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Building Construction II.

Roof Trusses

TUB, Dept. Of Building Constructions: Dr. Mária SzéllEnglish text: Miklós Svéd

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Roof Trusses

Contents:

1. Revision of the basics2. Timber roof constructions2.1. Structural variations, revision2.2. Roof truss design3. Transitional roof trusses4. Engineered trusses4.1. Rafter-type constructions4.2. Purlin-type constructions5. Roof truss substitutions

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1. Revision of the basics

Pitched Roofs were discussed in Fundamental Building Constructions class. The outline of the lecture was:

• The purpose of roof trusses, impacts and requirements• Roof structure, roof shape• Classification• Traditional trusses, solutions depending on building width:

– Rafter-type and– Purlin-type roof constructions

• Sructural components• Relation to the uppermost floor slab• Eave detail solutions

Students are expected to be familiar with these topics as theyare necessary in order to understand this lecture.

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2. Traditional roof trusses

Old attics: ↑↑↑↑Exposed structureTie-beams and floor slab filling →→→→

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2.1. Structural variations, revision

Rafter-type roof trusses, Variations as the span increases:c.1: empty; c.2: collar beam; c.3 – c.4: supported collar beam

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Structural variations, revision

Purlin-type roof truss, Variations as the span increases: d.1: one-; d.2: two-; d.3: three purlin supports

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Revision: roof truss with two purlin supports

Structural components:a) Wall plateb) Tie-beamc) Eaves purlind) Columne) Waist purlinf) Struttg) Angle braceh) Collar-tiesi) Rafterj) Ridge purlink) Tip platel) Battensm) Cladding

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Revision: roof truss with three purlin supports

a) b)

a) The column that supports the ridge purlin extends until the tie-beam(for example at roof tip)

b) The loads of the middle column are transferred close to the loadbearing walls by the strutts

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Eaves position correlated to the floor slab,raised eaves

1) eaves purlin, 2) rafter, 3) waist purlin, 4) collar ties, 5) waist purlin at 90° roof direction change, 6) column, 7) strutt 8) perimeter wall,9) thermal insulation + cladding (in case of living attic)

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2.2. Roof truss design

Two-step design process:- designing the primary truss on the cross section,- 3D truss design

Truss type is chosen according to:- building width;- possibilities of support positioning.

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Primary trusses asseen on the

buildicross section

The rafters in between theprimary trusses are laid out according to:

- building width,- support characteristics .

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The rules of rafter layout

• the distance between rafter axes is 0,8 – 1,0 m;

• rafters should be evenly spaced if possible

• under approximately every forth rafter pair there is a primary truss

(primary truss spacing: 3,5 – 4,0 m);

• rafters should be layed out starting from the critical positions

(for example: staircases, gable walls);

• a primary truss may be placed right next to or 1,0 m away from a gable

or fire wall, or the column of the primary truss may be substituted with a

pillar

• roof penetrations such as chimneys may not interfere with primary

trusses or purlins, but rafters may be cut and supported by secondary

beams running parallel to the purlins in between the two closest intact

rafters. In the latter case these rafters may be doubled / strengthened.

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3D layout

Rafter layout guidelines:

First design around the criticaldetails – for example staircase, gable wall, chimney penetrationetc.

The limits of rafter-to-rafterdistance: maximum allowedbatten/boarding span

Rythm of rafter layout (uniform, altering).

Positioning the primary trusses →→→→

maximum purlin span which maybe reduced by applying anglebraces

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Roof truss design,3D layout

a)

b) c)

Primary truss at gable wall:a) primary truss next to gable wall,b) primary truss 1 m away from gable wall,c) purlin supported by pillar

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Roof truss design,3D layout

Chimneys and vent ducts may not interfere withprimary trusses or purlins, but rafters may be cut and supported by secondary beams running parallel tothe purlins in between the two closest intact rafters. These rafters may be doubled / strengthened.

