taken to avoid damage when mechanically lifting on site and within the installation. Any damage to the I-Joist web or flange must be reported to the supplier as soon as possible to ensure the product is fit for use. The flanges of the I-Joist must not be drilled notched or altered, as this will critically reduce the joist stiffness.

 

All the system providers provide engineering support when things do not go as planned on site. Slender joists can be very unstable until the system is braced, suppliers will provide with deliveries and upon request details of temporary bracing required during joist installation. This information is a legal duty under the construction design and management regulations.

 

I-Joist Uses

Forestry Commission building at Smithton near Inverness 1700 metres of 195 mm I-Joists used in ground floors and roof purlins. 350 metres of 300 millimetre I-Joists used in the intermediate floors. 2000 metres of 195mm I-Joists used in the external walls.

 

Floor Joists

I-Joists can be readily specified for ground floors to. A ground floor is a service class to condition a common I-Joist depth used is 145mm. This depth is structurally possible due to the additional sleeper walls provided for support.

 

Floor Systems

 

Benefits of Floor Cassettes

Modern methods of construction MMC building systems have several advantages. Economic, cassettes have fewer defects and can be built more quickly. Environmental, structures can be more energy efficient, may involve less transport of materials and produce less waste. Social, fewer on site accidents and less impact on local residents during construction.

 

Onsite Screed Finished Floors

Screed systems can be used timber, concrete or steel constructions and new build or renovation work. The acoustic performance exceeds that required by part E by 5-10 decibels. As screed floors a shallower costs can be saved through less brick coursework, less scaffolding and less waste on site. In lightweight structures screed floors performed like solid concrete floors i.e. Minimising any sensation of movement and virtually eliminating any noise resulting from that movement. Other benefits to the homeowner include the ease of laying floor finishes directly on the screed finish and the environmental credentials of under floor heating. I-Joist and acoustic board used within the bespoke screed systems have environmental certification, with a screed largely consisting of recycled waste material from power stations.

 

Roof Rafters and Purlins

Benefits

I-Joist can be readily specified for both flat and pitched roofs. I-Joist rafter design can include or incorporate loose timber overhangs and there are several options for ridge and eve supports. For further detailing see the roof section within the system providers literature. The inherent shape of the I-Joist enables more insulation to be introduced into the joist depth zone. A typical U value for a 245mm I-Joist with standard mineral wool insulation is 0.2 watts per metre squared kelvin. The long lengths of I-Joists enable spans of over 11 metres to be achievable the flat roofs. Designers must consider the problem of water ponding at mid span on long joist spans. As with floors cassettes can be easily used to form the roof scheme. Cassettes may be simply dropped onto an already formed lattice of supports or may include support purlins and insulation within their design as part of a bespoke roof system.

 

Roof Systems

 

Panelised Roofs

Architecturally pleasing, open vaulted ceilings, excellent use of all available space with no interfering structural timbers, sales appeal.

 

Panelised Roofs vs Attic Construction

Panelised roofs have at lightning speed of erection when compared to attic trusses. A typical erection time for a panelised system is 4 hours, compared to two weeks for an attic truss rafter roof. The components within the panelised structure automatically provide the basis for the continuation of the build. Roof ready for tiling, gable wall build, installation of internal services and finishes.

 

Benefits of Onsite Panelised Roofs

Panelised roofs have many advantages when considering reducing safety hazards. Firstly, the majority of the build has been within a factory environment, not five metres up on a windy roof. Any further work required on site can be undertaken at ground level before the panels are craned into position. Inherent within the design of panelised roofs is an already constructed safety deck, the floor panel of the attic space. As these panels are usually first to be lifted onto the structure the builder has a safe working platform for the rest of the roof build. Panelised roofs are built in factory to precise dimensions using quality management systems thus, there is less scope for mistake.

 

The use of engineered wood products guarantees the quality of materials. Panels can be supplied to site fully insulated. This guarantees the consistency and fitting of the insulation as full access is available when the panels are manufactured. This guaranteed degree of insulation would reduce air leakage and cold bridging.

 

As the panels are delivered to site on the day of erection and remain on the lorry until they are lifted into place, security access condition and availability of materials are guarantees as no materials were stored on site. Panelised roofs are usually supplied as a one stop shop thus, there is a complete understanding of the system with regard to design manufacture and erection.

 

Video showing Construction Process.

 

Wall Studs

Typically, 145 and 195mm I-Joist studs are used for wall studs. However, there is no reason why 450mm I-Joists could not be used. The inherent shape of the I-Joist enables more insulation to be introduced into the joist depth zone. Thus, incredibly the timber becomes the weaker element within the panel with regard to thermal efficiency. A typical U value for a 245mm I-Joist with standard mineral wool insulation is 0.2 Watts per metre squared Kelvin.

 

I-Joists long lengths and lightweights enable the creation of big panels for tall walls. For further detailing see the wall section within the system provider's literature.