universita’ di napoli “federico ii” dipartimento di ... · dipartimento di analisi e...

1

Edifici a struttura portante in legno


Massimo FRAGIACOMO*

*Senior Lecturer, University of Canterbury, Christchurch, New Zealand

Email: [email protected]

Universita’ di Napoli “Federico II”Dipartimento di Analisi e Progettazione Strutturale

Corso di Costruzioni in Legno28 aprile 2006 – Seminario su

SUMMARY:

• Introduction on New Zealand• Main properties of timber (sustainability, anisotropy, shrinkage, mechanical properties)• Sawn timber, glulam, LVL, plywood• Industrial buildings (restraints, internal joints, and erection)• One- and two-storey houses


2

SUMMARY:

• Multistorey buildings:Ply shear wallsElastic and ductile designTraditional floors and diaphragm actionLateral load distribution into shear wallsConstruction techniquesHybrid buildings and composite floors


INTRODUCTION: NEW ZEALAND


23 hours

3



3 h



10 years of shallow (less then 40 km) seismicity

4



EXPORT:• wool (40 millions of sheep with 4 millions of residents)• milk, meet and dairy products• wood and wood-based materials



Kauri trees: 2 m diameter, 600 years old

5





Radiata pine: 1-2 cm thick growth rings per year Harvest after 25-30 years

6

PROS AND CONS OF TIMBER:

PROS:• aesthetic appearance• natural and sustainable material• high strength-to-density ratio


CONS:• cost• anisotropy• durability, if not adequately protected by the water

SUSTAINABILITY:

Why to use timber? Because it is a sustainable material and renewable resource.Embodied energy: energy consumed in the acquisition of raw materials, processing, manufacturing, transport to site & construction

7.94.62.510.1Embodied energy [MJ/kg]

14598222Embodied energy [MJ/m]

LVLGlulamTimber, kiln driedSteel


7

SUSTAINABILITY:

Embodied effects summary:

1.231.081Solid waste1.811.111Resources3.504.001Water pollution1.471.241Air pollution1.811.341GHG emissions1.571.261Embodied energy

ConcreteSteel TimberEnvironmental effect

(Canadian Wood Council)


TIMBER PROPERTIES:

NoNoYesHygroscopic behaviourYesNoYesTime dependent behaviourNoYesNoDuctilityNoYesYesTensile strength

YesYesNoOmogeneity

NoNoYesCombustibility

YesYesNoIsotropy

302108Elastic modulus [GPa]2405060Ratio ρm /σadm

24007850600Density ρm [daN/m3]

1016010Permissible stress (compr.) σadm [MPa]CONCRETESTEELTIMBERPROPERTY


8

ANISOTROPY:


ANISOTROPY:

AN

=

ll∆

=

0arctgE=α


9

Be careful of the connection design!!!

ANISOTROPY:


SHRINKAGE/SWELLING:

Longitudinal: Shr=0.1%

Radial: Shr=2.1%

Tangential: Shr=3.9%

Shr=(∆L/L)100 by drying the specimen from 30% to 12% of moisture content


10

12

3.9

2.1

0.1


SHRINKAGE/SWELLING:

Most shrinkage problems occur when timbermovement is prevented by stiffer elements:

SHRINKAGE/SWELLING:


11

SAWN TIMBER:


GLULAM:

Glue-laminated timber (glulam) is a solid wood member manufactured by gluing smaller pieces (planks) together.


12

WHAT IS GLULAM?

The idea is to:

• cut some planks (33 mm thick, 1500 to 5000 mm long) from a tree;

• join them lengthwise(finger joints);

• glue the laminationstogether. Edifici a struttura portante in legno

HOW IS GLULAM MANUFACTURED?

Jigs and pressing devices for straight members


13

HOW IS GLULAM MANUFACTURED?

Jigs and pressing devices for curved members


WHAT IS LVL?

• LVL is Laminated Veneer Lumber

• LVL is obtained by gluing together under pressures veneers of wood 2 to 4 mm thick produced by the rotary peeling of steamed logs.


14

HOW IS IT PRODUCED?

rotary peel lathe

veneer

drier

ultrasoundgrader glue

spreader

lay-up station

presscross-cutsaw

rip saw


WHY IS IT PRODUCED?

The better properties are achieved because the defects such as knots are smaller and spread throughout the beam volume. Therefore each defect is less critical compared to the case of sawn timber. LVL behaves almost as clear wood.


15

COMPARISON AMONG STRENGTHS:

28.813.286Modulus of Elasticity [GPa]

3421910Bending strength [MPa]

25342415Compression strength [MPa]

Concrete Grade 25

LVL (Hyspan)

Glulam GL8

Sawn timberm/c=16%


PLYWOOD:

T

L

R

R

T LR

L TR

T L

• Plywood is manufactured like LVL, but with the adjacent veneers laid at aright angle.


