Structural Analysis of Historical Constructions - Modena, Lourenço & Roca (eds) © 2005 Taylor & Francis Group, London, ISBN 04 1536 379 9

A strengthening technique for timber floors using traditional materiaIs

C. Modena, M.R. Valluzzi, E. Garbin & F. da Porto Department ofConstruction and Transportations Engineering, University of Padova, lta/y

ABSTRACT: In this contribution, experimental and numerical analyses for the validation of a strengthening technique for historic timber floors are presented. The proposed method consists in placing reinforcing planks above the existing floor beams and in fixing them with a 'dry' connection by means of wooden dowels. The results of push-out tests on the connections and of bending tests on the mechanically jointed 'T' beams are presented. Several variables were taken into account in order to characterize the connections, in particular the length and diameter ofthe dowels and the possible presence of glue and screws to improve the connection. The effect of steel dowels was studied for comparison. The results were analyzed according to the Mõhler theory and by means of finite element models. The main execution phases of the strengthening technique, with limits and advantages, are described. A case study of an historie building where this method has been already applied is illustrated.

TNTRODUCTlON

The main problems affecting the structural behaviour of historie timber floors are related to the lack of bending and in-plane stiffness and to the inadequate mechanical properties ofthe existing material. Several strengthening techniques have been developed in order to increase the bending stiffness and the load bearing capacity of existing timber floors , particularly in the case of simple frame floors made of rectangular section bearing timbers.

Among them, some are based on the introduction of strengthening elements at the floor intrados, whereas others are aimed at constituting a 'T' beam section, where the original wooden beam behaves as web and a new element, placed at the floor extrados and con­nected to the existing beam, behaves as flange. In this case, the new element constituting the tee beam flange can be of various possible types and materiais. AIso the shear connection between the existing beam and the strengthening element can be of various different typologies (Barbisan & Laner 1995, Parisi & Piazza 2003). In particular, when the existing boarding has to be maintained, special shear and flexural fasteners have to be used, in order to reduce the possible loss of stiffness at the connection.

Methods of this kind are, for example, those described in Tampone (1996), based on the use of steel beams coupled to the original beams, or those based on the use of reinforced-concrete slab collaborating

911

with the existing beams (Turrini & Piazza 1983a, b, c, Laner 1995).

An intervention technique based on the use of wooden elements, for both the flange and the dow­eis used for the 'dry' connections of the web and flange of the newly constituted tee beam, has been developed in order to preserve and strengthen the existing wooden floors. This technique is not inva­sive and can be easily removed, according to the basic principies of cautious repairs (Modena 1997). It can be applied on the floor extrados and it causes only small increases of the floor thickness and dead load. Furthermore, the use of traditional materiais, whose time-dependent behaviour is well known, and the use of 'dry ' assemblage methods, benefit the durability, compatibility, reversibility and/or recoverability ofthe intervention.

The behaviour ofthe flange-to-web connection and of the strengthened beams has been experimentally investigated in order to analyse the efficiency of the proposed technique. The studies presented in this con­tribution were carried out not only on 'dry ' wooden connections, but also, for comparison, on steel dowels and glued wooden dowels. The results of the exper­imentai tests were analysed according to the Mõhler theory (1956) and by means of finite element mod­eis. The described strengthening technique has been already applied in some historical buildings, for both the load bearing capacity and the seismic upgrading of existing floors (Modena & Tempesta 1998).

Figure I . Longitudinal cross section of the proposed intervention.

Figure 2. Axonometric view of the proposed intervention.

2 THE STRENGTHENING TECHNIQUE

The strengthening technique, described in detail in Modena et aI. (1997, 1998), consists in placing above each beam ofthe existing frame, at the floor extrados, a new plank connected to the beam by means of dow­eis. The dowels are 'dry' driven into the planks at a variable spacing (Figs 1 & 2). A 'T' beam compound section, whose web (original beam) and flange (new plank) are wooden made and with deformable cormec­tion between the flange and the web, is thus obtained. Web and flange are separated by the existing boarding, having a thickness ofabout 2- 2.5 cm, which is usually not considered as part ofthe compound section.

Before carrying out the intervention, the state of conservation of wood has to be analysed and, afier the floor and floor foundation have been removed, the real state of conservation of the load bearing struc­ture has to be analysed by means of inspections and visual examination. If any portion of the boarding and/or wooden beams is rotten or has lost its load bear­ing capacity, it has to be consolidated or substituted.