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Roof truss design,3D layout

Longitudinal bracing:a) Rafter-type roofs are braced by battens nailed diagonally to the rafters,b) Purlin-type roofs are braced by:the diagonal braces, changes in the roof slope direction, gable wallsreinforced with pillars

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Roof layout,changes in the slope

direction of rafter-typeroofs

Section: diagonal battens

Upper half of floor plan:-primary truss with tie-beans-secondary truss with shorteaves-beams supported byhorizontal beams attached tothe tie-beams

Bottom half of floor plan:- beams of timber uppermostfloor slab acting as tie-beams

Design principles for the trussunder the perpendicularlysloping roof section:- similarity to the regular roofsection,- place support under roof tip,- rafters at the intersection of the roof surfaces bear a lotmore load than regular rafters

tie-beams

timber floor-slab

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Roof layout,changes in the

slope direction of purlin-type roofs

Section: diagonalbattens;

Floor plan:- loadbearing uppermost floor slab;rafters under slopingridge and collar tiesare supported byauxiliarry beam underroof tip-diagonal battensnailed to the top surface of collar tiesload-bearing

floor slab

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Purlin-type roof.: truss with two columns (revision)

Roof truss with tie beam and eaves-beams on left side of picture ,Overhanging rafters at eaves on right side of picture

eaves witheaves-beams

auxiliarybeam

ridge purlin

rafter

eaves purlin

eaves with canti-levering rafters

tie beamsupport board

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Roof layout,change in the

slope directionof purlin-typeroof with two

columns

Floor plan:- tie-beam at primary truss,overhanging rafters at eaves;-secondary truss under roof tip;-column under 90° waistpurlin connection;-diagonal bracing at 90° waistpurlin connection;- connection of tie-beams undersloping ridge at column.

purlin ringbeam

tie beams; eaveswith canti-levering rafters

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Roof layout,change in the slope

direction of purlin-typeroof with three columns

Section:- slanting collar-tie under slopingridge

Floor plan: - tie beamseaves beams;- primary truss under roof tip;

purlin ringbeam

rafterbeams + auxiliary beams +eaves beams

eaves with eavesbeams

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Complex roof structure over L-shaped floor plan

Design principles:-start with the sectionhaving larger span / steeper roof angle;

-the section havingsmaller span / mellowerangle is designed to fit the section cerated first.

waist purlin →→→→purlin ringbeam

ridge purlin

A

B

A B

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Roof inconstruction

Beams with a curvededge are fastened inbetween the rafters of the conic tower roof.

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Further examples of traditional roof trusses: roofs witha mellow pitch

a) truss with one column

b) truss with slanting columns, strutts and raised eaves purlins

slantingcolumn

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3. Transitional roof trusses

Example: truss with slanting columns, strutts and raised eaves purlins

Detail:The waist purlin is connected to the columnand the strutt by angle braces. Thisconnection adds 3D rigidity to the roofstructure.

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4. Engineered roof trusses

Engineered timber joints:

• bolted,

• with connection component,

• nailed,

• glued connections

Further types:

• nailing plate,

• nail plate,

• bolted with diaphragm,

• Metal pins with diaphragm.

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Engineered timber joints

a)

b)

c)

d)

e)

Lengthenings: a) nailed; b) connection component and bolt; c) diaphragm + metal pinsd) hardwood block + bolt; e) ring + bolt;f) connection components : rings, Kübler disc; rings with spikes / teeth (Bulldog, Geka,

etc.)

f)

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Engineered timber joints

b) bolted connection

a) connection plate and metal pins →→→→

Remark: traditional timber joints were discussed inTimber, Clay and Stone Structures class, this lecture onlydeals with the up-to-date types.

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Engineered timber joints

a) connection profiles;b) nail-plates;c) glued, saw-toothed lengthening and connection with bolts

a) b)

c)

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Roof structure withnail-plate connections

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Engineered roof trusses

Characteristics:

− prefabricated, high strength components + standard profiles

− special connection profiles, high strength details,

− combination of timber and steel components,

− 3D loadbearing structures,

− körükben is megjelennek a sűrűállásos és ritkaállásos változatok.

Structural variations:

− Rafter-type : timber, steel or RC components

− Purlin-type roofs: glued timber, steel and RC components .

