Roof trusses
A roof truss is a large, vertical, load-bearing and stiffening component of a roof structure . Roof trusses with parallel or almost parallel upper and lower chords are often referred to as girders or trusses .
From a static point of view, the truss - like a truss - is made up of several triangular bars . If individual fields of the girder have four or more corners, these are ideally reinforced by the neighboring triangular fields. If necessary, the lack of rigidity of the girder must be compensated for by the support provided by the center purlin or, in the worst case, by the rafters, which are then subjected to bending stress as continuous beams . In the case of steel structures, it is also conceivable to form rigid corners ( nodes ) like a frame .
Traditionally, a truss consists of the roof beams, which form the roof structure, and, if necessary, of some additional beams inserted for stiffening.
In the case of a rafter roof , two rafters are connected to the ceiling beam ( collar beam ) via offset and bolting or nail plates to form the roof structure or rafter triangle . Since each of these bar triangles forms a truss, rafter and collar beam roofs, in contrast to purlin roofs , consist exclusively of trusses.
The purlin runs across the rafters on the ridge, on the floor or in the middle of the rafters. In principle, a purlin roof does not need any additional floor joists or collar beams as stiffening girders in addition to the rafters and purlins. If the ridge purlin settles, however, as with the rafter roof, shear stresses arise that can be absorbed by a truss construction. Alternatively, the rafters can also be connected to the purlin , provided that it is anchored to the ceiling joist so that it cannot tip over .
Traditionally, every third, fourth or fifth field in the purlin roof is stiffened with a truss construction. In the horizontal roof structure , the girders also support the ridge purlin and any central purlin. In the case of a standing roof structure , the girders should not experience any load with a symmetrical roof load and should only be used for stiffening when horizontal forces act on the roof structure due to one-sided snow load or wind pressure .
Roof trusses for smaller roofs can be pre-assembled in the joinery hall and completely transported to the construction site.
Construction forms
Depending on the application, the binders are also differentiated according to their shape:
- Parallel girders (e.g. truss or corrugated web girders )
- Gable roof trusses
- Shed roof trusses
- Trapezoidal binder
- Double trapezoidal binder
- Mansard truss
- Bow ties
Types of roof trusses
Rafter ties
The simple roof truss that is used on the rafter roof. It consists of two rafters and a ceiling beam and is also known as a gable roof truss.
Collar beam trusses
If the rafters of the truss exceed a certain length, it is economical to insert additional, mostly horizontally arranged pieces of wood into the truss in order to prevent excessive deflection of the rafters. These timbers then often form the attic ceiling, also known as the collar beam ceiling, in the case of loft spaces.
Plank binder
Around 1800, for the first time, larger roof structures with girders were made from planks or boards that were laid next to one another and nailed to one another, called plank trusses . In contrast to the half-timbered constructions that had been customary up to that time, it was also possible to manufacture trusses with a rounded upper flange for dome constructions.
Nail ties
These are trusses. The supporting structure consists of compression and tension rods that are positively connected at the nodes by means of wooden lugs and nails according to a precise nail pattern. As a rule, nail ties are made from several layers of boards. In this respect, they are similar to plank girders, which, however, are usually not designed as trusses from the outset, but rather stiffened by an additional truss construction after assembly.
Nail plate ties
Since the 1970s, nail ties have increasingly been manufactured as “nail plate ties” (NPB), in which the nodes are connected by standardized “nail plates”. These are sheet steel plates from which narrow pieces of sheet metal have been punched out and angled at regular intervals, which then protrude on one side like nails. The nail plates used in Germany require general building approval from the German Institute for Building Technology (DIBt). Nail plates are pressed into the wood on both sides with special tools above the wooden joints. As with conventional constructions, wood cross-sections and nail plates are statically measured. On the basis of the design standards DIN 1052 and DIN EN 1995 (EC5), nail plate trusses can be used in service classes 1 and 2.
For buildings, these supporting structures with nail plate connections are regulated as building products in accordance with DIN 1052 and listed in the building regulations list A Part 1, Edition 2012/2 No. 3.3.1.2.2. In the building regulations list, a requirement for the manufacturing company is monitoring and certification in accordance with the state building regulations. All nail plate binders must have an Ü mark. In addition to the manufacturer and manufacturing plant, DIN 1052 and the abbreviation of the certification body must be specified in the Ü mark.
Nail plate trusses are often used in halls and other single-storey buildings that are intended to span larger areas without supports, as is the case, for example, in large-scale retail operations such as supermarkets . Nail plate trusses are structural timber structures.
In addition to wide-span roof constructions, nail plate trusses are occasionally also used in roof constructions for residential buildings, in particular for flat pitched gable or hipped roofs. So-called studio trusses enable lofts that can be expanded flexibly for residential use and large, column-free interior spaces in the attic.
Manufacturers of nail plates and processing timber construction companies are united in the Association of Nail Plates eV (GIN).
A roof construction with nail plate trusses can pose an increased risk in the event of a fire. According to the NRW fire brigade institute, "if a nail plate truss fails, no load shifting is possible, ie a total collapse almost always follows"
Laminated beams
In technical terms, glulam beams or glulam beams for short. A wooden cross-section of any size can be created from approx. 40 mm thick wooden slats by gluing. Boards are glued together in layers, hence the name. Glulam trusses may only be manufactured in approved companies with strict manufacturing guidelines.
Other techniques
In modern roof construction, wood connections are used less and less to connect wood. Instead, angles, joist hangers or nail plates are used. Nevertheless, the lining is still necessary, but this work can usually be carried out with the alpha angle , and it is no longer necessary to untangle it on the drawing floor .
Nail ties
Nail plate ties
Nail plates detail
Joist hangers
Timber frame construction with dowels
crossed parallel ties
About history
One of the most important German pioneers in this field was the Weimar- based Grand Ducal court carpenter and entrepreneur Karl Friedrich Otto Hetzer (1846–1911). In a brochure in 1910 entitled Otto Hetzer Weimar - New wood construction , the company at 20 German and foreign cited patents , the roof constructions and structures concern. This means that Otto Hetzer is still considered to be the authoritative founder of glued wood construction .
A Hetzer truss is a roof truss with an I- shaped cross-section. It consists of various types of wood glued together (beech for the pressure zone, spruce for the tension zone) and, depending on the span, can be provided with wooden or iron ties . A roof or hall structure built with it is suitable for the column-free vaulting of large rooms. Thanks to their relatively flat and column-free roof construction, the halls are built as assembly or storage halls, for example.
See also
Web links
- Nail plate ties according to DIN 1052: 2008-12 (PDF; 5.6 MB)
- Nail plate ties, DIN EN 14250, fire resistance, fire behavior, strength, etc. At Bauprofessor.de
- Timber construction methods in Lueger's lexicon of all technology
Individual evidence
- ^ Weak point nail plate roof structures , Jan Helm, Institute of the North Rhine-Westphalia Fire Brigade