How does the timber trusses work ?
In the evolution of building there have been two great developments since man first used timber or stone to provide himself with shelter. These materials were first used as simple beams. The Romans are credited with the invention of the arch, and the truss was developed in Europe during the middle ages. A beam supports loads due to its bending strength. This is the way simple members such as rafters, battens, purlins, lintels and bressummers work. The top edge of a beam is normally in compression and the bottom edge in tension. These stresses reach a maximum near the middle of the beam’s span and for every doubling of span the strength of the beam must increase four times. Beams also tend to sag when loaded and sag is even more sensitive to increases in span than the requirement for increased strength.
Roman Arch Bridge
The Romans found that if they leant stones against one-another in the shape of an arch, they could span greater distances than by using the stone as simple lintels or beams. In an arch the stones are in compression. The arch will perform as long as the supports or buttresses at each end of the arch provide restraint, and do not spread apart. Timber beams can also be propped against one-another to form arches. The timber members will be in compression and will also act as simple beams.
To turn the arch into a truss, all that is required is to provide a tie between the two buttresses to stop them from being pushed apart by the arch. The arch, beam, tie combinations is self-supporting – we call this structure a truss. Gang-Nail trusses are based on these simple structures. All the truss members are timber, and the joints between the members are formed using Gang-Nail connector plates.
Single Truss with Arched Rafter and Tie.
The characteristic appearance of a truss is a framework formed by many small triangles. A triangle is a naturally stable shape, compared with say a rectangular framework which can be deformed unless its joints are rigid or it is braced from corner to corner. Such a brace would, of course, convert a rectangle into two connected triangles on a truss. The members forming the perimeter of a truss – the chords – usually act as beams as well as ties or struts. The shorter the distance between truss joints, the smaller the chord section required.
Common “A” Type Gang-Nail Truss.
However, the more joints there are in the truss, the more expensive it is to fabricate. The designer of a truss can choose the arrangement of the chords and webs and must balance structural efficiency against manufacturing efficiency in supporting the applied loads.
Roof truss systems
Advantages of Roof Trusses
Gang-Nail trusses are an economical constructionmethod for all types of roofs. The Gang-NailSystem also allows solutions to many problemsassociated with complex roofs.
In domestic construction only the perimeter wallsneed to be designed as load-bearing walls whenroof trusses are used. Internal walls becomesimple partitions and can be arranged without theneed to provide supports for propping beams,hanging beams, etc. The sub-floor structure issimplified as stumps and bearers don’t need to bearranged under internal non load-bearing wallsand where concrete slab floors are used, thearrangement of internal beams is simplified.
Trusses are designed to engineering standardswith a substantial factor of safety applying to everytruss in the roof. Traditional ‘stick built’ roofs arebased on historical carpentry practices, which areconservative, and so they use much greaterquantities of timber to achieve acceptable factorsof safety. The strength reserve of these traditionalroofs and their supporting walls is also variableand depends on the skill of the individualcarpenter.Truss roofs can be designed to resist uplift due towind suction and can be tied down to thesupporting frame with greater security than iseasily obtained with traditional roofs.
Trusses shorten the construction time. Withtrusses, most roofs can be installed in oneworking day. Delivery and erection can becoordinated with the completion of the frame. Sitelabour requirements are reduced, as is the impactof wet weather on the construction program.Because trusses are manufactured specifically foreach project, costly pilfering is virtually eliminated.Building Permits
The Gang-Nail Truss System is a proven methodof construction and is accepted by all buildingauthorities.SIA Freimans Timber Constructions manufactures tiber trusses according to MiTek Industries AB specifications and desgin criteria SIA Freimans Timber Constructions is MiTek licensed fabricators which can supply the necessary documentation for building permits.
Gang-Nail roof trusses allow just about any shape of roof to be constructed. However, there are number of standard roof types that remain popular for domestic construction.
These terms refer to the shape of the roof crosssection, and the detailing of the ends. All these roof types can be constructed in ‘L’ shapes, ‘T’shapes and combinations of these with trusses of varying spans.
With trussed roof construction, only external walls are load bearing,making the design of floor structure simple and inexpensive.
Basic truss mechanics
All trusses in a roof structure are designed for the worst possible combination of dead, live and wind loads. The individual truss members are designed to restrain the corresponding forces i.e.., tension or compression, or a combination of bending with either the tension or compression force.
Tension (pulling). With this type of force the member being pulled or subjected to a tension force is said to be “in tension”. The ability of a member to restrain tension forces depends on the material strength of the member and its cross-sectional area..
