Truss Roofs
What Is Truss Roofs?
If the span is more, a framework of slender members is used to support sloping roofs. These frames are known as truss roofs.
Before the tied rafter, timbers
(1) roofs had to be of sufficient strength to span quite tidy distances victimization nails and straightforward jointing techniques. If the timber couldn’t build it on its own, then sturdy beams
(2) can be wont to cut the spans of those timbers, and if that wasn’t enough additional
strong frames
(3) were created up and placed to interrupt the span of the beams.
so we tend to have:
(1) These timbers would be the rafters, ties, collars, and hangers of a standard roof.
(2) The beams used would be purlins to support the center of the rafters, and binders to support the ties, usually at their third points.
(3) The frames are referred to as trusses and will vary from king and post and alternative
wonderful trusses from medieval times, to the TRADA trusses of the latter half the
twentieth century.
Figure 6.4 shows some layouts of those timbers in
(1) within the higher than the list for varied types of roof. For identical timber sizes, roof C could be a larger span version of A, while B, the collared roof, is common on previous properties and farm buildings however aren’t as sturdy as roof A.
The provision of a tie in roofs A and C prevents the rafter ends at the wall head from pushing outward and pushing the wall over. whereas a collar can even try this, the lower half of the rafters are placed into bending yet as compression so a lot of stronger timbers need to be used for an identical span of the roof.
D is that the type of C with a vessel pitch and therefore the risk of getting attic rooms. With the steeper pitch, lighter section rafters, collars, and hangers will be used however the ties currently double up as floor joists so should be heavier and probably even supported at mid-span by a beam or partition.
Roof D conjointly shows a tool from the first roofs, the sprocket or bell forged piece. These were separate short lengths of timber connected at the overhang and forming a part of the projection of the overhang.
The pitch angle on them was typically but the most roof, that slowed the run-off of water from the slate or tile into the gutter. Figure 6.5 shows, within the small stages build-up
format, a roof victimization purlins and binders spanning from gable end to gable end and supporting rafters and ties.
Finally, Figures 6.6 and 6.7 show however a tied roof would be engineered up step by step. The truss form could be a post truss and it might be made up of significant natural timbers each in one piece, with the middle strut, the post, formed to require the struts and therefore the truss rafters.
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Note that the rafters that truly support the roofing material are referred to as common rafters and are set out across their supports, the purlins, while not taking any notice of the trusses.
The trusses are solely there to support the purlins. it’s attainable to try to while not trusses if the length between the gables isn’t too long, by victimization tied purlins or I beam purlins or box beam purlins.
Figure 6.8 shows sections through these, alongside the position between the gables that they’d take. Note that there’ll still be a plate on the prime of the wall for all the rafter feet, and a ridge wherever the pairs of rafters are part of.
The laminate I beam with a depth of roughly 900 and with four a hundred × fifty SC3 timbers as flanges was capable of supporting a covered roof throughout 4000. These beams will all be created on the spot, the fastening being either bolts and connectors or pattern nailing.
Figure 6.8 shows what’s meant by pattern nailing. alternative truss shapes are made up of solid timbers and Figures half dozen.9 and 6.10 show a variety of those. The post truss can span a lot more than the post truss and therefore the hammer-beam truss is more still.
Variations on the latter are used for several public and nonsecular buildings which is
probably the simplest place to ascertain them. From its form, it’s obvious that the post truss is very appropriate for holding attic rooms such a large amount of those are hidden from view, as is that the mansard truss.
The mansard truss could be a combination of a modified post truss with a reasonably pitch king past century – once it had been widely used for industrial buildings.
Figure 6.8 remarked the likelihood of layering timbers at the nodes of the tied
purlin. Indeed, this was the plan of action adopted by the Timber analysis and Development Association (TRADA) in their series of trusses for domestic and lightweight business roofs.
Figure 6.11 illustrates this kind of truss. Appendices C and G illuminate the timber connectors used.
The define shows the overall arrangement of the timbers. The combine of rafters is every
made from 2 items of timber, as is that the tie. there’s in-depth use of packing timbers or spacers that are wont to connect the pairs of timbers along.
The purlin struts are broad and have a sign in the highest to support the purlin. The long struts are barred to the ridge support at the highest and, though slightly out of alignment, are barred in between the tie-timbers.
Most of the node one details are given in Figure half dozen.11. Purlins are usually from one hundred fifty × fifty to two hundred × sixty-five because the trusses are set at 1800 centers with common rafters between them at 450 centers, thus there are 3 common rafters then a truss rafter, 3 a lot of common rafters, a truss rafter, so on right down the roof.
These trusses have rafters that take a share of the roof covering load along with the common rafters. Using trusses this way in the late 1940s and early 1950s was the first time that it had been done.
The author built one set of trusses in 1964, following the TRADA designs, which had
double timbers for the ties and triple timbers for the rafters. The purlin struts were double but the long struts were single.
The timber was Douglas Fir – stress grading was unknown at the time. The pitch was 22◦ and the roof was covered with aluminum on felt on wood wool slabs. The roof is still there with nothing other than routine maintenance.
About a year after the roof was completed and the house was occupied, the author had to go into the roof space and while there decided to have a closer look at the joints of the trusses.
To his dismay he found that every one of the nuts on the bolted connections was only fingertight at best and in many instances was loose. This was not caused by vibration but was solely due to shrinkage on the multiple layers of timber.
With packers and splice plates, some of the joints were some five layers thick, which represented something like 250 of timber.
The nuts were all retightened and while the author was in the occupation they were checked regularly but never came slack again. What was holding the trusses together? Only the Bulldog connectors’ teeth are embedded into the timbers.
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