Effective Effective connection connection methods methods

5.4 Effective Effective connection connection methods methods

As summarized by Madsen [90,91], certain general

As summarized by Madsen [90,91], certain general considerations pervade design and construc-considerations pervade design and construc-tion decisions related to structural timber connecconstruc-tions:

tion decisions related to structural timber connections:

•

• Strength Strength efficiencyefficiency: Connections must have the ability to : Connections must have the ability to resist potentially damaging effectsresist potentially damaging effects of individual or simultaneous flows of shear, axial, and moment forces from ends or of individual or simultaneous flows of shear, axial, and moment forces from ends or sur-faces of elements (e.

faces of elements (e.g. framewg. framework elementsork elements and diaphragms) to other parts of the systemand diaphragms) to other parts of the system..

•

•  Stiffness/flexibility  Stiffness/flexibility: Connections should be designed and constructed to have the ability to: Connections should be designed and constructed to have the ability to enable or limit deformations in ways that are consistent with the intended structural enable or limit deformations in ways that are consistent with the intended structural con-cepts for the behaviour of the completed building.

cepts for the behaviour of the completed building.

•

• FFailure ailure mechanismsmechanisms: It is always essential to be able to identify how connections might: It is always essential to be able to identify how connections might fail, if structural systems are overloaded. Some types of timber connections are prone to fail, if structural systems are overloaded. Some types of timber connections are prone to exhibiting brittle failure mechanisms and are, thus, inappropriate for construction of exhibiting brittle failure mechanisms and are, thus, inappropriate for construction of frame-works of large and tall buildings. Therefore, it is essential that engineers be able to reliably works of large and tall buildings. Therefore, it is essential that engineers be able to reliably identify how connections would fail under various scenarios, what residual capabilities identify how connections would fail under various scenarios, what residual capabilities connections would have after reaching their ultimate capacities, and what system level connections would have after reaching their ultimate capacities, and what system level design loads would be associated with connection failure. Modern Load and Resistance design loads would be associated with connection failure. Modern Load and Resistance Factor D

Factor Design/Partial Coefficients esign/Partial Coefficients Design (LRFDDesign (LRFD/PCD)/PCD) timber design timber design codes and codes and propri- propri-etary information belonging to connection hardware manufacturers support these needs.

etary information belonging to connection hardware manufacturers support these needs.

•

• FForce orce reversareversals:ls: Reversals of force flows through connections are common in practice un-Reversals of force flows through connections are common in practice un-der the effects of various normal and abnormal loads. Heights of multi-storey der the effects of various normal and abnormal loads. Heights of multi-storey superstruc-tures and their slenderness often mean that their total gravitational masses are insufficient tures and their slenderness often mean that their total gravitational masses are insufficient to prevent lateral forces associated with wind and seismic

to prevent lateral forces associated with wind and seismic loads from creating tension situ-loads from creating tension situ-ations in vertical members (i.e. superstructures respond like giant cantilevers). Thus, even ations in vertical members (i.e. superstructures respond like giant cantilevers). Thus, even connections that rely on simple bearing action should have some capability to resist the connections that rely on simple bearing action should have some capability to resist the effects of force

effects of force reversals.reversals.

5.4

5.4 EFFECTIVE EFFECTIVE CONNECTION CONNECTION METHODS METHODS  6565

•

• Simplicity Simplicity and and transparency transparency of of behaviour:behaviour: It is essential that the functions of individualIt is essential that the functions of individual structural connections be transparent, not only to the original designers but also to others structural connections be transparent, not only to the original designers but also to others who might have to perform structural assessments (possibly decades after buildings were who might have to perform structural assessments (possibly decades after buildings were constructed). Load paths and intended b

constructed). Load paths and intended behaviour of each connection within any superstruc-ehaviour of each connection within any superstruc-ture must be fully consistent with

ture must be fully consistent with the intent of the the intent of the design strategydesign strategy. If, for example, connec-. If, for example, connec-tions are assumed to be pinned, then that is what should be implemented. It is vital that the tions are assumed to be pinned, then that is what should be implemented. It is vital that the addition of secondary structural and non-structural elements during initial construction or addition of secondary structural and non-structural elements during initial construction or renovation does not transform the way in which the primary structural system works under renovation does not transform the way in which the primary structural system works under various design load scenarios.

various design load scenarios.

Traditional connection methods that have proven successful all adhere to these simple Traditional connection methods that have proven successful all adhere to these simple prin-ciples (

ciples (Fig. 5.4Fig. 5.4). To be effective, modern connection methods also need to do so. In large). To be effective, modern connection methods also need to do so. In large superstructure systems one must consider how elements will be manufactured and erected.

superstructure systems one must consider how elements will be manufactured and erected.