The example (Figure 1) shows that if the cross-sectional area of a member is doubled, the ability of that member to restrain the tension forces is also doubled.
Compression (stumšana). (pushing). When a structural member is subjected to this type of force it is sometimes referred to as a column. Unlike a tension member, the ability of a column to restrain compression forces is not simply a function of the cross-sectional area, but a combination of the material strength, the column length and the cross-sectional shape of the column.
If one tonne is the maximum compression force that can be supported by a piece of 100 x 38 mm timber, 1200 mm long without buckling, then the same force applied to a piece of 100 x 38 mm timber, but twice as long, would certainly cause it to buckle and possibly collapse. (2nd and 3rd figure) However, if we rigidly support the 2400 mm long column in the previous example at the centre, it would then be capable of withstanding the one tonne force. (Figure 4)
Where this rigid support is applied to a web member, it is called a web tie, which is used in conjunction with bracing. (See Figure 5A) Battens with bracing from the rigid supports are needed to restrain the truss chords from buckling sideways. (See Figure 5b).
The strength of a column is also dependent on the cross-sectional shape of a column. The squarer or more symmetrical the shape, the stronger the column, given that the cross-sectional area is the same. In the example of 100 x 25 member having a cross sectional area of 2500 mm is not as strong in compression as a 50 x 50 member, provided that the other factors of length and material strength are equal. Bending force, or more correctly bending moment, is the result of a force applied to a cantilever, for example: a diving board, or to a simple beam.
The load carrying capacity of a beam is dependent upon the strength of the material and also the cross-sectional shape of the beam. In the case of the beam, unlike the column, the deeper section having the same cross-sectional area will be the stronger member in bending. Beams subject to bending moments also require lateral restraints, as with columns. The deeper the beam the greater number of restraints required.
Forces in Members
In many common types of trusses it is possible to identify the type of force which is in any particular member without undertaking any calculations.
The example in figure 9 is a common ‘A’ type gable truss with a uniformly distributed load along the top and bottom chords. This is due to the transfer of the load of the tiles through the tile battens and the ceiling load through the ceiling battens.
This means that the chords are subjected to bending forces as well as compression and tension forces. This loading arrangement would result in the top chord restraining compression plus bending forces. The short web is in compression and the long web is in tension. The geometry of both ‘A’ &‘B’ type gable trusses is arranged so that under normal conditions, the longer webs are in tension and the shorter webs in compression. This is done to economise on the size of the timber required for the compression webs.
Wherever a member is subjected to a tension, compression or bending force (bending moment), the member is deformed by the force, irrespective of how strong the material is or how large the section. The amount of deformation does, however, depend on material strength and the size and shape of the section.
In Figure 10a it can be seen that the Oregon beam would deflect 32 mm soon after the one tonne point load is applied at a mid-span. If this load is maintained, the deflection may gradually increase to three times the initial deflection after a period of 20 to 24 months. This increase in deflection, with time, without increase in load, is called “creep”. This characteristic is significant with timber, but can be ignored in other structural materials like steel.
If the same load is applied to a steel universal beam (see Figure 10b), the spontaneous deflection is approximately 1 mm. The long term deflection will also be 1 mm.
The timber truss (See Figure 10c) will also deflect under the same load, but because it is braced by its triangular web layout, it is much stiffer than the heavier Oregon beam, and is nearly as stiff as a large steel beam which would weigh approximately three times more, and would probably cost five times as much as the timber truss.
From these examples, it can be readily appreciated that timber trusses are very effective structural components.
To compensate for deflection which occurs when loaded, trusses are manufactured with an upward bow which is called “camber”. Some deflection occurs as the truss is erected, more deflection will occur as the roof and ceiling loads are applied to the truss, and further deflection will occur over a period of time due to the “creep”.
Because the chords are subjected to a distributed load, they will also deflect in between panel points, in addition to the truss as a unit deflecting downwards.
This deflection of the chords is called “panel deflection” and cannot be compensated for during manufacture, as can be for truss deflection (camber). All standard truss layouts, are designed to keep panel deflection within acceptable limits.
Truss Analysis and Member Design
When the design loads are known and a truss shape has been chosen, the truss can be analysed to find the forces that will occur in each of its individual members. This process is done by computer using well-established methods of structural mechanics. The computer uses a process of analysis that is integrated with the selection of members of suitable size and stress grade and the calculation of expected deflection when loaded.
