Roof Truss Guide - Design and construction of standard timber and steel trusses (BASIN - SKAT, 1999, 187 p.): 6 STEEL TRUSSES: 6.2 System Options

Roof Truss Guide - Design and construction of standard timber and steel trusses (BASIN - SKAT, 1999, 187 p.)

6 STEEL TRUSSES

		(introduction...)
		6.1 Design Considerations
		6.2 System Options
		6.3 Materials
		6.4 Fixing and Jointing
		6.5 Finishes
		6.6 Angle Bar Truss
		6.7 Angle Bar Truss Samples
		6.8 Steel Tube Truss
		6.9 Steel Tube Truss Samples
		6.10 Reinforcing Bar Truss
		6.11 Rebar Truss Samples

(introduction...)

With the increasing scarcity of quality timber in some areas, steel structures are enjoying quite some popularity despite the relatively high cost of the material and the requirements on tools and equipment for fabrication. Unlike timber, steel is a homogenous and isotropic (same characteristics in all directions) material that allows an accurate structural analysis resulting in light and economical roof trusses.

6.1 Design Considerations

A number of design considerations need to be made before selecting steel as the basic material for the roof truss. Issues to be checked are the following:

· Availability of skills and equipment for fabrication:

Cutting steel bars and sections, welding, drilling and grinding for truss fabrication require power tools and the associated skills (qualified welders), which may not be readily available.

· Availability and cost of steel:

Steel is mass-produced in centralised factories. Due to transportation, costs increase with distance from such factories. The remoter the location, the less competitive is steel as roofing material. Local timber may then be more economic.

· Compatibility with other elements of the building:

Steel is a versatile material that can be combined with most other materials. When items such as rails for chain blocks, supported ceilings, ducts and piping need to be attached to the truss, the steel truss has advantages over timber. In any case, the structural analysis is to be modified accordingly. The standard trusses as presented in this manual may no longer be applicable.

· Fire rating required:

Steel structures rapidly lose their strength with high temperature. Avoid using thin sections (less than 6 mm thickness) if a high fire rating is required. A protective covering (insulating material, mortar, sprays or coating) may be employed to improve the fire rating.

· Corrosive environment:

With thorough wire brushing or sand blasting, several layers of primer and appropriate coating, steel structures can have remarkably long service lives even in corrosive (coastal) environments. The design of the building and the truss must allow for good accessibility of all members of the truss for repainting at regular intervals. When selecting the minimum thickness of the steel sections, allow for a loss of material from scaling (removal of oxide) and from corrosion. This allowance is usually in the order of 0.5 to 1 mm.

6.2 System Options

The options available for steel roof trusses are as follows:

· Angle and flat bar truss:

Small angle bars may be welded directly onto each other forming very light trusses up to about 12 m span. Due to the non-symmetrical shape of single angle bar truss members, the forces produce moments at the connections. Such eccentricities need to be considered in the structural analysis. The length of the compression members must be reduced as much as possible to avoid buckling. This results in trusses with a large number of diagonals (and connections) as for example with the Fink or Polonceau Truss.

For larger spans (> 12 m) and loads, double angle bar trusses with bolted connections on gusset plates are used. In some countries, angle bars are fairly expensive or not available in the required sizes and steel tubes may then be the preferred element.

Figure

· Steel tube truss:

Steel tubes are readily available almost throughout the world at reasonable prices since these are not only used for structural steel work but also for water supply piping systems. However, the jointing of round surfaces is difficult; butt and fillet welding of properly cut and shaped tubes is possible but slow and expensive. The use of gusset plates is almost inevitable if structurally sound and efficient connections are expected. When using tubes, the number of connections per truss should be reduced as much as possible.

Figure

· Rolled sections truss

Rolled sections other than angle bars used in truss designs are the channel and universal beams. Half-section universal beams are particularly useful in truss design but their availability is limited. Rolled sections are used in trusses of large spans (> 12m) and loads.

