Apart from the qualitative considerations recalled in the previous section, we can ask whether and to what extent existing and often very ancient timber structures can be able to exhibit a ductile and dissipative behavior under extreme conditions.
Unfortunately, answering this question is not an easy task, since the actual dynamic behavior of historical timber buildings is hardly predictable and it is usually documented, as far as for the past, only by empirical data. The task is even made more difficult by the different constructive techniques, wood species, kind and quality of infill materials and by the large variety of joints adopted in ancient structures. Some recent studies can be, however, exploited to have some clues. Experimental researches were carried out to assess the behavior of the Borbone constructive system, [ 16 , 17 , 29 ].
Particularly remarkable are the results provided in Ref. Experimental findings evidenced a non-linear behavior of specimens with comparatively high values of ductility. The latter was calculated as the ratio between the maximum displacement u max and the displacement at yield u y , namely. The quantity defined by Eq. To quantify the dissipation of energy in the plastic range, the hysteresis equivalent damping ratio was calculated in Ref.
Based on the Jacobsen approach [ 31 ], Eq. Pinched hysteresis loop third cycle : a dissipated energy for a half cycle; b potential energy of the equivalent oscillator for a half cycle. In the infilled specimens, the energy dissipation was found to depend mostly on friction and small ruptures in the masonry. In bare specimens, the almost wholeness dissipation was found instead to be concentrated in the connections, where two mechanisms occur contemporarily: the first one is related to the compression and crushing of wood grain with the formation of a cavity due to the presence of nails which is also responsible for the pinching effects discussed in the following ; the second one concerns the inelastic deformation of the iron nails, [ 16 , 17 ].
The dynamic behavior of the Pombalino timber-framed buildings in Lisbon was investigated through experimental tests on specimens taken from actual sites [ 28 ] or rebuilt in laboratory [ 19 , 27 , 32 ]. All of them assessed a rather good ductile behavior of Pombalino walls. In particular, the results given in Ref.
An introduction to timber frame building
The ductility ratio was found to be around 3. Significant pinching effects, related to gaps and residual displacements which form in the connection zones and between masonry and timber members were detected. A rather accurate non-linear hysteresis model was also developed in the same paper to capture the dynamic response of the walls. Similar experimental tests on real-size laboratory reconstructions of infilled and bare-framed Pombalino walls were carried out in Ref.
Experimental tests on eight different timber frames with and without infill brick and adobe or cladding, all reproducing portions of the typical himis -timber-framed Ottoman constructions, were carried out in Ref. The force-displacement curves obtained for reversed cyclic in-plane lateral loading showed stable hysteretic loops with highly dissipative behavior. The authors suggested to evaluate the cumulative energy dissipation capacity through the following empirical formulas [ 21 ]:.
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The highly dissipative behavior detected for Ottoman walls was found to depend on the dissipative plastic behavior of nailed connections, as also confirmed by other authors [ 22 ]. The results of in-plane quasi-static cyclic tests on full-scale dhajji-dewari walls wooden-braced frame system with masonry infill, typical of India and Pakistan were given in Ref. The tests highlighted a very strong resilience of the dhajji-dewari system against lateral loads, which is almost totally due to the performance of the timber framework.
Also in this case, a rather good ductile and dissipative behavior is found in the experimental tests. Finally, the data taken from [ 26 ] relevant to experimental tests carried out on half-scale quincha walls are also provided in Table 1. Quincha is a Peruvian traditional timber-framed constructive system consisting of an adobe-made ground floor and upper stories built with timber frames infilled with a weave of canes and mud. A qualitative diagram is provided in Figure 7 showing some of the typical features evidenced by the experimental cyclic tests on timber-framed walls, infilled with masonry and connected with metallic nails.
The main features evidenced in Figure 7 are a non-linear behavior; b almost symmetric curves; c indistinct yield point; d lateral stiffness degradation for increasing loading cycles; e pinching effect after the first load cycle; f strength degradation at the same deformation level for increasing loading cycles; g strength degradation for higher deformation; h fat hysteresis loops which implies large amounts of energy dissipated narrowed loop areas are generally found, however, for successive load cycles or for higher deformation levels ; i rather high values of ductility.
