Selection of Roof Trusses
The three basic types of trusses are bowstring, pitched, and flat. Architectural style, types of roofing material, methods of column framing support, and relative cost are the main factors influencing truss selection. In addition, the height and kind of side and end walls, as well as the roof design and bracing requirements, must be addressed.
If all other elements are equal, cost is the most important concern. Economy is determined by material efficiency in relation to truss type and proportions, as well as fabrication labour. Bowstring, tuned, and flat are the three primary varieties, in order of relative efficiency.
The purpose of a truss is to transfer load as quickly as possible from the point of application to the supports. A simple “A” frame is the most efficient for a concentrated load at the span’s centerline. A truss identical to the queen-post type is also the most efficient when only two equal and symmetrically arranged concentrated loads are involved. Without the use of web members, the load is carried to the support directly through the sloping top-chord members of both trusses.
An arch in the shape of a parabola is theoretically the most efficient for more or less homogeneous loads, which are commonly assumed in roof construction, because direct stress is created only in the arch and the tie member. Furthermore, a parabolic arch does not require a larger arch section to accommodate the bending moment, nor does it necessitate the use of web members to reduce the amount of bending. Web members are ideal because most buildings must withstand some imbalanced stress, and a circular arc is easier to fabricate than a parabolic one. As a result, the frequently used bowstring truss features a circular top chord and enough web members to maintain top-chord sizes manageable.
Bowstring trusses are typically examined for direct stress as though the top chord ran parallel to the panel points. Top chords can be glued-laminated to the curvature (Fig.1) or solid woods set to the curved pattern with or without their top surfaces sawed to the curvature (Fig.2) (Fig. 3.2). If the member’s center–line does not coincide with the expected direction of axial stress, the bending moment due to eccentricity between panel points must be addressed both for curved-laminated members and for members sawed to curvature. This secondary bending moment may be possible if joists are spaced along the top chord.
smaller member sizes than would be used in a true segmental, sawed timber top chord Furthermore, glued-laminated top chords and other glued-laminated members usually allow for smaller diameters because to their higher permissible unit stresses. They also reduce or eliminate the requirement for the seasoning maintenance that some sawed members require. They may, however, be more expensive than sawed members because to the additional labour involved in laminating.
Bowstring trusses are occasionally made from top chords that have been mechanically laminated using nails, bolts, or both. Although they are less efficient than glued-laminated or sawed members of the same size, they can be used provided the quantity of nailing has been calculated or specified based on experience to provide the needed strength of the built-up section. The section will often be greater than that required for glued-laminated members, but it will also be more suited to field lamination.
A bowstring truss can be constructed to resemble either a flat or a pitched truss, making it the most versatile of all truss designs. Proper lateral bracing is required for such structures.
Pitch and bowstring trusses are more efficient than flat trusses. They are only recommended if a somewhat level roof surface is wanted, especially on roofs with several spans.
They have the advantage of providing a bracing effect for lateral-bracing and column connections because both the top and bottom chords can be attached to the columns. Web-member stresses will be much higher for standard truss proportions than for pitched or bowstring trusses, and web connections will be more intricate and expensive.
The centre piece of the bottom chords is raised substantially above the level of the supports in raised-chord trusses. They’re commonly employed for aesthetic purposes or to provide more clearance. Bowstring crescent trusses, so-called cambered or raised-bottom-chord pitched trusses using Howe, Pratt, or Fink web systems, and scissors trusses are all examples. The effective depth-to-span ratios of simple trusses should be maintained unless these trusses are evaluated as arches and fixity or resistance to horizontal thrust is given correspondingly at the support. A raised-chord truss should be studied for the force on the walls caused by deflection, especially if the spans are longer than 50 feet, and the walls or columns should be constructed accordingly.
Special bearing details or wall framework should be provided if necessary.
If a truss is supported by masonry, deflection thrust can be reduced by using slotted anchorage connections. Except in broad spans where more positive free movement is required, roller supports at one bearing are uncommon. The provision for deflection thrust relief is especially critical during erection; later, the truss will have stabilised substantially. Furthermore, if the maximum vertical live load and wind are not expected to occur at the same time, the typical provision for wind loads on the supports is frequently thought to be adequate for vertical load deformation thrust.