Figure

· Rectangular hollow section truss

Rectangular hollow sections (RHS) provide a particularly neat appearance of steel trusses. Welded connections are common thanks to the regular shape of the RHS. But the RHS are expensive and not readily available in some countries. RHS trusses are not treated in detail here.

Figure

· Reinforcing bar (rebar) truss

Shallow trusses with parallel chords made of reinforcing bars can be used as joists in roof structures. The static system of the roof structure as a whole is then no longer a truss but can be a purlin roof (see Section 2.3), a double-hinged arch or bent.

The application of the rebar truss is limited to residential and commercial buildings with light loading (no snow loads). The rebar joist - if properly designed and fabricated - can offer an economic roof structure of very airy and attractive appearance.

Figure

6.3 Materials

With the exception of reinforcing bars, most commercial structural sections are mild hot-rolled steels with 0.15 to 0.25 % carbon content. Minimum yield strength is 200 MPa or slightly above. This manual uses a design yield strength of 200 MPa for all structural steel members.

When selecting a welded tube truss, do not use galvanised pipes. Black pipes should be bought or, if only galvanised pipes are available, the galvanisation must be removed before welding.

When welding over the galvanisation, harmful smoke is emitted seriously affecting the health of the welder, and, in addition, the weld itself is not sound.

Reinforcing bars of the rebar joist may have much higher strength (400 MPa) due to heat treatment, drawing or from the addition of manganese, vanadium and nitrogen. Such deformed bars should, however, not be used for rebar joists as some of these steels have poor weldability and are expensive. Plain carbon reinforcing bars (deformed or not) with yield strength of 200 to 250 MPa are preferred.

(introduction...)

Welding and bolting are the most common jointing methods used in structural steel work. Riveting is no longer applied.

6.4.1 Welding

Welding may be used for shop and field connections. The latter should not be used on roofs high above ground or in confined spaces where welders would not be able to produce good quality welds. Welding on site requires additional cleaning of weld surfaces and repainting for corrosion protection. This is a task often underestimated; mill and weld slag remaining on the surface is the principle source of corrosion. Also, corrosion attacks near welds soon after commissioning are often the result of scorched paint that has not been properly removed before repainting.

For light steel trusses, fillet welding is commonly used. These are welds placed along the edges of the elements to be joined.

Figure

It is important that fillet welds are neither undersized nor oversized.

Min. throat thickness: a_min = 3 mm

Max throat thickness: a_max = 0.7 d

where "d" is the smaller of the elements to be joined

min. length of fillet I_min = 8 a

max length of fillet: I_max = 100 a

Butt welds are welds that join two elements over their entire depth. Such welds require a preparation of the surfaces (bevelling) and are thus very labour intensive. Butt welds are not recommended for steel truss construction except for joining small sections with material thickness of less than 3 mm.

A short introduction in good welding practices is presented in Annex 3.

6.4.2 Bolted with gusset plates

For heavier trusses, site assembly of individual elements is inevitable. Bolted connections are then the preferred method of joining the truss members.

There are two types of bolts available for structural steel work:

- Grade 4.6 ("black or mild steel bolt")
- Grade 8.8 (high strength bolt)

Grade 8.8 is used for rigid, slip-free joints where forces are transferred from one element to the other through friction at the contact surfaces. The bolts need to be fully tensioned using a torque spanner. This type of connection is not recommended for small trusses because of the difficulty in obtaining the bolts and the high requirements on site supervision.

Snug tightened, hot-dip galvanised Grade 4.6 bolts are recommended for steel trusses. As the holes are generally drilled 2 mm larger than the bolt shaft, a slip of 1 mm in each joint will occur resulting in considerable deflection of the truss. Camber of about 1/150 of the span is applied to make sure that the lower chord has a neat appearance (no sag).