16

PLYWOOD:

Unlike LVL, Plywood experiences strength and stiffness comparable in any in-plane direction. It is produced in panels which are mainly used for 2D members such as floor deckings and shear walls.


Timber can be used for different types of buildings:

TIMBER BUILDINGS:

• one-storey industrial buildings• one- and two-storey houses• multi-storey buildings

Costruire con il legno

17

INDUSTRIAL BUILDINGS:


They are made of a series of main frames(in glulam or LVL) linked by longitudinal beams (purlins and girts) with a bracing system

MAIN FRAMES:

Portal frames: members and joints mainly subjected to bending; usually with pinned external restraints


ApexKnee

18

RESTRAINTS:

Pinned connection


Nailed steel gusset plates

RIGID JOINTS:


19

Nailed plywood gussets

RIGID JOINTS:


X-band LVL gusset for large moments

RIGID JOINTS:


20

ERECTION:


ERECTION:


21

Edifici multipiano in legno

• Timber is used in more than 90% of one- and two-storey dwellings in some countries such as Canada and New Zealand• Timber is also used for multistoreybuildings up to 5 to 8 storeys• Timber can be used for both vertical(walls and posts) and horizontal (floors and beams) structures

MULTISTOREY BUILDINGS:

ONE-STOREY HOUSES:


The floor is made of joists and particleboard sheathing, while the vertical structure is made of gypsum board shear walls

22

MULTISTOREY BUILDINGS:


The floor is made of joists and plywood sheathing, while the vertical structure is made of plywood shear walls

SUITABLE BUILDING TYPES:


• Apartment blocks • Hotels, Motels

23

TIMBER FRAMED HOUSE:

Top plate

Bottom plate

Outer stud

Plywood or gypsum board sheathing

Nailed connection

Holddown strapBolted connection to

foundation

Inner stud


SHEAR WALLS:


24

SHEAR WALLS:

Bottom plate

Holddown strap

Bolted connection to foundation

Plywood sheathing

Outer stud Inner stud


Gravity load induces compression into theinner and outer studs, while the other members (plates, sheathing, straps, nailed and bolted connections) are unloaded

bh

N N N

N N NN

N

SHEAR WALLS:


25

Shear walls subjected to lateral load behave like a cantilevered deep beam where:• shear is resisted by the panel sheathing andnailed connection to outer studs and plates• bending is resisted by the lateral chords in tension and compression• inner studs and nailed connection with sheathing prevent buckling of the sheathing

SHEAR WALLS:


b

h

P

T=Ph/b C=Ph/b

P

v=P/b

v=P/bT C

v=T/h=P/b

v=P/b

v=T/h=P/b

Free body diagrams:

s’

Ap Ast

s

SHEAR WALLS:


26

ULS design of shear walls:• top plate-compression: σc,0,d=P/Ap≤kc,yfc,0,d

• outer studs-compr.: σc,0,d=(P+N)h/bAs≤kc,yfc,0,d

• sheathing-shear: τd=P/bt≤fv,d

• nailed connection-shear: F=vs=Ps/b≤Fv,Rk

• bolted conn. to found.-shear: F=Ps’/b≤Fv,Rk

• holddown strap-tension: σs=(P-N)/As≤fs,dEdifici a struttura portante in legno

SHEAR WALLS:

Each unit can be assembled in a larger wall member:P

HB B B B

T=PH/3B C=PH/3B C=PH/3BT=PH/3Bv=P/3B v=P/3B

Nogging

SHEAR WALLS:


27

SLS design of shear walls: 4 components of deflection:

Base Rotation

28%

Nail Slip

59%

Panel Shear

1%

Flexural Deformation

12%

SHEAR WALLS:


SLS design of shear walls:

SHEAR WALLS:


28



SHEAR WALLS:



SHEAR WALLS:

29


SHEAR WALLS:

Racking test on single panel:


Racking test on complete assemblage:

SHEAR WALLS:

30

Resistance Deflection


SHEAR WALLS:

Cyclic quasi-static test:

Design strength S+

Mean strength P+=2S+

Overstrength factor Φ=P+/S+=2

Remarks:• ductility provided only by the plasticization of the nailed connection, with every other component (studs, plates, sheathing, straps and bolts) characterized by brittle failure• high overstrength factor (Φ=2)


SHEAR WALLS:

31

SEISMIC DESIGN:

Elastic design: each member is designed under the action Ee evaluated from the inelastic spectrum with ductility µ=1.5 according to EC5 for timber structures

Ee

TTs

µ=1.5

µ=5


SEISMIC DESIGN:

Ep

TTs

µ=1.5

µ=5

Ductile design: the nailed connection of the shear walls is designed under the action Ep evaluated from the inelastic spectrum (µ=5). Every other element is designed under the action ΨΦΕp, Ψ being

the overdesign and Φ(=2) the overstrengthfactor of the nailed connection


32

SEISMIC DESIGN:

Ductile design: 5- to 8-storey stick framed buildings are designed so as the plastic hinge occurs in the first to second storey.