As soon as the contact with the surrounding wood is reestablished, at increased deformation levels, the stiffness rapidly increases which leads to the typical pinched shape of curves in the load-displacement diagram. It can be noted, finally, that features very similar to those illustrated in Figure 7 and discussed before were also detected for modern timber walls plywood shear, strand-board diaphragms , as can be inferred by examining, for instance, the experimental load-displacement curves provided in Ref.
Experimental studies were carried out to assess the dynamic behavior of traditional Chinese [ 35 — 37 ] and Japanese [ 38 — 40 ] constructions. Of particular interest to the purpose of the present study are the results provided in Ref. They are relevant to scaled specimens, reproducing a prototype of a historical Chinese timber palace, subjected to cyclic lateral loading. Stable and large-area hysteretic loops were found in the tests, associated with rather high values of ductility 9—19 and energy dissipation not quantified.
It is worth noting that Chinese and Japanese constructions were found able to exhibit a rather high ductile response and to dissipate large amount of energy, despite they typically avoid metallic connections. Energy dissipation was found to be due, in fact, to friction and embedding between structural elements at the contact interfaces inside the wooden joints. Further experimental studies can be also quoted, for example [ 41 ], confirming a rather good ductile and dissipative behavior of ancient timber constructions.
Ductile behavior and energy dissipation are two key points of the modern aseismic strategy adopted by current standards. Experimental findings showed that historical timber buildings can generally be able to meet these modern requirements. Table 1 substantiates statement i. With reference to the classification given by EC8 [ 1 ], recalled in Table 2 , three kinds of the traditional timber structures referred to in Table 1 may be assigned to the high ductility class DCH , namely the Borbone , the Pombalino and the quincha walls all in the case of infilled timber frames.
A low ductility class DCL should be assigned instead to the dhajji-dewari walls.
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The class of ductility of the Ottoman traditional walls cannot be assessed through the available data. Although giving an interesting portrait of the ductile and dissipative capacity of traditional timber buildings, the data collected in Table 1 should be compared with caution due to the different settings laboratory set-ups, loading procedures, recorded data analysis, yield and ultimate deformation evaluation adopted in the experimental studies mentioned. For instance, the total amount of energy dissipated can be strongly dependent on the load protocol, which is usually different from an experimental research to the other.
In addition, it can be noted that a different ductile behavior was sometimes detected in experimental tests from positive to negative direction of load, although such an aspect has not been evidenced in Table 1 for the sake of brevity. This is in fact a very significant parameter to be considered when large displacements are involved. Despite the extreme attention paid by the authors in reproducing, as faithfully as possible, the in-situ conditions timber species, infill materials, geometry, ground constraints, connections, and so on , the results obtained in laboratory on rebuilt models of parts of ancient constructions should be obviously used with great care to predict the actual dynamic behavior of existing whole buildings, also in view of all the aspects affecting the behavior of real structures material degradation, efficiency of connections, internal damage, workmanship irregularities, tridimensional behavior of the building.
A crucial role in the ductile behavior of ancient timber structures was found to be played by connections, since timber elements generally do not exploit their latest strength resources [ 21 , 42 ]. Based on this statement, some retrofitting solutions were also proposed to improve the dynamic behavior of ancient structures under dynamic loads [ 27 , 42 ]. Ductility classes and behavior factors q for different typologies of timber structures, according to EC8 [ 1 ]. Besides the more common single-family and low-rise houses, spectacular and daring-shaped modern timber buildings may be even encountered nowadays in many countries, as the few instances of Figure 8 let imagine.
A feeling for eco-friendly and renewable materials, together with the easy of production and transportation from the past, adds new motivations to the construction of wooden buildings. As discussed in the introductive section of this chapter, modern structures are required to be ductile and dissipative, particularly when they are built in seismic areas.
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While timber structures are uniquely recognized to be able to meet such requirements provided that they are regular, hyperstatic and connected with ductile fasteners as also confirmed by Table 2 , most of the issues related to evaluating and modeling this ability are still under discussion. Connections in modern timber buildings are metallic devices ensuring transmission of forces between structural elements. Their design is the most strategic part of the structural project of a timber construction, since from the characteristics of the connections type, mechanical properties, geometry, spacing, assembly techniques may strongly depend the stiffness, the strength, the ductility and the energy dissipation of the whole structure.
