MiTek Roof Truss Design
The MiTek 20/20 roof truss Engineering software has long been established as the most widely used trussed rafter design package in the industry.
MiTek 20/20’s Business Application, MBA,
provides the desk top to all the applications
a truss plant needs and enables:
- Overall Project Management
- Project Overviews
- Detail Costing & Time estimates
Layout: A fully 3D system to frame even the most
complex roof shapes.
MiTek 20/20 Layout allows the user to:
- Frame accurately so that trusses fit precisely
- Check dimensions of all components
- Prepare fully dimensioned erection details
Engineering: A state of the art timber and steel
engineering system allowing the designer to:
- quickly design even complex frames
- handle multiple load cases automatically
ONTRACK: A state of the art resource planning and monitoring system allowing the designer to:
- control production per workstation
- allocate call-offs to different workstations
- Monitor and control loads
- get production reports on utilization of production
THE EFFICIENCY OF THE MiTek CONNECTOR PLATE
MiTek connectors are engineered to achieve greatest resistance to nail withdrawal, combined with maximum strength to resist both lateral and shear forces – thereby optimising plate efficiency.
MiTek connectors are manufactured from prime quality, galvanised steel. Strict quality control is undertaken prior to receipt of steel from the mill and quality is checked throughout the manufacturing process. A wide range of sizes and types is manufactured to enable our structural engineers to use the most economical size of connector for any particular joint. MiTek connector plates have been tested by the Council for Scientific and Industrial Research (CSIR) and research reports are available on request.
MiTek trusses are manufactured in a factory, under controlled conditions by trained personnel using unique high-speed equipment. Trusses are assembled in jigs, ensuring tight, accurate joints and uniformity.
Only selected stress-graded structural timber complying with SABS codes 563, 1245 or 1460 or New SANS1563, SANS11245, SANS11460 is used.
TRUSSES DELIVERED TO SITE WHEN REQUIRED
Complete roof sets of MiTek trusses are delivered to site, ready for erection, and are less likely to be pilfered than loose timber. No more choice lengths of structural timber in the night watchman’s fire.
REDUCED CONSTRUCTION TIME
Since the trusses are manufactured in a jig, they are identical. They may therefore be lined up quickly prior to fixing the purlins or battens. This allows the accurate scheduling of ceiling erectors, electricians and other trades who must proceed under cover.
Through MiTek, the MiTek System provides the contractor with a pre-fabricated truss unit, pre-cut, assembled, and delivered to site. Labour costs are built into the truss cost. The labour is paid for on normal terms of credit, as opposed to payment of weekly wages for labour making trusses on site. The automated manufacturing process lends itself to repetitive fabrication of identical trusses where substantial savings are realised. The prices of complex roofs requiring multiple jig-set changes may not reflect such dramatic cost benefits, but your guarantee of expert design and fabrication and delivery when required will prove a major advantage over on-site carpentry.
MiTek trusses are individually designed strictly in accordance with the code of Practice for General Procedure and loadings (SANS 10160) and the Code of Practice for the Design of Timber Structures (SANS 10163). The designs are prepared using advanced computer design and drafting systems supported by MiTek’s professional engineers. The design approach has been verified through a large number of truss tests conducted by the National Timber Research Institute of the CSIR. On small projects such as domestic dwellings, the fabricator can supply full working details directly off his own computer system while on large or complex projects, the fabricator will request MiTek’s professional engineering team to prepare full working drawings showing all the truss designs, connection details and bracing.
All timber used, both for the trusses and the secondary timbers is in accordance with either:
- SANS 1783 for Stress Graded Softwood General Structural Timber.
- SANS 1783 for Stress Graded Softwood Engineering Timber.
- SANS 1460 for Structural, Laminated Timber.
- SANS 1783 for Battens and Branderings.
These specifications cover structural grades S5, S7 and S10
- Structural grades, 5. 7 and 10 are now the only legal grades that are used in South Africa.
- The South African Bureau of Standards has developed a system of proof grading which has been adopted by some saw-mills. These proof grades generally are designed to develop the same minimum grade stresses or the equivalent Structural grades.