Only the nailed connection of the shear walls at 1st and 2nd

floor are designed under the reduced action Ep, while every other member is designed under the action ΨΦEp


TRADITIONAL FLOORS:


33

TRADITIONAL FLOORS:


The panels must be staggered and nailed (100 to 150 mm c/c on the edges, 200 to 300 mm c/celsewhere) or glued to the joists and blockings in order to achieve the diaphragm action.

flooring panel

joist

blocking

TRADITIONAL FLOORS:


34

TRADITIONAL FLOORS:

Plywood sheathing

Hy-beams


TRADITIONAL FLOORS:

I-shaped joists in LVL and plywood:

LVL Flange Plywood

Web

Resorcinol web to web joint

Resorcinol flange to web joint


35

TRADITIONAL FLOORS:

Hy-beams

hangers

particleboard



DIAPHRAGM ACTION:

36

DIAPHRAGM ACTION:

The sheathing must also be checked under in-plane shear in order to achieve the diaphragmaction. If the shear walls are elastically designed:

dEldlEw u

u ==

2* wlVp =

dwl

dMNN tc 8

2*** ===

Vp* is resisted by the sheathing

Nc*, Nt

* are resisted by the top and bottom chords

Seismic action

Edifici a struttura portante in legnow

lH HNt

* Nt*

Nc*Nc

*

Vp* Vp

*d

If shear walls are designed for ductilebehaviour, capacity design shall be applied. The nailed connection is the ductile element in the shear walls.

Φ=Qnu/φQn: overstrength factorδ

Q

Qn

Qnu

QQ

δ

Qn=nkQk: nominal strengthQnu= actual mean strength =1.6Qn

Φ=1.6/φ=2 for nailed connections0.8

φQn

S*<φQn: S* is the design shear loadφQn: design shear strength

S*


DIAPHRAGM ACTION:

37

The floor diaphragms shall be designed for the overstrength of the shear panels:

lV

w p*2

=′np QV Φ=*

dlw

dMNN tc 8

2*** ′

===

The uniformly distributed seismic action w’ can also be obtained as:

wlEdldE

SQ

QQw u

un

n

nu ΦΨ=ΦΨ=φ

φ=′

*

S* is the design shear load on walls, Ψ is the overdesign factor (Ψ>1 if S*<φQn), and Φ is the overstrengthfactor.

ΨΦ

Edifici a struttura portante in legnow’

lH HNt

* Nt*

Nc*Nc

*

Vp* Vp

*d

DIAPHRAGM ACTION:

LATERAL LOAD DISTRIBUTION:

How do you determine how much of the total lateral load at each level gets into each shear wall?

In the case of rigid floor diaphragms: in direct proportion to the stiffness of each shear wall.


38


In the case of flexible floor diaphragms: in direct proportion to the tributary area and mass of the floor on the shear wall.The timber floor diaphragms are somewhere in between rigid and flexible, which makes the problem of the lateral load distribution pretty complex.Approximation commonly adopted to solve the problem: Edifici a struttura portante in legno

Vi

Pijlij

Vi=Total lateral load at ith storey

Pij=Shear force at ith storey in the jth

shear wall

lij=Length of the jth shear wall at the ith storey

i

jij

ijij V

ll

P∑

=



39

CONSTRUCTION TECHNIQUES

There are two commonly used methods for shearwallbuildings:• platform construction• balloon construction Edifici a struttura portante in legno

CONSTRUCTION TECHNIQUES


40


TIMBER FRAMED HOUSE:

Typical Wall/Floor Detail Where JoistsRun Perpendicular to WallBALLOON CONSTRUCTION


41

Bottom Plate

Seating Block

PlyTop Plate

Blocking

BALLOON CONSTRUCTION


Typical Floor Support

200 deep engineered floor joists with proprietary fixings to ribbon plate

Seating Blocks

BALLOON CONSTRUCTION


Plywood

42

HYBRID BUILDINGS:

The limit for multistorey buildings with plywood shear walls and traditional timber floors is 5 to 8 storeys.

The limit span for traditional timber floors is 6 m.

Is it possible to overcome those limits? Yes, it is, using hybrid and composite systems.


Kiln dried timber wall framing

Plywood flooring over Hybeam joists

Structural steel internalgravity post and beam system

Structural steel perimetermoment resisting frame

HYBRID BUILDINGS:


43

HYBRID BUILDINGS:


HYBRID BUILDINGS:

Another possibility: using LVL posts and beams gravity load system, timber floors, and steel bracing and/or concrete shear walls for the lateral load resisting system.