6.5 Finishes

Surface preparation

Removing rust and grease from steel surfaces and roughening these are the main tasks of surface preparation. Sand blasting would be the best method but is often not available. Wire brushing (by power tool or manually), scrapers and hammers are the alternatives. Scale, rust and foreign objects are to be completely removed. After preparation, the surface should have a light metallic shine. The first coat of primer must follow immediately (within the same day).

Priming and Painting

A paint system for steel protection consists of a primer or rust inhibiting coat and a protective or weather coat, often called finishing coat. Primer and finishing coats cannot randomly be selected but must be compatible with each other. This is basically a question of the binder used in the primer (for details refer to supplier's specifications).

For relatively humid indoor conditions (60 to 80 % relative humidity) as encountered in roof truss applications the following two priming systems are in use:

- 2 coats of 30 mikron (mm) of red oxide primer, or
- 2 coats of 30 mikron (mm) of zinc based primer

Tar epoxy paints, which may be primer and finishing coat all in one, are seldom used for roof trusses due to their rough appearance.

The primer is preferably applied after all fabrication work is completed but prior to delivery to the site. The finishing coat is applied at site after erection where damaged primer is first repaired and the coverage of unprimed surfaces such as bolts and site welds are completed.

Steel work that is to be grouted in concrete such as anchors and the like are not painted.

6.6.1 General

Pros and cons

Single angel bars have limited capacity against buckling. In order to shorten buckling length a large number of web members and associated connections are required. This results in increased labour input for truss fabrication. Where labour costs are relatively low, economically competitive roof trusses can be fabricated.

The use of double angle members reduces eccentric connections but brings about inevitable corrosion problems as the space between the angle bars cannot be cleaned and repainted.

6.6.2 Design

An overhang of 10 % of the span but at least 0.8 m can be accommodated with the proposed designs.

A sample structural analysis is presented in Annex 2.

6.6.3 Details

No gusset plates are used for the single angle bar truss. This makes its application particularly economic.

Do not weld too close to the root of the angle bars as this warps the section. Internal stress from fabrication is highest in the root zone and welding (heat introduction) provokes a release of this energy.

Seal the contact surface: Weld all around to avoid water entering the contact surface between the angel bars. This results in much longer welds than statically required but corrosion protection takes priority.

Figure

In order to keep stocks of different bar sizes to a minimum and to avoid confusion in the workshop with too many different designations, only a limited number of sizes are used for the sample trusses presented in this manual. The standard ones are as follows.

EA 100 × 100 × 6
EA 75 × 75 × 6
EA 65 × 65 × 6
EA 50 × 50 × 5
EA 30 × 30 × 3
EA 25 × 25 × 3

Dimensions and design information of these bars are presented in Annex 4.

The supports rest on two bearing plates of which the top one has a slotted hole to allow horizontal movements from thermal expansion. A lot of grease must be applied to the sliding surfaces of the bearing plates to reduce friction.

Figure

6.6.4 Bracing

Bracing against overturning of the individual trusses due to wind loads on gables and buckling of the compression chords is accomplished by welding X-braces onto the top chord of the truss in every fifth field. These X-braces form lateral restraining (bracing) trusses together with the purlins.

For MCR roofing material, the purlins have a spacing of 400 mm and must be comprised of equal angle bars of at least EA 50 × 50 × 5 sizes. The X-brace uses the same angle size and a flat 50 × 5 as a cross brace. These cross braces are welded to the top chord members and also to the purlins at each intersection. As the purlins are welded with the leg facing upwards, a support made of a flat (from cuttings) is to be added to secure the connection at each truss.