MiTek connector plates are fabricated from prime structural hot dipped galvanised steel with guaranteed properties. A range of connector plates is available for different applications. The design values for all MiTek connectors have been determined through joint tests at the CSIR’s FORESTEK Division under the following contracts:
M20 C/WOOD 49
Design data is available on request.
Where highly corrosive conditions are involved MiTek can provide 430 grade stainless steel connectors. These plates are sourced from our European Manufacturing facilities.
TIMBER AND GRADING
If timber is to be used in construction, it must be graded for strength and stiffness. Grading into different strength classes allows the best possible use to be made of the various timber dimensions. This ensures that timber that is unsuitable due to low strength, twist, warp or springs is not used structurally.
- Three methods of grading structural timber are employed:
- Machine stress grading
- Visual grading
- Proof grading
MACHINE STRESS GRADING
Research has shown that timber strength is directly related to its stiffness – the stiffer it is, the stronger it is likely to be. This principle has been applied to the development of machinery and systems which accurately and consistently classify South African pine timber into structural grades.
The graph deals with the relationship between strength and stiffness. This characteristic strength value is taken at the 5th percentile of the strength of the material, that is the value which is exceeded by 95% of the timber tested. A safety factor of 2,2 is then applied to give the working stress used for design purposes.
Small static machines in which each piece of timber is individually tested, or high-speed dynamic testing machines which grade timber at speeds of up to 60m per minute, may be used for grading. Both these work on the same simple principle (shown below).
Stress-grading machines measure the deflection of the timber at a certain load and, given the dimensions of the piece, the stiffness can be calculated. In the simple static machines this is done by setting the scale on the dial gauge. The dynamic machines use computer systems to calculate the stiffness and indicate the grade, e.g. M4, by automatically spraying the timber with coloured dyes which are coded to the various grades (see below)
Machine stress-grading of structural timber is covered by SABS specifications1783 and the SABS code of practice D149.
HIP END DETAILS
Various standard hip layouts are commonly used depending on truss span, loading and pitch. Typical examples are:
- Hydro Hip System – Small hip system.
- Truncated Hip System – Truncated girder system suitable for larger spans but not well suited to low pitches.
- Dutch Hip System – Dutch or louvered hip system.
- 45o Hip System – A versatile full girder system simple to erect but not generally viable at spans over 10m (under tiles).
Where the cost of the hip system may have a significant effect on the total roof cost, (i.e. pyramid roofs or large multi-hip projects) the specifier should contact MiTek for advice as to the best system to use.
|45 Degree Hip System||Truncated Hip System|
|Hydro Hip System||Dutch Hip System|
There is NO Steel Truss System that is easier to Design, Cut, Fabricate and Ship!
Recently, in a timed comparison in the US, the Ultra-Span Light Gauge Steel Truss System was priced 20% lower for like material, took 33% less time to cut, and 36% less time to fabricate than its closest light gauge steel competitor. Using crews with no experience on either system, the MiTek Ultra-Span truss system clearly outperformed the competition in every category from cutting to shipping.
The clearest advantage was the unique member profile shape that MiTek has developed. Purposely designed with less steel per metre than any other system, the Ultra-Span truss shape is fabricated with only three components (chords, webs and single size connector). This simple engineering system allows the fabricator to utilize the MiTek 20/20 software suite to lay out, engineer, cut and estimate every type of truss from simple common truss shapes to complicated long clear span trusses.
In the shop the Ultra -Span truss system allows for one-sided fabrication (no flipping of the truss) and the connections are made with an inexpensive #10×16 self-drilling galvanized hex head screw. Our customers are able to buy these screws from MiTek or a quality supplier in their area. No special, expensive, or difficult to apply connection devices are necessary to fabricate the Ultra-Span Truss system.
As per the design the Ultra-Span truss system shape sits flat in the jigging fixture with all of the connection surfaces easily accessible and flush for a quality screwed connection every time. The system also features same gauge connectors for faster screw throughput, same gauge top and bottom chord designs to minimize cutting and handling, and a true truss shape every time including scissor trusses and tray ceiling with no job site modifications.
The Mitek Ultra-Span Light Gauge Truss System for non-combustible roof framing of any type.