In this way it is possible to build multistoreybuildings up to 15 to 25 storeys.


44

HYBRID BUILDINGS:


COMPOSITE FLOORS

In order to overcome the span of 6 m, stressed skin panels can be used.

T cross-section (I -200)

3D-view of SSP elements

box cross-section (I-200)

222

1 2 0 0

2220

0

3 7 7 3 77 3 7 7

237

1522

200

3 7 7 3 7 7 3 7 7

1 2 0 0

393

1522

356

5 0 0 5 0 0

box cross-section (I-356)

(all dimensions in mm)


45

COMPOSITE FLOORS

OSB panel glued to I-beams


COMPOSITE FLOORS

Concrete slab with steel mesh

Timber beam

Connection system (steel mesh glued into the timber beam)

Steel mesh

Glued rebars

Timber decking


46


The End

ERRORS:


47

ERRORS:

?


ERRORS:


48

ERRORS:

• Bolt end & edge distances?

• Washer sizes? (85mm dia. for >M20 bolt)

• Timber/steel dimensional change with moisture?

• Joint durability?

??

?

??Edifici a struttura portante in legno

ERRORS:

• Epoxy grouted rods in exposed environment ?

• Rod edge distances ?

• Timber dimensional change with moisture vsrigid steel bracket?

?


49

RIPAIR:

25025

4525

90

45

90

570

800 to 810 varies

490 to 500 varies

755 to 765 varies

760 to

770 va

ries

EXISTING APEX GEOMETRY

30 3075

135

existing 12.7mm steel fin plate

15mm x 135mm s/s platerebated into first laminate

M16 threaded bars in 20mm dia holesepoxy injected with Hilti CA-80inject epoxy from top of beam

low profile M16 nuts, 5mm x 40mm square washers

M16 counter sunk thread12mm deep - 4mm TIG fillet weld

drill epoxy bleed holesplug after injection

CROSS SECTION - NEW SHEAR REINFORCEMENT

The problem: longitudinal splitting cracks, maybe due to timber shrinkage prevented by the fin steel plate Edifici a struttura portante in legno

RIPAIR:


50

RIPAIR:

REPAIR DETAILS FOR APEX CONNECTIONS(TYPICAL DIMENSIONS)

85115

75 50

50

replace existing boltswith M24 s/s dowels

new M24 s/s dowels(4 per side)


M16 threaded bar in 20mm dia holeepoxy injected with Hilti CA-80

275

271,7 275

271,7

150,2150,2


inject epoxy from bottom of beam

drill epoxy injection holesplug after injection


30 3075

135



M16 threaded bars in 20mm dia holesepoxy injected with Hilti CA-80inject epoxy from top of beam


M16 counter sunk thread12mm deep - 4mm TIG fillet weld

drill epoxy bleed holesplug after injection

CROSS SECTION - NEW SHEAR REINFORCEMENT


RIPAIR:


51

RIPAIR:


Specification / Schedule:1. Install scaffold incorporating props at either end to provide support to the roof beams approximately 1.2m either side of the joint interface.2. Loosen existing bolts3. Drill 20mm diameter holes vertically through the members4. Rebate bottom of beams to accommodate 15mm bearing plate for stitch bolt assembly5. Manufacture stitch bolt assemblies plug welding M16 threaded bars to the s/s bearing plate (grade 304 s/s)6. Install M16 threaded bar assembly into holes from underside, install top plate and tighten nuts to close delamination gaps7. Drill 8mm diameter injection (bottom - face of beam) and bleed holes (top of beam) and inject Hilti CA-80 or equivalent epoxy (from bottom) 8. Install wooden plugs into injection holes9. Manufacture grade 304 s/s dowels (1.5mm chamfered edges) for new dowels and to replace all existing bolts10. Locate and drill new M24 holes using template and install snug fit s/s dowels as indicated in Details 11. Remove all existing bolts and ream holes to 24mm diameter12. Inject epoxy as necessary to fill any elongated holes13. Install replacement M24 snug fit s/s dowels14. Remove scaffold

Notes:1. The Contractor must confirm all final dimensions on site. All existing dimensions are nominal only - minor variations for each connection have been observed2. These details are not to be modified without written approval from the Consulting Engineers3. All details have been prepared in accordance with the design provisions of AS1720.1 - 1997 for joint group JD2.

RIPAIR:


52

RIPAIR:

The problem: use of recycled notched hardwood beams with longitudinal splitting cracks


RIPAIR:


53

RIPAIR:


RIPAIR:


54

NOTCHED BEAMS:

Because of the notch, some tensile stresses perpendicular to grain will develop

hαh


NOTCHED BEAMS:


55

NOTCHED BEAMS:


RIPAIR:


56

RIPAIR:


RIPAIR:


universita’ di napoli “federico ii” dipartimento di ... · dipartimento di analisi e...

Documents