Figure 3. Placing the screws and drawing the dowels' position.

Furthermore, preventive measures such as the prop­ping ofthe wooden beams, also aimed at reducing the vibration induced damage caused by the insertion of the dowels to the possible intrados decorations, have to be taken.

The stiffening planks have to be sawn before being placed on the boarding, and their intrados has to be levelled in order to allow the perfect adhesion with the existing surface. The planks can be fixed to the existing boarding by means of screws in order to facil ­itate the following intervention phases and, in case that the floor beams have not been propped, to provide adhesion between the straight planks and the perma­nently deflected floor beams. The screws have to be placed any four to six dowels, in pre-drilled counter­sunk holes. The planks have to be placed with the pith upward, in order to maintain the screws in tension even afier the wood shrinkage. Subsequently, the position where the dowels have to be inserted can be drawn onto the plank by means of a template (Fig. 3).

The dowels ' position is staggered of about half diameter from the longitudinal beam axis and the dow­eis are not aligned in order to increase the strength of the system to the longitudinal splitting (see Figs I & 2). The pre-bored holes are about 1- 2 mm smaller than the dowels' diameter. They are cleaned with compressed air before forcing the dowels by means ofhammer.

It is possible to use planks shorter than the floor span, avoiding their insertion into the load bearing walls. This solution, still being statically admissible, allows placing ali the technical installations in the floor thickness (Fig. 4) and avoids the creation of dan­gerous horizontal chases in the walls. The placement of installation and/or thermal and sound insulating panels in the empty space between adjacent planks, together with other finishing, are the final operations related to the execution ofthe described strengthening technique.

912

Figure 4. Technical installations placed between two planks.

3 EXPERlMENTAL INVESTIGATIONS ON THE BEHAVIOUR OF THE CONNECTlONS

3.1 Experimental program

A comprehensive experimental campaign aimed at characterizing the mechanical behaviour and at determining the parameters needed for a proper design of the upgrading of existing wooden floors with the proposed technique, was carried out at the Labora­tory for Structural Material Testing of the University ofPadua.

The experimental program was divided into two main phases: the first started in 1996 and was aimed at evaluating the efficiency of various types of connec­tion. Different bench wood dowels , dry driven into the tee beam section or glued with vinyl , phenol, resor­cinol or epoxy resins , fixed with or without the use of screws, were studied. Other variables in the tested specimens concerned the diameter (20, 22, 25.4 mm) and the length (110, 130, 185 mm) of the dowels. Two specimens were also tested without the board­ing between the beam and the plank. A comparison between the efficiency of bench wood dowels and stainless steel dowels was also carried out. The charac­terization was performed by means of push-out tests on the different connection types, carried out under loading control and according to the UNI-EN 26891 standard (see, for example, Fig. 5). In some cases, not only the efficiency ofthe connection, but also the behaviour ofthe entire 'T' beam was studied. Bending tests were thus carried out according to the UNI-EN 380 standard (Fig. 6). The results ofthis experimental phase can be found in Modena et aI . (1997 , 1998), and are briefly summarized in the first part ofTable I.

The second phase of the experimental program started in 2003 and was planned on the basis of the first phase results. This second part was focused on the optimization of the diameter of dry bench wood dowels. The main aim was to obtain the best mechani­cal performances from the fasteners that, despite being less performing than the glued fasteners (Modena et a!. 1997), are more easily removable and represent thus

SEZIONEA-A 4> 26 s;:: 130mm

o

~

Figure 5. Geometry and push-out test set-up for speci­men L26.

Figure 6. Geometry and bending test set-up for the 'T' beams.

the best option in the conservation viewpoint (Pomare 2003). This second phase ofthe experimental program started again with push-out tests on the connections. The studied fasteners were ali 130 mm long and they had four different diameters. Furthermore, smooth stainless steel dowels with three different diameters were tested for comparison. The connections that pre­sented the best behaviour where used to build three tee beam specimens, which were tested under flexure .

The specimens were ali made ofred pine, with dow­eis and screws spaced of five diameters, according to the requirements of the EC5. The moisture con­tent of the specimens was measured according to the UNI 9091 after their construction and before carrying out the tests. The small differences in moisture con­tent did not cause the presence of any clearance at the dowel-hole interface.

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Table I . Tested eonneetions and data obtained during the first and seeond experimental testi ng eampaigns.