Figure

6.7.1 Assumptions and Limits of Application

· Yield strength of sections 200 MPa (N/mm²)
· Loads are permissible loads (not ultimate capacities)
· Loads are applied through the top chord

6.7.2 Double Pitch Roof; Single Angle Bars, 6 m span

Roof Slope min 19°, max. 30°

Bracing system according to Section 6.6.4 above

Spacing of truss 2.5 m

Total load 1.0 kN/m²

Figure

6.7.3 Double Pitch Roof; Single Angle Bars, 8 m span

Roof Slope min 19°, max. 30°

Bracing system according to Section 6.6.4 above

Spacing of truss 2.5 m

Total load 1.0 kN/m²

Figure

6.7.4 Double Pitch Roof; Single Angle Bars, 10 m span

Roof Slope min 19°, max. 30°

Bracing system according to Section 6.6.4 above

Spacing of truss 2.5 m

Total load 1.0 kN/m²

Figure

6.7.5 Double Pitch Roof; Single Angle Bars, 12 m span

Roof Slope min 19°, max. 30°

Bracing system according to Section 6.6.4 above

Spacing of truss 2.5 m

Total load 1.0 kN/m²

Figure

6.7.6 Single Pitch Roof; Single Angle Bars, 6 m span

Roof Slope min 19°, max. 30°

Bracing system according to Section 6.6.4 above

Spacing of truss 2.5 m

Total load 1.0 kN/m²

Figure

6.7.7 Single Pitch Roof; Single Angle Bars, 8 m span

Roof Slope min 19°, max. 30°

Bracing system according to Section 6.6.4 above

Spacing of truss 2.5 m

Total load 1.0 kN/m²

Figure

6.7.8 Single Pitch Roof; Single Angle Bars, 10 m span

Roof Slope min 19°, max. 30°

Bracing system according to Section 6.6.4 above

Spacing of truss 2.5 m

Total load 1.0 kN/m²

Figure

6.7.9 Flat Roof; Single Angle Bars, 6 m span

Roof Slope min 19°, max. 30°

Bracing system according to Section 6.6.4 above

Spacing of truss 2.5 m

Total load 1.0 kN/m²

Figure

6.7.10 Flat Roof; Single Angle Bars, 8 m span

Roof Slope min 19°, max. 30°

Bracing system according to Section 6.6.4 above

Spacing of truss 2.5 m

Total load 1.0 kN/m²

Figure

6.7.11 Flat Roof; Single Angle Bars, 10 m spar

Roof Slope min 19°, max. 30°

Bracing system according to Section 6.6.4 above

Spacing of truss 2.5 m

Total load 1.0 kN/m²

Figure

6.7.12 Single Pitch Roof; Single Angle Bars, 12 m span

Roof Slope min 19°, max. 30°

Bracing system according to Section 6.6.4 above

Spacing of truss 2.5 m

Total load 1.0 kN/m²

Figure

6.7.13 Double Pitch Roof; Double Angle Bars, 14 m span

Roof Slope min 19°, max. 30°

Bracing system according to Section 6.6.4 above

Spacing of truss 2.5 m

Total load 1.0 kN/m²

Figure

6.7.14 Single Pitch Roof; Double Angle Bars, Span 12 m

Roof Slope min 19°, max. 30°

Bracing system according to Section 6.6.4 above

Spacing of truss 2.5 m

Total load 1.0 kN/m²

Figure

6.7.15 Flat Roof; Double Angle Bars, 14 m span

Roof Slope min 19°, max. 30°

Bracing system according to Section 6.6.4 above

Spacing of truss 2.5 m

Total load 1.0 kN/m²

Figure

6.8.1 General

Tubular sections offer up to 40 % lower weight as compared with mild-steel angle bars of the same strength. Similar ratios apply for the surface areas of tubular sections compared with angle bars. Therefore the cost of total steel mass, surface preparation, priming, coating and repainting is reduced when using tubular sections instead of angle bars as discussed in the previous section.

But joints of tubular sections are more difficult than those of angle bars and the advantage of reduced weight and surface area is partly lost on increased labour costs for the connections.

6.8.2 Design

All members are single tube sections with varying diameters. In order to avoid confusion and errors in the fabrication process and difficult stock keeping, only a limited number of different tube types should be used.

The tubes used in this manual are black mild steel pipes according to British and related standards (Australian, Indian) and the ISO recommendation R 65 and 336).