As illustrated by example projects in Chapters 8 and 10, timber/glulam framework elements As illustrated by example projects in Chapters 8 and 10, timber/glulam framework elements of large multi-storey superstructures are nearly always manufactured off-site in factory of large multi-storey superstructures are nearly always manufactured off-site in factory con-ditions. This includes the exact, final element shaping and the cutting and drilling ditions. This includes the exact, final element shaping and the cutting and drilling associ-ated with connections.

ated with connections. SimilarlySimilarly, metal connections , metal connections are nearly always manufactured off-site.are nearly always manufactured off-site.

Therefore, for building superstructures of types discussed in this chapter, the elements are Therefore, for building superstructures of types discussed in this chapter, the elements are received as parts intended to fit together exactly. The situation is in no way akin to traditional received as parts intended to fit together exactly. The situation is in no way akin to traditional timber construction practices, where elements

timber construction practices, where elements were typically manufactured from raw materi-were typically manufactured from raw materi-als such as

als such as pieces of lumber. In modern practice, large tipieces of lumber. In modern practice, large timber framewmber frameworks are often orks are often partiallypartially preassembled before being lifted into place, as illustrated in

preassembled before being lifted into place, as illustrated in Fig. 5.7 Fig. 5.7 . In principle, on-site. In principle, on-site assembly practices are similar to those that apply to structural steel and precast RC element assembly practices are similar to those that apply to structural steel and precast RC element frameworks.

frameworks.

Joints made using generic mechanical fasteners are pervasive in modern timber construction.

Joints made using generic mechanical fasteners are pervasive in modern timber construction.

The term mechanical fastener encompasses all devices that are inserted into two abutting The term mechanical fastener encompasses all devices that are inserted into two abutting mem-bers in ways that result in transferred forces flowing across joint interfaces via those devices.

bers in ways that result in transferred forces flowing across joint interfaces via those devices.

Most generic mechanical fasteners have been mass-produced since the 19th or early 2

Most generic mechanical fasteners have been mass-produced since the 19th or early 20th centu-0th centu-ries, with wire nails, steel bolts, and plain

ries, with wire nails, steel bolts, and plain steel dowels being the most common. The popularitysteel dowels being the most common. The popularity of joints using g

of joints using generic fasteners reflects severeneric fasteners reflects several factors including that al factors including that the fasteners are typicallythe fasteners are typically inexpensive and easy to install, and that joint strength can be reliably predicted using easily inexpensive and easy to install, and that joint strength can be reliably predicted using easily accessed information (i.e. in codes and handbooks). In the context of large frameworks, joints accessed information (i.e. in codes and handbooks). In the context of large frameworks, joints

(

(aa)) ((bb))

Fig

Fig. 5.7: . 5.7: Installation of glulam superstructure framework seInstallation of glulam superstructure framework segments (further details in Chaptergments (further details in Chapter 8): (a)

8): (a) two-storetwo-storey segment; (b) iny segment; (b) interconnection of segmentsterconnection of segments

in connections are typically made using laterally loaded steel dowels and bolts. As indicated in Fig. 5.8, joints made with generic dowel fasteners often interconnect timber elements with metal link elements that bridge connections, with the metal parts being at the hub. The slenderness of metal dowel fasteners normally determines the flexibility and degree of ductility of joints and connections employing them. Reliance on skin friction around dowel fasteners for primary force resistance is something structural engineers usually try to avoid because of its relative lack of reliability after timber members have dried or dried and then been rewetted. Therefore, with the exception of bolts, plain dowels, and some types of screws, emphasis tends to be on using dowel fasteners in situations where force transfer does not involve the possibility of pulling them out of or through joined materials. In the context of structural frameworks formed from large tim-bers, all mentioned types of generic fasteners have been employed. However, experience sug-gests that using plain metal dowels and metal bolts are most appropriate and economical when large forces need to be transferred.

There exists extensive literature on design and construction of joints employing generic dowel fasteners. Nearly all general textbooks and design aids on timber engineering devote substan-tial attention to the appropriate use of various fasteners and provide sample design solutions [92,93]. This stated, designers who intend system responses to remain in the elastic range should verify the response state associated with design capacities for recommended dowel fastener connections [93].

(a)

(b) (i) (ii)

Gripping element Forces on Wood gripping element

Resistance from wood (bearing + skin friction)

Wood Steel bolt

Steel plates

Wood

Wood

Steel link element (embedded) Connection

interface

Steel dowel (embedded)

Sacrificial wood plug

Fig. 5.8: Joints with laterally loaded fasteners: (a) principle of gripping action; (b) classifica-tion of mechanical joints with metal parts: (i) exposed metal, (ii) concealed metal

5.4 EFFECTIVE CONNECTION METHODS  67

Many types of proprietary joint/connection hardware are highly suitable for making connec-tions in timber framework elements, for attaching diaphragms to frameworks, and for attaching frameworks to shear walls and foundations. In all cases, necessary design information about  joint/connection stiffness, strength, and failure mechanisms is outside the scope of design codes

and must be obtained from the specific manufacturer.