Fastener type: 0 Length Fmax Fesl F1mm Vi,mod V06 Ufail Ks K06 K 1mm

Name, materi al and eonneetion mm mm N N N mm mm mm N/mm N/mm N/mm

First experimental phase Se20 wood vi nyl resin 20 130 86 12 8326 4535 0.62 1.48 11 .9 1 572 1 3603 4542 Srm20 wood phenol resin 20 130 856 1 8243 4047 0.77 1.79 7.2 1 4400 2880 4047 Sn20 wood resorein resin 20 130 9245 9667 3882 1. 16 2.44 15.00 3336 2278 3882 Sev20V wood vinyl+serews 20 130 10894 10895 5738 0.65 0.9 1 10.04 6757 6100 5738 S20 wood dry 20 130 6189 6070 1780 2.40 4.62 14.63 1033 808 1780 S20V wood serews 20 130 9042 8542 4192 0.84 2.05 15.00 4068 2650 4192 St20' wood dry 20 110 7325 7438 3482 0.87 1.78 10.96 3436 2469 3482 Set20V' wood vinyl+serews 20 110 15783 15292 0.65 0.9 1 10.04 8011 9675 Sev20" wood vinyl 20 130 4192 4236 1776 0.97 2.03 10.59 1866 1305 1776 Sv20** wood dry 20 130 4725 3772 1705 1.01 2.89 11.0 I 1495 982 1705 Se22 wood vinyl resin 22 130 9129 957 1 4139 0.99 2.04 10. 13 3973 2835 4139 S22 wood dry 22 130 6067 6625 1592 3.06 4.35 15.00 867 838 1592 SeM22 wood vinyl resin 22 185 6800 6875 28 16 1.07 2. 17 8.50 2608 1906 2816 Se26 wood vinyl resin 25.4 130 9472 9535 45 19 0.85 1.97 14.15 4933 3248 4520 S26 wood dry 25.4 130 8633 8358 1392 3.99 6. 15 15.00 837 842 1392 Be26 wood vinyl resin 25.4 110 8263 7925 3790 0.74 1.35 15.00 43 13 3686 3790 B26 wood dry 25.4 11 0 7635 7528 2090 1.73 3.73 15.00 1788 1230 2090 B26V wood serews 25.4 11 0 8415 8688 2612 2.16 3. 17 15.00 16 14 1366 26 12 Be26V wood vinyl+serews 25.4 110 8843 873 1 4002 0.79 2.09 15.00 4404 254 1 4002 Sa20 steel dry 20 130 833 1 7868 2334 2.46 5.10 15.00 1332 983 2334 Pre l4 steel epoxy resin 14 180 12374 13859 6827 0.72 1.23 15.00 7785 6060 6827

Second experimental phase L20 wood dry 20 130 5606 5994 1845 1.92 2.64 10.07 1270 1285 1845 L23 wood dry 23 130 838 1 7972 11 53 2.62 4.57 13.82 1220 1129 1153 L26 wood dry 26 130 9979 9278 2342 1.59 3.28 14.86 2337 1829 2342 L29 wood dry 29 130 10207 9833 279 1 2.46 4.25 14.6 1 1600 1445 2791 A I2 steel dry 12 130 6213 597 1 1367 3.15 4.40 15.00 759 847 1367 A I6 steel dry 16 130 7456 75 19 1273 2.88 4.48 15 .00 1065 1004 1273 A20 steel dry 20 130 93 10 9358 1204 3.86 5.78 15.00 999 974 1204

• tested without boarding in pure shear (adhesion); •• tested without boarding leaving a empty spaee.

3.2 Experimental evaluation of parameters (EA)s = EIAI + E 2A 2 (4) needed fo r the design

As above mentio ned, the push-out tests on the con- -(I 2 (EA)p Si r (5) y- +7t ·--·--2 nections were a imed at determining the parameters (EA)s Ki ·I needed for the design of the interventions. The sim-plified design method for mechanically jointed beams Pedix 1 and 2 respectively stand for the stiffening with deformable fasteners is that developed by Mõhler plank and the existing beam , d is the distance between (1956) and reported also in the EC5 (2003). In particu- the barycentre of the plank and of the beam. y is the lar, the deflections are calculated by using an effective coefficient of effic iency for the connection given by bending stiffness (EJ)ejJ determined in accordance Equation 5, where I is the beam length, Si is the dowels ' with Equation 1: spac ing and Ki is the value ofinstantaneous slip mod-

(EJ)'jf = (EJ)o + r ~~~: · d2

ulus. The coefficient of effic iency y has null value in

(1 ) case of infin itely deformable connection; it is equal to one in case of infinitely rigid connection and assumes values between O ::: y ::: I in any other case.

where: In particular, conventional values of slip modulus

(EJ)o = EIJI + E2J 2 (2) Ki for the serviceability limit state (Ks) and for the ultimate limit state (Ku) calculations were determined

(EA)p = EIAI . E 2A 2 (3) from the s lope ofthe secants to the experimentalload-d isplacement (F-v) curve (Fig . 8).