There are light, medium and heavy walled pipes on the market but the designation is sometimes misleading. It is important to always verify (by measuring with a vernier gauge at the dealer's warehouse) the actual wall thickness of the pipes.

Spans from 6 to 20 m are possible but spans above 15 m usually require either bolted connections (and scaffolding) or a crane for installation due to the weight and size of the truss.

Designation:

Figure

6.8.3 Details

Joints

Where small tubes are to be joined with a larger tubular member, direct connection without gusset plate is possible as pipe end preparation is not too difficult. In order to reduce the end preparation for tube to tube connections, the tube ends of struts and ties between chord members may be flattened using a press or a simple vice.

Figure

Where two tubes of similar size need to be connected it is better to use gusset plates. These are welded onto the circumference of continuous members or into slots cut into the end section of diagonals / web members.

Figure

Support details

The same approach as with the single angle bar trusses is proposed. A support plate is resting on two bearing plates. The upper one is meant to slide on the lower one, the anchor plate, when movements occur due to thermal expansion. A stiffener plate welded laterally onto the support plate is designed to avoid buckling especially in those fields with the bracing system.

Figure

Fixation of battens

Fixing battens on to a tubular section is difficult since a tube provides only a very short point of support. It is recommended that short piece of flat bar (the same size as used for the X-brace) be added to make the connection more rigid.

Figure

6.8.4 Bracing

The bracing or lateral restraining system that avoids overturning of the individual trusses is the same as proposed for the angle bar truss.