Allowing for lateral movement between column and wall when erecting trusses supported on free-standing columns with masonry side walls is a good idea. The connections between the column and the wall can be strengthened after the initial lateral movement. Deflection calculations can be used to estimate required clearance, but it is typically set haphazardly based on experience.
There are numerous more varieties of trusses, as well as combinations of conventional forms, that provide unique benefits for specific situations. The same dimensions, spacing, and other design features suggestions apply in general. Bowstring-flat and pitched-flat trusses with a two-span width are common combinations.
Outside walls need to be drained. With flat trusses, Pratt and Howe web systems are typically combined. The common saw-tooth truss, cantilevered trusses, and inverted trusses are examples of special types.
In timber, indeterminate structures like as rigid frames or continuous trusses are rarely employed. Erection issues are common with these trusses, which raise prices more than the material savings.
MAXIMUM ROOF TRUSS SPAN
The maximum economical span of any type of timber truss varies depending on the material available, loading conditions, spacing, truss type, labor-to-material cost ratio, and fabrication methods.
Roof trusses for pitched and flat roofs
For spans beyond 80 feet, pitched and flat roof trusses with average loading and spacing of 15 to 20 feet are rarely employed. The available widths and lengths of solid sawed or glued-laminated timber, as well as the theoretical capacity of the web-member connections, are usually the limiting factors for cost-effective spans. Larger spans can be built with the same relative member sizes and joint details if the loading and spacing are smaller.
Bowstring roof trusses
Bowstring trusses are cost-effective for spans of up to 250 feet. Bowstring trusses with glued-laminated members are typically shop-fabricated and should not be attempted in the field unless competent supervision is provided and practically the same quality control is applied as in the shop. Even while original designs may call for a flat or pitched type truss, it may be profitable to explore as an option because many fabricators have standardised on the bowstring type.
On the basis of experience, certain effective depth to span ratios are indicated as satisfactory. The use of deeper trusses is more desired as the span grows larger. Although trusses with less depth may be acceptable, particular consideration should be given to the risk of increased deflection and secondary stresses. The following practises can help keep deflection in trusses of less-than-average depth to a minimum: (1) conservative design, (2) use of low or intermediate grades of material, (3) use of a minimum number of chord splices (by using the longest available lengths), (4) use of fastenings with the smallest deformation, and (5) use of as few panels as possible. Members that are more stout are also obtained, as well as
The top chord of a bowstring truss should be built with a radius that is roughly equal to the span. The effective depth-to-span ratio is considered to be between 1:6 and 1:8. With a radius equal to the span, the ratio will be somewhat higher than the recommended minimum. An effective depth-to-span ratio of 1:5 to 1:6 is recommended for pitched trusses, with a minimum of 1:7 unless special emphasis is given to deflection. For the sake of beauty, far deeper trusses can be employed, such as in churches with steeply pitched roofs.
A minimum depth-to-span ratio of 1:8 to 1:10 is suggested for flat trusses, with deeper trusses preferred for longer spans. For proper drainage, roofs should have a minimum slope of 14 in. per ft, though steeper slopes are often preferred. Flat roofs with no drainage slope are not recommended unless the design allows for possible water accumulation due to a stopped drain or natural deflection. Flat roof drains should be placed at the lowest places. If the truss is built flat, these are at the middle of the span.
Secondary deflection stresses are likely to be more important at longer spans. Because these stresses are difficult to calculate precisely, the bigger depth-to-span ratios should be employed for trusses with such spans. Even though the inherent deflection of free-span trusses is normally well within acceptable limits, care should be taken to ensure that the natural deflection does not interfere with auxiliary framing. Suspended ceilings are very popular. There should be plenty of space between trusses and non-healing barriers or plate glass windows. If there is a chance that deflection will interfere with the correct operation of truss-suspended doors or machinery, provision should be made for hinge level adjustment.