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V1ail Slip(mm)

Figure 7. Experimental curves for the definition ofthe slip moduli.

Z12,-------------------------------~

~ -g 10

.3 8

6

4

2 I"'A 12c~A 16c. A20c'" L20<> L23-<> L26 <> L291

°0~~1~2~~~·~~~~~~~~~~ 3 4 5 6 7 8 9 10 11 12 13 14 15

Slip[mm)

Figure 8. Experimental behaviour of the different connections.

From the push-out tests, the following main parame­ters were thus evaluated: maximum load at failure Fmax and maximum estimated load Fesl> modified initial slip Vi ,mod, equal to 4/3 of the difference between the slip at 40% and 10% ofthe maximum estimated load and corresponding slip modulus Ks = O, 4Festl vi mod (parameters required by the current version or' the EC5), slip at 60% of F max V06 and corresponding slip modulus KO,6 = O, 6Frnax1vo6 (parameters required by the version of the EC5 issued in 1987). Finally, also the load (F lmm ) and the slip modulus (K lmm ) corre­sponding to a slip of I mm and the slip at failure Vfail were determined, following the experiences made by Turrini & Piazza (1983a, b, c). These parameters, evaluated for ali the push-out tests carried out dur­ing the first and the second experimental phases, are summarized in Table I.

3.3 Test results

In the first experimental phase, the connections lhat showed the best behaviour were those without any boarding. They presented apure shear behaviour and the experimental results showed a very good

915

agreement with the formulation proposed by the EC5 (2003), which actually refers to direct connections between flange and web. When an empty space was left instead of the boarding, the failure turned into a combined shear-flexural collapse with lower values of slip mudulus and of coefficient of efficiency.

The glued dowels, in particular when vinyl resins were used, gave better results than dry dowels. The use of screws improved the behaviour for small displace­ments (higher initial slip modulus), but their contribute was lost for displacements higherthan 10 mm. The best results in terms of connection efficiency were obtained with the glued steel dowels, even if they presented a stronger tendency to the splitting of the hole. Among the dry connections, those made of steel and bench wood gave similar results.

In the second experimental phase, dry connections made ofbench wood dowels (L) with diameters equal to 20, 23, 26, 29 mm and length equal to 130 mm were tested. The dry connections did not proved to be the most effective during the first experimental phase, nevertheless they were sufficiently efficient and more easily removable and therefore they represent the most suitable solution to the conservation cri teria. For this reason it was decided to optimize the main parameter ofthe fastener, namely the dowel diameter. A compari­son was carried out with smooth stainless steel dowels (A) with diameters of 12, 16 and 20 mm.

The failure ofthe different bench wood connections can be seen in Fig. 10. The failure ofthe wooden dow­eis was characterized by a combined shear-flexural behaviour. The dowels with smaller diameters were characterized by dominant flexural behaviour, whereas the dowels with larger diameter were characterized by dominant shear behaviour. The best results were obtained on the specimen L26 (dowels with 26 mm diameter).

In this case the maximum load at failure Fmax was lower than for the L29 specimens, but the slip mod­uli presented the highest values, leading to a stiffer and more efficient connection. In average, the connections made with dry bench wood dowels showed a better behaviour than those made with steel dowels. The dif­ference is related to the different failure modes (see Figs 8 to 10). The wooden dowels failed in a combined shear-flexural state, with a slight splitting ofthe hole for the larger diameters, whereas the connections made with steel dowels failed for the splitting of the hole, before any deformation ofthe dowels could occur.

It has to be noted that the values of slip modulus are lower in the case of combined shear-flexural failure than in the case of pure shear failure. The latter can be evaluated by means of Equation 6, given by the EC5 (2003):

(6)

I c) d)

Figure 9. Combined shear-flexural fa ilure of bench wood fasteners: a) L20, b) L23, c) L26, d) L29.

