Structural Analysis of Historic Construction – D’Ayala & Fodde (eds)© 2008 Taylor & Francis Group, London, ISBN 978-0-415-46872-5

Experimental assessment and modelling of in-plane behaviour oftimber floors

M.R. Valluzzi, E. Garbin, M. Dalla Benetta & C. ModenaDepartment of Structural and Transportation Engineering, University of Padua, Italy

ABSTRACT: In the paper, the assessment of the in-plane behavior of timber floor samples is proposed byexperimental laboratory tests, aimed at characterizing the basic properties of simple construction floor typologies,common in existing masonry buildings, for the proper selection of structural improvement solutions. On the basisof the analysis of the testing procedures and systems proposed in literature, a specific set-up has been designedand realized, in order to minimize undesirable and/or uncertain effects, able to simulate both monotonic andcyclic shear behaviour. The first experimental results on unreinforced floors are discussed, together with thecalibration of a preliminary FE model.

1 INTRODUCTION

The adoption of timber for floors and roofs has beencontinuously considered to realize horizontal struc-tural components, both in old and current buildings,thanks to their good structural and comfort perfor-mances, and the easy production and workmanship(Tampone 1996, Giordano 1999, Barbisan&Laner1995). Nevertheless, especially in seismic area, dueto the low in-plane stiffness and the probable lack ofeffective connections to the main walls, timber floorsand roofs have been subjected to obtrusive upgrad-ing interventions or even overall substitutions, mainlyobtained with reinforced concrete slabs. Experiencesacquired during the on-site inspections after the mostrecent earthquakes in Italy, demonstrated the high inef-ficiency of this kind of generalized interventions, asthey caused often new and more brittle collapses (Giuf-frè 1993, Modena 1997, Valluzzi et al. 2001, Bindaet al. 2006).

Therefore, in the last decade, many research workshave been focused on the characterization of morecompatible techniques, able to strengthen and reha-bilitate original timber floors (Modena et al. 1998 and2004, Giuriani 2004, Liberatore et al. 2004, Piazzaet al. 2005, Corradi et al. 2006, Gattesco et al. 2007,),and to provide a suitable “box” behaviour (Tomaževic1999, OPCM 3431 2005).

In this context, the study of the influence ofdeformable floors on the seismic behaviour of existingmasonry buildings has not been sufficiently inves-tigated up to now, both at experimental (Tomaževic1991, Mezzina&Dentamaro 1997, Galasco et al.2001, Liberatore et al. 2004, Abrams&Tena-Colunga

1994, Corradi et al. 2006) and numerical level(Moon&Lee 1994, Cohen et al. 2001, Lee et al. 2002,Kim&White 2004).

Moreover, only few works are available on therole of the connections between vertical and horizon-tal structural elements in the dynamic behaviour ofmasonry buildings (Tomaževic 1999, Giuriani 2004,Foraboschi 2005, Sorrentino et al. 2007).

In particular, some laboratory experimental cam-paigns have been performed on full scale floor sam-ples or elements, subjected to in-plane monotonic orcyclic loads (Modena et al. 1998, Giuriani & Plizzari2003, Peralta et al. 2004, Corradi et al. 2006). Theypropose various set-up, related to different basis con-ception. In Modena et al. (1998), a simple diago-nal test on square samples of timber floor portions(125 × 125 cm), rotated of 45◦ and positioned ver-tically under a press, was used. This test, usuallystandardized for masonry walls, is very easy to per-form and avoids undesirable confining and frictionaleffects; however, it is not completely representativeof the real floor shear behaviour, due to the lim-ited geometrical dimensions of the sample, the wayhow the load is applied, and the probable instabilityof the boarding. Moreover, only monotonic test canbe easily performed. Alternatively, test set-up usingfull-scale specimens laying horizontally were devel-oped by Giuriani & Plizzari (2003) and Corradi et al.(2006). These shear testing systems allow monotonicand cyclic tests without instability of the boarding andby minimizing frictional effects. Anyhow, they seemproviding a not negligible confining effect, due to thepresence of active steel frames, that force the shearbehaviour all around the floor specimen. In Peralta

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et al. (2004), a monotonic and cyclic bending testset-up on horizontal floor samples, treated as simplesupported or double fixed ends beam, was presented.This procedure enables the investigation of the shearbehaviour of the floor as a part of the flexural defor-mation, as in the beam model of Timoshenko (1934).By this system it is possible to perform tests withoutthe disadvantage above, but it requires specimens inreal scale with large dimensions (about 4 × 8 m, as inPeralta et al. 2004).

