Lecture Notes 7 Design of Truss I

Lecture Notes #7 Design of Truss Structures Struct ures

Professorr Guowei Professo Guow ei Ma Office: 160 Tel: 61-8-6488-3102 Email Ema il:: ma@ ma@civi civi l.uw a. a.edu.a edu.au u

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Trusses • Fabri Fabricated cated from variou various s steel steel sections sections availa available, ble, jointed jointed together by welding or by bolting usually via gusset plates. • Pla Plane ne trus trusses ses and spa space ce trus trusses ses.. • Bri Bridge dge tru trusse sses s and and roof roof trus trusses ses.. • Mem Member bers s supp support orting ing hea heavy vy loads loads • Mem Member bers s hav having ing lon longer ger spa span. n. • Sa Savi ving ng in we weig ight ht..

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Type of Trusses Roof truss

Supporting truss

Bracing truss

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Truss System Space truss

Singapore Esplanade theatre

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Truss Analysis • Pin-joint truss analysis 

method of joint, method of section, numerical

simulation 

several analyses may be needed for different load

combinations • Analysis of load bearing members such as rafters • Assessment of stresses due to eccentricity of the connections • Assessment of the effects of joint rigidity and deflections 5

Roof Truss 

Roof rafters spanning more than 20 m can be designed

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Usual span-to-depth ratio of steep roof trusses is 7.5 to 12

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Panel width should be constant

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Even number of panels avoids cross-braces

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Diagonal web members should be in tension under worstcase loading

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Inclination angle of the diagonals should be between 35° and 50°

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If at all possible, the purlins and verticals should closely coincide 6

Roof Truss

Usual range of depths of r oof t russes 7

Approximate mass for roof trusses

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Approximate mass for roof trusses

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Out-of-Plane Load • Wind loads acting on the upper half of the end walls • Frictional drag effects on the roof, and • Accumulated “ lateral” bracing system restraint forces

Forces in the longitudinal bracing system in the plane of the compression chords

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Design of Tension Members Cl. 7.1, Design for axial tension *

 N  ≤ φ  N t  ф = the capacity factor, see Table 3.4, ф=0.9

Nt  = the nominal section capacity in tension

 N t 

=  Ag f  y

and

 N t 

= 0.85k t  An f u

 Ag = the gross area of the cross-section  f y  = the yield stress used in design k t  = the correction factor for distribution of forces  An = the net area of the cross-section  f u = the tensile strength used in design 11

Design of Compression Members Cl. 6.1, Design for axial compression *

 N  ≤ φ   N s

and

*

 N 

 N c ≤ φ 

ф = the capacity factor, =0.9 Ns = the nominal section capacity determined in accordance with Clause 6.2 Nc = the nominal member capacity determined in accordance with Clause 6.3.

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Truss Node Connections 

Direct connections • Members are welded directly to one another, without the need for gussets or other elements (e.g. tubular joints). • When the chords are made from large angles or tee-sections, it is possible to connect angle web members directly to the chords.

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Gusseted connections • Predominant when rivets and bolts are used for connections. • Transfer of forces is indirect and not aesthetically pleasing. •  Advantage: easier to make all members intersect at the theoretical node point—in contrast to direct connections, where some eccentricity is unavoidable.

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Pin connections • Generally used when aesthetics are important

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Open Sections Gusset-Free Connections

(a) centre of gravity li nes intersect at the node; (b) eccentric co nnection can be a practical way of detailing but addition al bending st resses are indu ced 14

Typical Sections

(a) to (f ): comm onl y used in w elded cons tru cti on (thou gh (a), (c), (d) and (e) may be bolted) (g) to (k): comm on section s us ed fo r c hord and w eb/diagonal members 15

Node Connections for Rolled Sections

(a) Gussetless co nst ruc tio n using Tee-chords; (b) guss ets are required where diagonals carry l arge forces; (c) Tee-diagonals and cho rds , gussetless; (d) and (e) node detail for heavy t rus sw ork , and (f ) riveted/bolted nodes 16

Connections of Rolled-Steel Sections

(a) portal-type Pratt trus s (b) Fink truss with large eaves overhang (c) alternative chord cros s-sectio ns 17

Closed Sections Splices for Tubular Truss Members

(a) sandw ich pl ate spli ce; (b) sandw ich pl ate splic e at c hord reduction; (c)  jacket splice; (d) welded butt splice; (e) w elded butt splice with reducer, and; (f ) flange spl ice. 18

Connections for Tubular Sections

(a) Direct contact overlap connection witho ut eccentricity; (b) direct contact overlap connectio n with eccentri city; (c) direct contact gap connection with/without eccentricity (with chord face reinforcing plate shown—without reinforci ng plate is very commo n); (d) T-joint w ith cho rd face reinforcing plate (for very heavy loads—otherwise no reinforci ng plate is also popul ar); (e) connection d etail at support (note vertical stub port ion w ith flange splice for liftin g onto support ); (f ) concentric reducer where chord s ection is stepped down (alternatively, if the overall section is not stepped down then the wall th ickness is reduced—the latter applies for RHS/SHS); (g) slotted-gusset connection s; (h) flattened end connections, and; (i) slit tube connection s. 19

Example

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