Figure 10. Splitting failure of the hole for stainless steel fastene rs: a) A 12, b) A16, c) A20.

where Pm = density expressed in kg/m3 , ri> = diameter ofthe fasteners expressed in mm.

lt is thus possible to define a q factor, equal to the ratio of the initial slip moduli in case of combined shear-flexural failure and of pure shear failure:

K s,shear+bending q=

K s,shear (7)

Figure 1 I. Beam A at co ll apse.

The values of q found for the tested connections vary between 0.12 and 0.22. The slip modulus Ks for a combined shear-flexural fai lure is in average equal to 13-:-- 16% of the slip modulus for apure shear failure .

3.4 Bending test on mechanically jointed beams

The behaviour ofmechani cally jointed 'T' beams was experimentally analysed during both the experimen­tai phases (Modena et aI. 1997, Pomare 2003) . In the first experimental phase two tee beams with bench wood fasteners 125 mm long and 20 mm of diameter, glued wi th vinyl resins and fixed with screws, were tested.

The results showed that there is a good agree­ment between the theoretical value of the coefficient of efficiency y obtained by Equation 5 and that experimentally obtained by inverting Equation I. This confirmed the possibility of carrying out push-out tests in order to characterize the mechanical behaviour of the mechanically jointed beam with deformable connections.

In the second experimental phase the attention was focused on the connection that gave the best result during the push-out tests . Therefore, three tee beams (beam A, B, C) with dry bench wood faste ners 26 mm of diameter were built and tested.

The three specimens were loaded up to about 37 kN; specimens A and C were subsequently loaded again up to lhe maximum strength. A fairly elastic behaviour was observed during the fi rst loading cycle. In the second cycle, the fai lure occurred in the field of large displacements, at an average Ioad of 43.90 kN. In par­ticular, the beam collapsed at midspan and a crack, following the fibres direction, developed through the beam (Fig. 1 I). The chosen L26 fasteners showed a good behaviour. Only at the maximum load they presented small vertical displacements and a slight splitting of the holes. The global flexural behaviour of the tested specimens is shown in Figure 17.

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Z 40

'" :; 35

'" .3 30

25

20

15

10

O O 2 4

___ o--'

<> Slip ai end af beam

<>Slip ai lhe laad painl

o Slip in belween

8 10 12 14 16 18 20 22 24 26 Slip[mm]

Figure 12. Relative horizontal displacements between the beam and the plank.

Figure 13. Fasteners along the beam afterthe test, it can be noted the deformation of the dowel c10se by lhe beam end.

During the test, the relative displacements between the beam and the stiffening plank were measured. According to the adopted design method, the fasteners should be ali stressed by equal shear forces, which imply equal displacements. Conversely, the relative horizontal displacements measured under the loading points (position 4 and 5 in Fig. 6), at the beam ends (pos. 8 and 9 in Fig. 6) and in between the loading points and the beam ends (pos. 10 and Ii in Fig. 6) were smaller close by the loading points, probably due to some friction (Fig. 12). The fasteners close by the beam ends presented larger deformations than those placed at midspan, confirming the presence of a frictional component (Fig. 13).

Again, a comparison was carried out between the valueofy obtained by Equation 5 assumingKj = KS.L26 and that experimentally obtained by inverting Equa­tion 1. Figure 14 shows the comparison among the coefficient of efficiency of the cOlmections. It can be noted that the actual efficiency for this connection is similar to the theoretical one in the serviceability state (applied load smaller or equal to 10 kN). The nega­tive values of y means that the experimental bending stiffness is lower than the theoretical bending stiffness, evaluated under the hypotheses of Equation 2.

A trend very similar to that detected in the case of the coefficient of efficiency was also found for the bending stiffness (Fig. 17). A comparison was made

?- 0.5

0.4

0.3 o o-

0.2

0.1

o

-0.1

-0.2 1-0 Theoretic 11 Beam A

o 10 15 20 25 30 35 40 Load [kN]

Figure 14. Comparison among the coefficients of efficiency.

Table 2. Material properties.