Within this context, an experimental program hasbeen planned at the University of Padua, in order toevaluate the shear behaviour of original and strength-ened timber floors, by using traditional materials. Anew test set-up has been designed and realized, inorder to perform both monotonic and cyclic tests, andby minimizing many undesired effects and the dimen-sional limitations. The system has been preliminarytested with two pilot floor samples, made by fivebeams and two types of boarding (raw-finished andtongue-and-groove shaped in the thickness). Follow-ing this preliminary phase, one of the two boardingsystems will be used as basic sample for variousstrengthened conditions (double-boarding, diagonalmetal stripe or timber board, etc.).

In the paper, the description of the test set-up andthe results of the preliminary pilot samples are pre-sented. The local behaviour measured during the testshas been used to calibrate a numerical FE model, ableto reproduce the global behaviour of the floor, to beused for predictive analysis on the further plannedtests. The validation of the system is also discussed onthe basis of reliable drifts expected in masonry build-ings subjected to horizontal cyclic loads, as availablein literature.

2 EXPERIMENTAL PROCEDURE

2.1 Test set-up

For real scale floor specimens, a specific testingmachine was on purpose designed and realized, toadequately simulate the monotonic and cyclic shearbehaviour of floors.

Starting from the typical advantages of the verticaldiagonal test, the proposed system consists in a ver-tical shear-bending test similar to the configurationsused for testing woodframe shearwalls (Gatto & Uang2003). The structural scheme provides three simplesupports applied at the floor base (Figure 1); it is equiv-alent to a cantilever beam, which can apply bendingand shear deformations, as set in theTimoshenko beamtheory (1934). Thanks to this structural scheme, thegeometrical dimensions of the floor specimens couldbe almost half of those used in Peralta et al. (2004),and about a quarter of those usually present in existingmasonry houses.

Figure 1. Design of test set-up and structural schemeprovided by steel frame.

The steel articulated quadrilateral was designedsuch that out-of-plane motion at the specimen bound-aries (bottom, lateral and top) is prevented, as ithappens in buildings. On the contrary, it allows upliftand in-plane deformations of the sample, by minimiz-ing frictional effects. Floor specimens are inserted inthe steel frame with the beams placed horizontally,thus allowing the free rotation of the boarding.

The bottom joist is fully fixed to the bottom steelreaction beam by means of bolts. The steel reactionbeam is connected to the steel basement trough threeload cells by means of mechanical sliding connections,thus providing the three simple supports. In the hori-zontal direction, a hydraulic actuator was used, to loadthe specimens at the level of the top beam.Tests are per-formed in displacement control, and the force appliedat the top of the samples is measured through a loadcell. Finally, horizontal and vertical load cells apply theshear and the bending moment to the floor specimenin its plane.

In Figures 1 and 2, the main characteristics of thetest system and some views of the experiment aregiven.

A suitable instrumentation was placed on both sidesof each specimen, to capture the global and localshear-bending behaviour. Several transducers wereused to record horizontal, vertical and diagonal relativedisplacements (Figure 3).

2.2 Floor specimens

The pilot tests were performed on two timber floorelements (2,2 × 2,2 m), namely F1.M and F2.M, built

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Figure 2. View of testing machine with sample, and detailof connection of base beam to frame.

Figure 3. Position of transducers on samples: local (a) andglobal (b) quadrilateral on boarding side; global quadrilateralon back boarding side (c).

Figure 4. Dimensions of floor specimens F1.M and F2, asconnected to steel frame.

Figure 5. Detail of boardings: (a) common raw-finished(F1.M sample); (b) tongue-and-groove shaped thickness(F2.M sample).

with components in real size. The samples reproducea common typology of mono-directional floors, com-posed by simple supported beams and a transversalboarding. In particular, five beams 12 × 14 cm in sec-tion, and a basic boarding 13,5 cm wide and 2 cm thick,were used (Figure 4). Spruce wood was used for theconstruction of all the elements. Each board was joinedto every beam by means of 2 nails Ø2,75 × 60 mm, fora total of 32 nails for each beam and 10 nails for eachboard.

Specimen F1.M is made with common raw-finished boarding, whereas F2.M has a tongue-and-groove shaped connection in the boarding thickness(Figure 5).

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Figure 6. Geometry of the push-out samples and view oftest.

Figure 7. View of sample after test: steel connections failunder pure shear followed by unthreading of nails at ultimatestage.

3 EXPERIMENTAL RESULTS AND ANALYSIS

3.1 Characterization of materials and connections

Preliminary laboratory tests were performed to char-acterize the materials of components and their connec-tions, made by steel nails. The mechanical propertiesof the spruce wood used in the floor specimens were:455 kg/m3 as volume mass, 44 N/mm2 as compres-sive strength, 66 N/mm2 as flexural strength, whereasa longitudinal elastic modulus of 11000 N/mm2 wasestimated, in agreement with Giordano (1999).