Glued-in, rod joint systems involve the embedding of metal or other types of reinforcing bars into structural elements. This creates local continuity akin to connections in RC frameworks, or diffusely transfers forces into or from ends of members to other parts of connections (e.g. roll-ers, pins, and swivels). Reinforcing rods are inserted into predrilled oversized holes in glulam or other large timber members with gap-filling adhesives or grouts employed to bond the rods. For economy, rods are normally of the types employed in RC construction processes. Considerable research has been devoted to developing methods in Scandinavia, Russia, New Zealand, and Canada. Most of the research has focused on the behaviour of joints/connections under static and seismic loads for situations where glulam columns and girders intersect or where columns connect to foundations. As expected, certain connection configurations, details, and fabrica-tion methods are superior to others. Generally, it is preferable that reinforcing bars be installed into timber members off-site under controlled factory conditions and then fastened to hub-like connection devices by on-site bolting to create arrangements such as that illustrated in Fig. 5.9.

The illustrated arrangement would achieve a self-bracing frame action and steel connecting assemblies could be designed to yield in desired ways, if the system was overloaded. Using glued-in rod approaches can create force flows consistent with almost any desired structural system behaviour [91].

A discussion of standard timber jointing methods would not be complete without mentioning glued joints. Rigid adhesives are employed under factory conditions to create intra-component  joints in glulam members, multilayered timber-based composites such as laminated veneer lum-ber and CLT, box-girders, and stressed-skin panels. On-site gluing of large timlum-bers has been practiced on a limited basis to create long members for low-rise construction and conceptu-ally such technology could be applied to construction of relatively tall multi-storey framed superstructures. However, the technology has not been proven suitable for construction of super-structures of types that are the focus of this chapter, and hence, is not recommended at this point of time.

Glued-in rods

Steel connecting assemblies Glulam members

Fig. 5.9: Illustration of glued-in rod connections suitable for multi-storey framework applications

(a)

(c)

(i) (ii)   (iii)

(b)

Glulam beam

Glulam column

Surface or embedded energy dissipation elements (absorb energy via plastic work during load cycling)

Unbonded post-tensioned tendon (compressing connection to enforce self-centering on release of  external loading)

Plastic response Elastic response

θ 

 M 

 Elastic

θ 

 M 

Plastic

θ 

 M 

Plastic  Elastic

Fig. 5.10: Logic of self-centring frame connections, based on Buchanan et al [16]: (a) compo-nents of a frame connection; (b) deformed connection during seismic action; (c) logic of flag-shaped hysteresis loops (moment M versus rotation q : (i) effect of post-tensioning, (ii) effect of energy dissipation elements, (iii) combined response: flag-shaped hysteresis loop

On a final note, researchers from New Zealand and Italy have been performing experiments and undertaking numerical modelling to define self-centring connection technologies suitable for timber heavy-framed building superstructures. Focus is on creating frameworks that efficiently resist the effects of seismic actions [16], based on adaptation of what have become fairly common practices for post-tensioned prefabricated RC frameworks [94]. The principle is that by suitably arranging post-tensioning tendons and sacrificial metal energy absorbing elements (that deform plastically, if overloaded) it is possible to create a self position-restoring system response with-out sustaining more than minimal damage (Fig. 5.10). Essentially the post-tensioning forces that do not exceed the elastic range of the tendons pull the system back into its original position when external forces are removed. The energy dissipation elements can undergo plastic deformation while absorbing considerable amounts of system-level inertial work. If properly designed, the dissipation elements can be easily replaced during post-disaster repair work. Buchanan et al.

[16] have also developed post-tensioning connection systems suitable for minimizing damage in timber buildings containing massive wall panels. They claim that their connection methods are suitable for constructing building superstructures with 10 or more storeys in high seismic risk locations. No explicit proof exists yet of satisfactory field performance of post-tensioning

5.5 ADDITIONAL COMMENTS  69 connection systems, but it can reasonably be presumed that they will work well, because what is done replicates in principle the approaches to seismic design employed in traditional bearing wall and timber infill-frame buildings (Figs. 2.3a and 2.3b). The modern approach is to achieve via post-tensioning and energy absorbing elements what was formerly achieved by overburden pressures, frictional damping, and carpentry details.

5.5 Additional comments

This chapter is not intended to be read alone. Readers are strongly encouraged to pay close atten-tion to Chapters 3 and 4 that respectively address fire and durability performance and design of large and tall building superstructures employing timber as a primary construction material. It is also advisable to pay close attention to the discussion of the example project of a six-storey hybrid building in Quebec City in Chapter 8, the case studies in Chapter 10, and future possibili-ties discussed in Chapter 12.

Acknowledgements

This chapter draws on input from Drs. Mohammad A.H. Mohammad, Marjan Popovski, and Chun Ni and Mr. Sylvain Gagnon of FPInnovations, Canada.

71 Chapter

6