Although some constructive typologies such as moment-resisting timber frame systems, timber shear panel systems and cross-laminated panel systems are indicated as being particularly able to ensure a ductile behavior under extreme dynamic lateral loads [ 43 ], it is the connection design that eventually decides the ductility resources of a timber structure. The same structural type may be, in fact, assigned to different ductility classes in dependence of the rotational ductility capacity of its connections, as can be inferred, for instance, by the classification done by EC8, recalled in Table 2.
The most common connections in modern timber structures are the dowel-type mechanical fasteners nails, screws, dowels, bolts, rivets which deeply penetrate into the wood to transfer the load by means of wood bearing and connector bending.
Dowel-type connectors can be used alone or in combination with metal predrilled plates. Joints with dowel-type fasteners are expected to be ductile due to the highly nonlinear behavior of the wood under embedding stresses and the plastic behavior of the steel fasteners in bending [ 44 ]. Nevertheless, they can sometimes be affected by sudden and brittle failures like block shear or splitting [ 45 ].
Ten different types of failures six in single shear and four in double shear are considered by the European standards for dowel-type timber connections [ 46 ]. As a matter of fact, timber members and metallic joints play different roles in the seismic behavior of timber structures. Since the failure mechanisms of wooden elements are mostly brittle, the timber members are required to remain in the elastic range even under very strong events.
The task of satisfying the demand of ductility is entrusted instead to the metallic connections which are expected to sustain large inelastic deformations while preventing collapse. The ductile behavior of connections is influenced both by metallic fasteners which may behave in a ductile or brittle way depending on whether plasticization is attained or not and by the strength properties of the wood surrounding the connection zone direction of the grain with respect to the load direction.
Preventing brittle failure may guarantee an adequate ductility to the whole structure. Complying some strength hierarchy rules can assure a ductile behavior to timber structures. In particular, it is essential to design the fasteners to be weaker than the wood members they are connecting, so that they can yield and dissipate great amount of energy.
On the other hand, the weaker the fasteners, the lower their bearing capacity. A way of ensuring both adequate ductility and sufficient bearing area is using a large number of weak fasteners. Some alternatives to improve the performance of dowel-type joints are discussed in Ref. Although the plastic properties of the steel fasteners alone are well-known and their behavior under cyclic loads easy predictable, the non-linear response of the assembly of metallic connectors and surrounding wood is rather difficult to predict, since it is not a cross-section property as for reinforced concrete.
In fact, the behavior of the timber connections depends from several factors, some well-known as the strength properties and the geometric configuration of involved materials, others affected by uncertainty as the influence of neighboring metallic fasteners or the interaction between fasteners and surrounding wood.
Systems in Timber Engineering
This makes rather difficult to develop an analytical model able to reproduce the behavior of a timber connection. Most of the features evidenced in Figure 7 and discussed in Section 2. In particular, two phenomena were found to be typical of the hysteretic response of steel dowel-type connections, as recalled in Ref.
The first one is the pinching effect implying different hysteretic curves from the first to the subsequent load cycles see Figure 9. The second one, referred to as the memory of material , is due to a dependence of the load-slip curve from the loading history. This paper shows how an understanding of the material leads to an appreciation of engineering properties and shows showing how to create clear concepts for design.
This paper Part 1 of 2 addresses the material and its properties. AB - Although new courses on timber engineering design have recently begun or are being started in most of our universities, it has not been common for engineers to be taught it in their undergraduate studies. R Harris. Abstract Although new courses on timber engineering design have recently begun or are being started in most of our universities, it has not been common for engineers to be taught it in their undergraduate studies. Fingerprint Timber.
A glued laminated member is made up of these lamellas, laid up so that the grain is parallel to the longitudinal axis. The lamellas are end-jointed by the process of finger jointing and therefore each individual laminate acts as a continuous structural element. Beams wider than the normal commercially available lamellas can be manufactured by edge-gluing after finger jointing. The lay-up of these wide laminates is arranged so that the edge joints are staggered about the central vertical axis of the component.
Glulam is one of the most widely used engineered timber components used in specialist timber framed construction projects because of its unique advantages which include:.