Truss members are subjected to combinations of bending, shear and compression or tension. The combinations can vary during the life of the structure as different loading conditions occur and every foreseeable situation has to be considered. Timber members are chosen so that they meet the strength and serviceability requirements.
Gang-nail connectors - how they work
A Gang-Nail connector is a steel plate with a collection of spikes or nails projecting from one face. The spikes, or teeth, are formed by punching slots in steel but leaving one end of the ‘plug’ connected to the sheet. The teeth are then formed so they project at right angles to the plate. During this process the teeth are shaped to produce a rigid projection. When the teeth of a connector plate are pressed into timber laid end-to-end, the plate ‘welds’ them together by forming a Gang-Nail joint. Connectors are always used in pairs with identical plates pressed into both faces of the joint.
The concept is simple but the design of efficient Gang-Nail connectors requires careful balancing of tooth shape and density, connector plate thickness and ductility. An ongoing commitment to research and development ensures that MiTek’s licensed truss fabricators have the most efficient truss system at their disposal.
Performance criteria for Gang-Nail connectors
It is not economical to have a single connector that gives optimum performance under all loading conditions, for all of Australia’s wide range of commercial timbers. MiTek Australia Ltd. has developed a complementary range of connector plates of varying plate thickness (gauge), tooth layout and tooth profile. These are:
- GQ – 20 gauge (1.0 mm thick) galvanised steel. General purpose connector. Many short, sharp teeth - 128 teeth in a 100 mm x 100 mm area.
- GE – 18 gauge (1.2mm thick) galvanized steel. Similar to GQ. For use when additional steel strength is required.
- G8S – 18 gauge (1.2 mm thick) stainless steel. This connector is only used when the environment is highly corrosive. 70 teeth in a 100 mm x 100 mm area.
- GS – 16 gauge (1.6 mm thick) galvanised steel. Heavy duty connector. 144 teeth in a 100 mm x 190 mm area.
Corrosion of Roof Truss Gusset Plates
Deterioration of metal truss gusset plates is a major concern in buildings that contain high humidity and corrosive environments. Many of these buildings show severe corrosion within 5–10 years. Normal galvanized steel plates exposed to moisture, condensation and ventilation air containing manure gases will corrode rapidly. This corrosion can weaken the building and could potentially lead to structural failure.
Truss plates are light-gauge metal plates used to connect prefabricated wood trusses. They are produced by punching light-gauge galvanized steel (normally 16-, 18- or 20-gauge) so teeth protrude from one side, as shown in Figure 1. The truss plates can be galvanized prior to punching, leaving numerous unprotected metal edges. During truss fabrication, these truss plates are pressed into the lumber with either a hydraulic press or a roller to fully embed the teeth.
The buildings most affected by this corrosion are cold, naturally ventilated beef and dairy barns with slatted floors and deep manure storages. Also affected are warm, naturally ventilated swine barns. These buildings often expose the entire truss assembly to a potentially wet service condition. In most cases, farm trusses are designed for a dry service condition.
Figure 2 shows the degradation that can occur when the roof trusses are included in the environmental air space of a building — the gusset plate is beginning to rust. In contrast, Figure 3 shows a gusset plate that has been in place for a longer period of time, but only used in an equipment storage building. This joint is still in good condition because it has not been exposed to corrosive environmental conditions.
Truss plates often show the greatest deterioration near the building air exchange openings, typically at the heel and peak joints of the truss — the areas of greatest air mixing and temperature change, producing high humidity and condensation problems. Unfortunately, these are also very critical joints in the structural integrity of the truss.
When installing rooftop solar modules, be sure to check these connectors during the full facility review. Even if the building has been securely in place for a number of years, do not assume that corroding gusset plates can handle additional loads.
CAUSES OF CORROSION
There are many potential causes of corrosion in animal buildings. Animals exhale large quantities of moisture into the air, creating high relative humidity in the building if the moisture is not properly vented. High humidity increases the potential for condensation, which wets the entire truss assembly.
Ammonia gas, typically found in animal environments, combines readily with this moisture and becomes ammonium hydroxide, a chemical that attacks most metal surfaces. Free moisture on poorly protected steel will also initiate the rusting process. The moist wood accelerates the corrosion of the metal fasteners since wood itself is slightly acidic. In addition, long-term wetness can raise wood moisture content above 30% and accelerate wood decay.
Dust, commonly found in animal environments, provides a surface on which acids and gases can react, significantly accelerating the rate of corrosion. Common bacterial colonies found in barns tend to form biofilms on building and equipment surfaces, allowing bacterial growth and the production of other corrosive acids.