EI Element [MPa]

E2 [MPa]

Beam Plank

11000 367 11000 367

E) [MPa]

367 367

GI2 [MPa]

687 687

VI 2.!3

0.5 0.5

0.03 0.03

among the bending stiffness determined according to Equation I , assuming the longitudinal Young modulus E = II GPa and the initial slip modulus Kj = Ks.L26,

and the bending stiffness that can be deduced from the experimental data, expressed by the following Equation 8:

where a is the shear span, equal to 130 cm for the assumed loading condition (se e also Fig. 6), I is the loaded span, equal to 380 cm, Fn is the maximum load at the n-step and fm,n is lhe deflection at lhe n-step, from which the components due to wood crushing at lhe sup­ports and to shear deflection (generally negligible) are subtracted.

4 NUMERICAL MODELLING

A finite element model was developed in order to idenlify the behaviour of the strengthened beam. The beam was modelled with four nodes plate elements. The beam and the plank were modelled using the same orthotropic elastic linear material, assuming lhe properties of red pine (Table 2). The boarding, whose influence is included inlo the connection behaviour, was not modelled.

The connection between lhe beam and lhe strength­ening plank was modelled by means of non­linear springs with six DOE The law deriving from

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Z12,----------------------------------, ~

~ 10 .3

8

6

4

2 --- L26 a 1 --- L26 a2 D L26 c1 -o- L26 c2

o~----~--~---=~~~~----~ O 5 10 15 20 25

Figure 15. Law for the L26 bench wood dowel.

.(I.6201Jo9 .­

.O_S\l:n.~PU! )

Figure 16. Numerical mode!.

Slip [mm]

N-100,---------------------------------~

~ 90 z 80 -o 70

'àl 60 ;;] 50

40

c

30 20i ~----------------------__,

-o-Theoretic - /I- Beam A 10 -oBeam C -<> Fem model O ~~~~~~==~====~------~

o 5 10 15 20 25 30 35 40 Load [kN]

Figure 17. Comparison between experimental and calcu­lated effective bending stiffness.

push-out tests (Fig. 15) was assigned to the two directions that correspond to the relative displacement between beam and plank.

The numerical model (Fig. 16) gave conservative results. The obtained bending stiffness was lower than the experimental one measured during the first loading steps, whereas the behaviour ofthe strengthened beam was very well simulated during the last loading steps before the collapse (Fig. 17). This difference between the numerical model and the experimental behaviour is probably due to the relevant frictional component which is developed in the real beam during the first loading steps. The law assumed to model the behaviour ofthe connection, in fact, is obtained by means of push­out tests, during which friction gives no contribute.

~ 40 ,----------------------------------,

~ 35 o

...J 30

25

20

15

10

5 o Beam B -o-Beam C -o-FEM Beamj

0~~==~~==~==r==T==~~==~~ o 10 20 30 40 50 60 70 80 90 100

Def lection [mm]

Figure 18. Loads versus displacements diagrams. Compar­ison between experimental and calculated defection.

The model hence gives a first interesting evalua­tion of the behaviour of a strengthened beam, even if it needs further improvement in order to better repro­duce the beam flexural behaviour. Micro-models of the connections and macro-models ofthe strengthened beam, where friction and plasticization are taken into account, are currently under development.

5 INTERVENTION ON BUILDINGS

5.1 In-plane stiffening and details

When the intervention has to be applied on the floors of an existing building, not only the load bearing capacity and flexural behaviour of the floors have to be taken into account, but also the influence that the floors have on the global behaviour of the building. In particular, a common requirement for floors of buildings placed in seismic prone areas is that of high in-plane stiff­ness. Actually, on-site surveys on buildings damaged by earthquakes and large experimental investigations (Tomazevic et aI. 1994, Benedetti & Pezzoli 1996) demonstrated that, besides the in-plane stiffness ofthe floors , also other requirements, such as good connec­tions between floors and walls and between orthogonal walls and a good overall quality of the construction, are ofbasic importance.

In this framework, some simple developments ofthe proposed strengthening techniqne were proposed for those buildings where not only an upgrade ofthe load bearing capacity or an improvement of the flexural behaviour ofthe floors was requested. These develop­ments were still based on the use of dry wood connec­tions, limiting the presence of other materiais. The use of stainless steel was proposed for anchoring ties.

To increase the in-plane stiffness, the application of a second boarding onto the planks was conceived (Fig. 19a, b). AIso in this case, the use of dry wooden fasteners was proposed to make the existing beam and boarding collaborating with the new plank and

91 8

b)

c)

ISOI "" I 100 1 '00 I 100 I 100 I 100 I "" lso l 80 I

I "" 1 _

a)

stalnless steer tle bar nalleel to lhe pranck 5ma!! beech wood pin beech woocI pin

Figure 19. Use of a second boarding for the in-plane stiffening of the wooden floors.

