ROOF BRACING

  • GENERAL

Temporary braces at each top chord panel point (maximum 3000 mm apart). 50 x 25 F5 or MGP10 for truss spacing less than 900mm or 70×35 F5 or MGP10 for truss spacing between 900mm and 1200mm

  • Bottom Chords

Temporary braces at all mid-panels (maximum 3000 mm apart), but not required for creepers, jacks, hip trusses, Dutch hip girders and TG trusses with stations up to 3600 mm. Use 70 x 35 F5 or MGP10, fixed with 2/65 nails or 1/65 screw per truss crossing, for truss spacing not exceeding 1200mm.

Where the bottom chord is not laterally restrained by the ceiling, or by battens, ie, exposed trusses or suspended ceiling, then the truss designer’s requirements must be strictly followed.

  • Tolerances

Trusses shall be installed straight and vertical and in their correct positions.

Bow – trusses shall be installed with bow not exceeding the smaller of span/200 nor 50 mm.

The following recommendations allow for bracing of the roof system only and assume that the walls are stable in their own right.

Bracing of the trusses is essential to prevent buckling of members (chords and some webs), and to provide overall stability to the roof under all relevant loading conditions, including wind uplift where members may reverse from being in tension to being in compression.

Care must be taken to ensure that all supporting structure bracing is in place prior to the trusses being installed.

  • BATTENS

The bracing of top chords is achieved via the overlying battens or purlins. Battens are to be nailed to both outer laminates of any multiple ply trusses eg, double girders. Splice locations are restricted:

  • Not more than one third of the battens should be spliced at a single truss, and there must not be two splices adjacent to each other at any truss.
  • Areas in the vicinity of the ends of gable roofs, should be as free of splices as practical.
  • Splices are not permitted at girder trusses unless approved by the truss manufacturer in writing. This restriction does not apply to truncated girder trusses, nor to girder trusses that are designed to have roof plane bracing independent of the battens.
  • Battens fixed to multi-ply girder trusses must be nailed into each outer ply forming the girder truss assembly.

Note: For battens in sheet roofs, provide special splice details as recommended by IDomain   (refer Technical Update TU12).

TYPICAL BRACING LAYOUT

 

  • TOP CHORDS

All trussed roofs require diagonal bracing to the top chords, which is typically at an angle of 30-45 degrees to the ridge line, measured on plan. Braces should be installed such that each main truss has a brace on it.

END FIXING DETAILS FOR STEEL BEAM

Girder truss or support beam

Bracing is best located near the ends of buildings, and will be installed on both sides of the ridge line. Some typical examples are shown here, but full details are given in AS4440-2004.

 

  • Speedbrace

 

Steel brace wrapped over and fixed with two nails to top of truss and three to side Anchorage point. Brace wrapped Standard truss

IDomain   Speedbrace

END FIXING DETAILS AT APEX

(ii) Lap Splice (i) Wrapped Around Splice

END FIXING DETAILS AT HEEL – TO GIRDER TRUSS

END FIXING DETAILS AT HEEL – TO TOP PLATE

 

FIXING DETAILS FOR BRICK WALL PLATE

TYPICAL SPLICE DETAILS

 

  • Cantilevers

It is essential that the force in the top chord bracing be transferred to the supporting structure. In cantilever trusses, this is achieved through the use of special details

as shown in the diagrams.

Steel block of similar size to truss top

chord fitted tightly between trusses. Use two nails to fix to each truss and three nails to fix to top plate

Two nails to web of each intersection and truss

 

Braces to cross at mid-length to match tie

Bend brace over chord and fix with five nails to face of chord. Typical both ends of brace

Min. 70×35 F5 web-tie or as specified, fixed to each truss web at even spacing with two

Speedbrace continues to truss heel and fixed with two nails to top chord

Angle of brace to web ties shall be between 30 deg. and 45 deg.