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4.1. Engineered rafter-typeroof trusses

Timber trusssolid web, nailed connections ↓↓↓↓

Glued trusses with laminatedwood web ↑↑↑↑ Rafters with wavy web ↑↑↑↑

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Engineered rafter-type timber roof

trusseswith laced rafters

Timber laced beamvariations←←←← ↓↓↓↓

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Timber laced beam with nailed connections

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Halls withtimber laced beam

roofs underconstruction

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Timber and steel combined,3-hinged design with tensile rods

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Prefabricated rafter-type roof trusses

Reasons of popularity: − living attics are popular, − old flat roofs converted to pitched roofs, − adding extra floor space to already existing

buildings− expensive imported timber material, − decline in the carpenter craft.

Advantages: − light, − high degree of prefabrication, − easy assembly on site, − efficient material useage and easier

manufacturing due to engineering.

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Prefabricated rafter-type roof truss ,(GANG-NAIL)

Characteristics:- L = 8,70 – 10,50 m;- material: 50 mm thick. pine;- connections: rings inserted from two sides,- assembly: the two halves of the truss are connected by profiles and

bolts on site.

statical model

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Positioning theprefabricatedGANG_NAIL pyramid roof

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Prefabricated rafter-type roof structure

(ALBA)

Characteristics:- AZ1 and AZ2 are open

frames;- AH: closed frame, no

uppermost floor slab is constructed but falseceiling with thermalinsulation;

- material: pine;- connections: laminated

board blocks or metal diaphragms, nailed orbolted;

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Prefabricated rafter-type roof structure(ROTIP)

Characteristics: - RN: a = 8,90 – 10,70 m; RV: a = 6,00 – 13,00 m- material: 3 ∗∗∗∗ 22 mm (wattle – hardwood – wattle);- connections: glued, nailed,- assembly: the two halves of the truss are connected by profiles and

bolts on site.

statical model

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Roof design applying ROTIP trusses

flo

or

slab

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Roof design applyingROTIP trusses

component list

number nominal quantitywidth

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Transportationof prefab. rooftrusses using

special vehicle

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Prefabricated rafter-type roof structure

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Prefabricated roofstructure

sloping ridge

score

← Eaves withringbeam blocksand fixing profiles

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Examples

↑ a) Pitched roofconnected to flat roof

b) Lightweight timber skeletonframe and truss→→→→

a)

b)

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Rafter-type steel roof truss(Stran-steel)

Nailable!

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Rafter-type steel roof truss(SRK)

Battenconnection →→→→

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Rafter-type steel roof truss(FILIGRÁN)

Characteristics:

- material: cold-formed steelprofiles

- connections: rivets.

Types:

a) living attic;

b) steel lacedbeam.

a)

b)

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Reinforced concrete roof trussCharacteristics:- rafter-type,- ridge-piece is supportedby beam;- eaves-piece is anchoredto ringbeam.

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4.2. Engineeredpurlin-type roof

trusses

a)

b)

c)

d)

Glued-laminated timber:a) frame with 3 hinges ; b) arch with three hinges; c) primary truss with there

hinges + purlins + rafters;

d) for buildings withoutuppermost floor slabs

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Engineered purlin-type roofs

Glued-laminated timber frames

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Steel and reinforced concrete hall roofs

a1)

a2)

b1)

b2)

a1) steel frames + purlins; a2) steel laced beams + purlins;b1) RC laced beams + purlins; b2) RC solid web beam + purlins

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5. Roof substitutingsolutions

Rafters of tower roof aresupported by steel lacedbeams and RC walls

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Roof substitution

↑ window hole in slanting RC slab,

thermal insulation between rafters →→→→

RC „coffin” roof supported bytransversal RC walls

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Roof substitution(revision)

Example: Ytong – Xellaroof panels, a) horizontal planksb) slanting planks

a) b)

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Bibliography(in Hungarian and German)

1. L. Gábor: Building construction III. (pg. 1-102.)2. L. Széll: Building Constructions II.3. Frick, Knöll, Neumann, Weinbrenner:

Baukonstruktionslehre 2. (pg. 11-102.)