PREVENTION OF CORROSION
Properly Ventilate the Building
A good ventilation system should move enough fresh air through the building to reduce the levels of moisture, gas and dust to acceptable levels. A well-designed system will minimize corrosion problems. Proper ventilation requires good building design and good ventilation management. A ventilation specialist, equipment supplier or building contractor can help ensure that ventilation issues are not contributing to the corrosion problem.
Often, an owner tightens up a barn to raise the building temperature or to save on supplemental heat. Unfortunately, this reduces the ventilation rate and allows the humidity level to increase. In fan-ventilated barns, keep at least one exhaust fan operating to constantly remove the respired moisture. Similarly, naturally ventilated barns require a constant exchange of air to control moisture. Additionally, barns with continuous ridge ventilation provide a condensing surface at every peak truss connection along the barn. Situating chimneys intermittently between the trusses keeps this ventilation air away from the truss plates and reduces the potential for corrosion.
Apply a Protective Coating to Metal Truss Plates
Apply a protective coating to the plates by brush either before or after truss installation. Surface preparation is important. Roughen and clean the substrate and cover each metal plate, including its edges, with the coating. One of the recommended epoxy coatings that is lead- and chromate-free is Epoxy-Polyamide Primer and Topcoat (SSPC Paint No. 22 or CGSB Paint No. 1-GP-146). SSPC refers to the Steel Structures Painting Council (U.S.), and CGSB refers to the Canadian Government Specifications Board.
This epoxy paint is a two-component product that requires specific mixing and application expertise. Prepare the plates with a cleaning solvent or by sandblasting prior to coating or as per the requirements of the SSPC SP16 procedure. This task could be undertaken by a commercial painter, building contractor or the owner. It is a labour-intensive job that will be only as effective as the quality of workmanship employed. A truss manufacturer or industrial paint supplier can help find this coating material or an equivalent product. In some cases, the extra step of boxing each coated connector with plywood has been taken. Ultimately, the person responsible for your roof design will have to specify what needs to be done.
Use Pre-coated or Stainless Steel Truss Plates
Various pre-coated and stainless steel truss plates are available, however, they are expensive. Coated truss plates are almost five times the price of standard G90 galvanized plates. They must be larger than standard plates because of their slippery surface. Also, because they are coated before truss assembly, they may get chipped before they are installed in your building, creating opportunities for corrosion.
Stainless steel plates are also quite expensive, which can make a significant difference in the total cost of the truss. Not all suppliers stock these plates, so factor in delivery time. In all cases, it is important that the truss designer and manufacturer understand your service conditions. When this is known in advance, the proper materials can be selected for your project.
Install Ceiling with Insulation and a Vapour Barrier
If the truss assembly is completely partitioned out of the animal environment, it will not be influenced by the high humidity condition and related problems. A 4- or 6-mil polyethylene vapour barrier where the seams are taped is necessary to prevent moisture migration into the attic space. Steel or plywood ceilings by themselves do not provide an adequate vapour barrier. The sheet seams, fasteners and material porosity will allow moisture to pass through. A minimum amount of ceiling insulation is also required to prevent condensation from occurring on the ceiling surface itself.
To mitigate the impact of barn air entering the area, ventilate the attic area to allow moisture to escape by allowing fresh air to enter along both the eave and soffit areas. It is still possible to allow the air to escape by installing exhaust chimneys through the attic space between the trusses, as shown in Figure 4. For more information, see OMAFRA Publication 833, Ventilation for Livestock and Poultry Facilities, Chapter 8, Attic Ventilation, and consult a ventilation designer for the correct size and number of chimneys required.
- If the truss deterioration is minor, employ one or more of the prevention procedures outlined above to halt further corrosion..
- If the deterioration is more severe, consider a structural repair. Have a consulting engineer conduct an assessment and repair specification. The Ontario Building Code requires roof systems to be engineered..
While the extent of the truss deterioration problem is not known, it is a very serious situation for those buildings affected. Owners should inspect their buildings periodically for signs of wetness and corrosion on the truss plates. Initiate a repair if necessary. A building contractor, truss manufacturer or engineering consultant can assist you. The building repair or retrofit expenditure undertaken now will most likely be cheaper than replacing the structure prematurely.
This Factsheet was revised by Dan McDonald, P.Eng., Civil Systems, OMAFRA, London, and reviewed by Daniel Ward, P.Eng., Poultry & Other Livestock Housing & Equipment, OMAFRA, Stratford.