Pionta o 10 20 30 ~o ~ : : !: :

Figure 20. Steel anchors for the floor-to-walls connection.

boarding (Fig. 19c, Fig. 20). To improve the connec­tion between floors and walls , the use of simple steel anchors fixed with steel anchor plates or bonded to masonry was proposed (Fig. 20).

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Figure 2 I. Tripartite plan with simple frame wooden floors.

5.2 Case study: Ca' Duodo Palace

The presented strengthening technique was applied for the first time in 1996, in the framework of some conservation work on the fifteenth-century complex Ca'Duodo. The palace is placed in the province of Padua and is a relevant example ofVenetian late gothic style (Modena & Tempesta 1998). The building has three floors and a typical plan divided into three parts (Fig.21).

The strengthening intervention was carried out on ali floors at first and second floor, for a total sur­face of 553 m2

. The 240 red pine beams presented a slender section, with width and height measuring respectively 11-:-12cm and 23-:-24cm. The floors ' span varied between 412 and 546 cm; the distance of beams varied between 54 and 58 cm. The floor intrados was provided withjoint covering strips and interesting tempera paintings representing geometrical, floral and mask pattem (Fig. 22). Preliminary investigations con­firmed that the wood was in good condition, also near by the beams' ends. The admissible stresses and the strength class were assigned according to the UNI EN 338 standard.

The design method adopted for the strengthening intervention is that given by the EC5 for the mechan­ically jointed 'T' beam. The connections between beams and planks were ma de by means ofhard-wood dowels with diameter equal to 22 mm coupled with

Figure 22. Intrados of the fl oors at Ca' Ouodo.

screws with diameter of 10 mm and length of200 mm. The beams with 560 cm span were strengthened by means of 8 x 25 cm planks. The design load was a uniform distributed load equal to 3.65 kN/m2 and the section was verified placing 30 fasteners with spac­ing variable from 11 to 30 cm. The beams with 430 cm span were strengthened by means of 5 x 25 cm planks. The design load was a uniform distributed load equal to 4.80 kN/m2 and the section was verified placing 24 fasteners with spacing variable from 11 to 30 cm. The intervention procedure followed the steps described in section 2.

6 CONCLUSIONS

In the present contribution, a strengthening technique fo r the upgrading of the load bearing capacity and for the improvement of the flexural behaviour of existing wooden floors is presented. The proposed technique is completely based on the use oftraditional materiais such as wood and on 'dry' installation procedures. lt thus respects the conservation criteria, gives warranty of complete compatibility with the historic structures and at the same time it ensures durability, reversibil ity/ removeability and low invasiveness. In particular, the proposed strengthening technique can be very useful when it is necessary to preserve decorations at the floor intrados.

The technique, which consists on the use of stiffen­ing planks connected by means of wooden fas teners to the existing beams, preserve the thickness and the lightness of the existing structures and allows a strong reduction of dead load if compared to similar techniques baseei, fo r example, on the use of an extra­dos reinforced concrete slab. Also the placement of

technical installation can be easily solved when this technique is adopted.

The efficiency of the proposed technique was assessed by means of experimental tests, through which it was also possible to evaluate and compare dif­fe rent typologies offasteners. Ali the main parameters needed for the design of the strengthening interven­tion were defined. The behaviour of the strengthened beams was also studied by means of finite element analyses.

If simple improvements to increase the in-plane stiffness of the strengthened floors and if lhe use of steel anchors is taken into account, the proposed tech­nique can be applied also fo r the seismic upgrading of buildings.

The research is still in progress with the main aim of calibrating the design parameters by means of experimental tests and numerical analyses. Further­more, the short and long term behaviour under static and cyclic loading is currently under study, together with the possi ble application of mixed strengthening techniques.

ACKNOWLEDGEMENTS

The authors want to acknowledge the company 'F ll i Bozza Legnami ' that supplied the wood and prepared the specimens used for the experimental research. Eng. Dario Francescato and Carlo Bettio and ali the tech­nical staff at the Laboratory for Structural Material Testing at lhe Department of Structura l and Trans­portation Engineering at the University of Padua are gratefullyacknowledged.

REFERENCES

Barbisan U. & Laner F ( 1995) "I solai in legno", Franco Angeli , Milano

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