6.9.1 Assumptions and Limits of Application

Ditto angle bar truss

6.9.2 Double Pitch Roof, Tube Truss, 6 m span

Roof Slope min 19°, max. 30°

Bracing system according to Section 6.6.4 above

Spacing of truss 2.5 m

Total load 1.0 kN/m²

Figure

6.9.3 Double Pitch Roof, Tube Truss, 8 m span

Roof Slope min 19°, max. 30°

Bracing system according to Section 6.6.4 above

Spacing of truss 2.5 m

Total load 1.0 kN/m²

Figure

6.9.4 Double Pitch Roof, Tube Truss, 10 m span

Roof Slope min 19°, max. 30°

Bracing system according to Section 6.6.4 above

Spacing of truss 2.5 m

Total load 1.0 kN/m²

Figure

6.9.5 Double Pitch Roof, Tube Truss, 12 m span

Roof Slope min 19°, max. 30°

Bracing system according to Section 6.6.4 above

Spacing of truss 2.5 m

Total load 1.0 kN/m²

Figure

6.9.6 Single Pitch Roof, Tube Truss, 6 m span

Roof Slope min 19°, max. 30°

Bracing system according to Section 6.6.4 above

Spacing of truss 2.5 m

Total load 1.0 kN/m²

Figure

6.9.7 Single Pitch Roof, Tube Truss, 8 m span

Roof Slope min 19°, max. 30°

Bracing system according to Section 6.6.4 above

Spacing of truss 2.5 m

Total load 1.0 kN/m²

Figure

6.9.8 Single Pitch Roof, Tube Truss, 10 m span

Roof Slope min 19°, max. 30°

Bracing system according to Section 6.6.4 above

Spacing of truss 2.5 m

Total load 1.0 kN/m²

Figure

6.9.9 Flat Roof, Tube Truss, 6 m span

Roof Slope min 19°, max. 30°

Bracing system according to Section 6.6.4 above

Spacing of truss 2.5 m

Total load 1.0 kN/m²

Figure

6.9.10 Flat Roof, Tube Truss, 8 m span

Roof Slope min 19°, max. 30°

Bracing system according to Section 6.6.4 above

Spacing of truss 2.5 m

Total load 1.0 kN/m²

Figure

6.9.11 Flat Roof, Tube Truss, 10 m span

Roof Slope min 19°, max. 30°

Bracing system according to Section 6.6.4 above

Spacing of truss 2.5 m

Total load 1.0 kN/m²

Figure

6.9.12 Flat Roof, Tube Truss, 12 m span

Roof Slope min 19°, max. 30°

Bracing system according to Section 6.6.4 above

Spacing of truss 2.5 m

Total load 1.0 kN/m²

Figure

6.10.1 General

Pros and cons

The reinforcing bar truss is not the most economical option for roof truss design as solid bars do not make efficient compression members. Rebars have poor strength against buckling due to the concentration of the mass in the centre (tubes are much better in that regard). Nevertheless, reinforcing bar trusses may be selected for basically three reasons:

- Their airy and light, open appearance is highly attractive from the architectural point of view;

- The reinforcing bars can easily be cut and welded on site. The rebar truss is thus preferred by local workshops with limited (planning) capacities for pre-fabrication in the workshop and with few skilled personnel. Welding solid bars requires less skill than welding thin-walled tubes or angle bars.

- Thanks to its low weight, the rebar truss can easily be handled at the construction site and put in place by manual labour without the use of lifting devices.

6.10.2 Design

Rebar trusses come in the form of joists with parallel chords. These are used in roofs as simply supported beams between walls (single pitch roof) or between wall and ridge purlins (double pitch roof). The ridge purlins, which span the distance between gable or intermediate walls, cannot be designed as rebar joists. The load is too large for the slender compression members. Tube trusses with parallel chords are proposed instead.

Only diameters 8, 10 and 12 mm are normally used for the simple rebar joists. This is because larger diameter bars cannot be easily cut on site using cutters. Only one bar size should be used for a particular truss. The use of two or three different bar sizes would result in confusion and errors on the construction site. Assumed yield stress of the rebars is 200 MPa.

The depth of the joists as proposed in this manual is limited to 300 mm. The spacing of the web members is 400 mm for all joists proposed. This is to make sure that the battens, which have a spacing of 400 mm for MCR tiles, coincide with the panel points of the joist and do not provoke bending of chord members due to loading off the panel points.

An overhang of maximum one metre can be accomplished with the proposed designs. The standard joists consists of two longitudinal bars for the top chord and two longitudinal bars for the lower chord. Web members are welded in between the two chord members. For the sections with increased stresses such as at mid-span and at the supports, additional bars are welded in parallel to the standard bars where required.

6.10.3 Details

Joints

The rebars are jointed by fillet welding without the use of gusset plates.

Support detail

A vertical support plate is welded in between the lower chord bars. The support plate rests on two bearing plates, which are anchored into the masonry wall with a concrete pad to distribute the load. The top one of the two bearing plates has a slotted hole to allow horizontal movements due to thermal expansion of the steel bars. A lot of grease must be applied to the sliding surfaces of the bearing plates to reduce friction.

Battens

A standard truss spacing is 1.50 m. If rebars are also used for the battens then their diameter must be at least 16 mm. The use of light weight angle bars (EA 40 × 40 × 6 or EA 45 × 45 × 5) may be considered.

6.10.4 Bracing

X-bracing in end fields of the roof (or at least in every fifth field) is proposed to avoid overturning of individual trusses and to provide a lateral restraining system against buckling of compression members. The X-braces consist of rebars welded diagonally onto the top chord of the truss. They form bracing trusses together with the battens.

In addition to the X-bracing, lateral support structures are needed at the support and at the free overhanging end of the joist. These support structures are similarly built bar joists but with single chord members.

6.11.1 Assumptions and Limits of Application

· Yield strength of bars is 200 MPa (N/mm²)
· Loads are permissible loads (not ultimate capacities)
· Loads are applied through the top chord

Designation:

The bar joists are designated as follows:

- The depth of the joist in mm is preceding the abbreviation "BJ" for bar joist
- The diameter of the rebars used is added to BJ.
- A "300BJ08" is a joist made of 8 mm rebars and with a depth of 300 mm