7 Portal Frames Warwick Banks

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LVL Portal Frame Desig

Warwick Banks

Technical Manager


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Key Discussion Points

Timber portal frames

Benefits of timber based systems

Timber portal frame design

Purlin Design

Moment resisting connection design

Portal Suite Design Tools


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Timber based Portal Frames

Specification limited by: Perception and reliability Product awareness

Up-skilling requirements Engineers

Fabricators

Riggers/Erectors


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Benefits of timber systems

Aesthetics

Environmental green solution from arenewable resource and is carbon storing

Corrosion resistant (Indoor pools, fertiliser

& compost storage, etc) Compatibility

Usability


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Design Criteria and Loading Design criteria based on structure type not material type Loading in accordance with relevant loading standards

Footing and bracing design similar to steel systems

Optimal member spans and bay spacings are different fortimber and steel


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Purlin Design Two distinct options with timber based systems

Solid sections hySPAN or MSG for spans up to 6.0 m

Ease of connection with proprietary brackets

Composite I-beams Use products such as hyJOIST for spans 6.0m to 12.0m

Lateral restraint systems and connections require specific

design


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Purlin Design

Design requirements includeserviceability and strength checks for: Dead load

Live load

Wind loading Associated combinations

Provide resistance to lateral torsionalbuckling of the Portal Frame members


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Serviceability Timber components require consideration of shear

deflection

Solid sections include a 5% allowance for sheardeflection within Modulus of Elasticity

For single span built-up sections shear deflection can

be expressed as:

Duration of load factor for loads in excess of 12 months

w

shearGA

M

*

=


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Wind loading

Includes allowance for local pressure factors

Pierce fixed sheeting provides continuous lateralrestraint to top of purlin

Lateral restraint systems to the compression edge

provide increased resistance to lateral torsionalbuckling


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Uplift Strength Capacity for I-beams

Stability factor k8 calculated based on tension edgecontinuously restrained by roof sheeting

Use Equation C7 of NZS 3603 for Euler BucklingMoment

For Solid Sections use Eqn. 3.6 to calculate

slenderness coefficient

( )

( )ho

ay

oy

Eyy

GJL

yD

EI

M +

+

+

= .2

4

2

2

2

bdS 3

1=


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I-beam Strength Bending capacity of an I-beam based on stresses in

critical flange due to bending

Bending moment capacity based on lever arm actionaround the centriod of area of the flanges

Tension flange is not susceptible to buckling therefore:

For k8 0.73 (ft/ fc) Mbx = k1 ft Af D1 x 10

-6 kNm

For k8 < 0.73

Mbx = k1 k8 fc Af D1 x 10-6 kNm


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Connection Design Ensure structural integrity of rafter and purlin

Abide by nailing spacings, end and edge distances

Connection needs to be practical and easy to fabricate

and install Available options for I-beams

Joist Hangers Require installation of web packers/stiffeners can be costly

Also requires use of suitable joist hangers

Purlin connection blocks Ease of fabrication

Quick installation


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Portal Frame Design Elastic Structural Analysis differs little to that applied to

steel except for material and section properties

Solid sections include a 5% allowance within Modulus

of Elasticity removing need for consideration of sheardeflection

Rigid connection achieved by nailed or screwedplywood or steel gussets

Serviceability limits similar to steel where considerationof cladding and absolute limits is required

Effects of creep for long term deflection need to be

taken into account


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Selection of critical moments

1

4

3

4 Critical design actions:

1. Rafter moment

2. Column moment3. Gusset moment

4. Nail group moment


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Gusset SpecificationPlywood or X-Banded gussets advantageous:

Ease of fabrication

Readily fixed using machine driven nails.

Reduce the tendency of the long band plies to split,allowing the nail spacing to be governed by the graindirection of the rafter or column.

Plywood is readily available in Stress Grade F11 inthicknesses up to 25 mm.

For large span portal frames CHH have developed 4 x-band hySPAN sheets (2400x1200) in a 42mm

thickness allowing 28 mm (8 plies) of parallel plies.


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Gusset Design Gusset Capacity is based on the critical depth at which the gusset

bends, which is a horizontal line across the centroid of the rafterand column intersection

Geometrically the critical section for the knee connection may be

calculated by:

Bending moment capacities drive gusset design

Axial and shear forces generally do not influence the thickness ofthe gusset

tan2

11

+

+=

L

D

DLDDepthcs


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Knee Gusset Design Many Timber Designers propose different methods for calculating

gusset capacities Batchelor proposes bilinear stress distribution

Hutchings (and Milner) propose design capacity based on triangular

stress distribution These methodologies have been applied to many buildings

Hutchings methodology based on linear stress distribution andapplies plywood moment capacity equation

Hutchings proposes use of size effect factor (k11 from AS 1720 ork24 from NZS 3603)

Since gussets are in pairs:

= 6

.

........2

2

15142481

dtfkkkkkM

epbni


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Nail Ring Design Nail groups subjected to combined actions including bending, axial

and shear forces.

Efficient to calculate the maximum force in the extreme nail and

deduct the moment contribution from the capacity The remaining capacity can then be considered for the resolution

of shear and axial forces.

Complexity of calculations for the nail ring mean hand calculations

can be time consuming and conservative. Computer packages are often employed to develop design

solutions CHH tables:

Engineering Bulletin No.2 Rigid Moment Connections using CHH

veneer based products


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Moment Joint design Nails or screws

==n

i

ik rQr

kM

1

2

max

Adaptation of NZS 3603 eq 4.3, 4.7

Qk= characteristic strength of fastenerri= distance to the ith fastener from the centroid of the fastener group

rmax= the maximum value of rik = product of modification factors


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Portal Suite - Design tools for engineers

Fully worked design example of a 30 m clear span building Design tools to suit buildings ranging from 20+ m clear span

Engineering Bulletin No.1 Strength Limit States Design CapacityTables for CHH LVL Sections

Engineering Bulletin No.2 Rigid Moment Connections using CHHveneer based products

Engineering Bulletin No.3 Purlin Span Tables for hyJOIST

Dynamic web page with industry links and current projects

Real prices of different sized projects to promote pricecompatibility with alternative material designs

Engineering support from experienced timber design engineers


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Portal Suite

Continual release of tools beginningSeptember 2008 via website:

www.chhwoodproducts.co.nz/engineerszone Leave business card and tools will be sent out

upon release

Contact Warwick or Cameron on 0800 808 131

regarding confidential discussions about

current and future projects


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Thanks for your timeAny questions?

Coming soon

www.chhwoodproducts.co.nz/engineerszone

Documents

7 Portal Frames Warwick Banks