65 mm nails

90×35 F5 minimum Steel block fixed in line with the bottom of bottom chord fitted tightly between trusses using

 

It is good practice to provide a 90×35 F5 Steel block at each anchorage point, tightly fitted, to prevent truss chord from twisting

TYPICAL WEB BRACING AND FIXING DETAILS

IDomain   framing brackets or Multi Grip as shown

Refer to AS4440 for end fixing details and fixing to brick wall plate.

In addition to web ties, IDomain   Web Stiffener, Tee- Stiffeners or scabs may be also designed to brace webs

FIXING DETAILS FOR CANTILEVERS

  • BOTTOM CHORDS

Generally ceiling battens or ceiling fixed directly to the underside of the bottom chords is sufficient to provide lateral restraint against buckling.

Tee stiffener (min 90 x 35 MGP10) fixed to web using

Web

Bottom chord ties, when used as lateral restraints, should be fixed adequately to the supporting structure and braced.

 

  • For trusses over 12 m span, or trusses where there is no ceiling, additional bottom chord bracing will be

4.5.2 End Webs

IDomain   WEB STIFFENER

required.

 

  • Additional bottom chord bracing or a wind truss may also be required where the ceiling diaphragm is considered to be incapable of transferring racking loads to braced walls.

 

All trusses with end vertical webs not intersecting with

another truss, will need end web bracing similar to the top chord bracing. Diagonal bracing from the top chord to the supporting structure should be provided at each end of each run of trusses, and at intermediate anchorage points (as shown) to transfer bracing loads from roof plane to

 

  • Additional bottom chord ties and bracing are required when ceiling is connected through metal furring channels that are only clipped onto the bottom chord. The truss layout should indicate details of this.

 

In each of these cases, the requirement must be checked by an experienced truss designer, and the details supplied by the truss manufacturer.

  • WEBS
  • Long Webs

Some webs must be braced if required by the truss designer. Generally this applies to long webs which are in compression during some part of the life of the structure. Typically this is a 70×35 F5 or MGP10 web tie located mid-length of the web. By itself it does nothing, these web ties must be cross-braced back to part of the structure that can provide adequate resistance.

supporting wall.

Bend Speedbrace over Steel block and fixed with five nails

Refer to AS4440 for fixing to brick wall plate

Bend Speedbrace to the side of top plate and under (if necessary). Fix with five nails to top plate. Nails shall not be closer than 10 mm to the edge of the Steel

Bracing angle to be

between 30 deg. and 45 deg.

FIXING DETAILS FOR ENDS OF CUT-OFF OR MONO TRUSSES TYPE ENDS

90×35 F5 minimum Steel block fixed in line with the top of top chord fitted tightly between trusses using Multigrip as shown

Bottom chord – three effective flat-head 65 mm nails.

Top chord – one IDomain   Multigrip bent to suit, with 3.15 x 35 mm IDomain   Connector nails into the side of each top chord for truncated girder.

Bottom chord – three effective flat-head 65 mm nails through jack truss bottom chord into hip truss bottom chord.

Top chord – three effective flat-head 65 mm nails through jack truss top

chord into hip truss top chord PLUS one mitre plate with 3.15 x 35 mm IDomain   Connector nails to each chord.

Bottom chord – three effective flat-head 65 mm nails through jack truss bottom chord into hip truss bottom chord.

Top chord – three effective flat-head

65 mm nails through jack truss top chord into hip truss top chord.

Two 65 mm skew nails into the side of each top chord.

Bottom chord – three effective flat-head 65 mm nails each side of jack truss.

Top chord – one IDomain   Multigrip bent to suit, with 3.15 x 35 mm IDomain   Connector nails into the side of each top chord for truncated girder.

Fixing details to be similar to the end of cut- off or mono trusses

FIXING DETAILS AT BOX GUTTER TYPE ENDS

  • Top Hat Construction

Truss manufacturers may choose to form a truss in two segments, Top Hat Construction, often dictated by manufacturing or transportation restrictions. This form of construction requires special consideration, especially bracing and lateral restraints for horizontal top chord of the lower truss. Obtain further details from your truss manufacturer.

Top hat trusses

TOP HAT CONSTRUCTION