The characterization of the connection was per-formed by means of push-out tests carried out underloading control and according to the UNI-EN 26891standard (Figure 6). The spacing of the nails was inagreement to the requirements of the Eurocode 5.The moisture content of the specimens was measuredaccording to the UNI 9091 after their constructionand before carrying out the tests. The mean value wasaround 12%.

To simulate the connection made by nails betweenthe beam and the basic boarding, three specimensbuilt with raw-finished boards where tested. Theconnection behaves under pure shear, as shown inFigure 7. According to Eurocode 5, the followingmain parameters were evaluated: maximum load atfailure Fmax and maximum estimated load Fest, modi-fied initial slip νi,mod, and corresponding slip modulusKs = 0,4Fest/νi,mod. Results are given in Table 1 and in

Table 1. Results obtained from tested connections.

PO.F1.M.a PO.F1.M.b PO.F1.M.c Average

Fmax (N) 576 575 561 571Fest (N) 592 592 592 592νi,mod (mm) 0,351 0,489 0,587 0,476Ks (N/mm) 748 539 477 588

Figure 8. Experimental behaviour of connections.

Figure 9. Deformation of specimen F1.M at maximumdisplacement.

Figure 8. The connection show a good initial stiffness,followed by a sudden lower branch, due to the yieldingof the nails at the beam-boarding interface.

3.2 Test results and analysis of floor specimens

Two preliminary monotonic tests were performedto validate the new test set-up and investigate thebehaviour of the unstrengthened floor specimens F1.Mand F2.M, that constitute the basic floor configurationto which the strengthening methods will be applied.Also in this phase, the moisture content of the floorspecimens was measured; according to UNI 9091,after their construction and before carrying out thetests, the mean value was around 12%.

The final phase of a test is depicted in Figure 9,whereas the load-displacement trend of the two

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Figure 10. Comparison among load displacement curves ofspecimens F1.M and F2.M, and schematization proposed byUNI EN 12512.

Table 2. Results of shear test on floor specimens.

F1.M F2.M

Fmax (30 mm) (N) 1047 1435Fy,est (N) 774 901Vy,est (mm) 8,60 2,67Ki = tgα (N/mm) 80,93 288,29tgβ (N/mm) 13,49 19,22tgα/tgβ – 6 15

samples is graphically compared in Figure 10. Bothspecimens showed a failure characterized by a relativerotation between the boards and the beams (Figure 9),which involved the resistant mechanism of the momentof the couple provided by the pair of nails fastened ateach beam.

The contribution of additional friction due to thetongue-and-grove connection during the relative slipat the board sides, influenced the better performance ofthe F2.M sample, in comparison with the F1.M one. Byconsidering the conventional displacement of 30 mmas reference (UNI EN 12512), an increment of strengthof 37% was found (Figure 10 and Table 2).

To analyze the results, by assuming that monotoniccurves usually constitute the envelope of cyclic tests,for general timber structure built with metal fasteners,the UNI EN 12512 standard can be also used (Ceccottiet al. 2005).

The results are summarized in Table 2 and repre-sented in Figure 10, where symbols are as follows:Fmax is the maximum load at 300 mm of displacement,Fy,est is the estimated load at yielding, Vy,est is the dis-placement correspondent to Fy,est , and α and β are theangles of the slope of first and the second branch ofthe rectified curve, respectively, built as described inthe box of Figure 10.

Also in terms of yielding load the F2.M sample per-formed better (Table 2), with an increment of 16%of the reference load. Moreover, it showed an initial

Figure 11. Comparison between in-plane floor and in-planemasonry wall deformations.

and an ultimate stiffness respectively 256% and 42%higher than those provided by F1.M.

A first estimation of the effect of the in-plane defor-mation of timber floors in the behaviour of masonrysubjected to lateral actions can be also suggested,by considering a simplified case, represented by amasonry box having common dimensions of walls(4 m long and 3 m high, as in Figure 11) and coveredby a plain floor. The estimation of drifts (δ) normal-ized to the height of the wall (H) for masonry piersavailable from literature, led to the following values:0,2 ÷ 0,3% is the common range for the achievementof the first cracking, 0,4% for the shear cracking,0,5 ÷ 0,6% for the attainment of the maximum load,whereas 0,8% refers to the rocking behaviour (da Porto2005, Tomaževic et al. 1996, Tomaževic & Lutman1996, Tomaževic 1999).

By dividing the shear load by the span length(L = 4 m), and referring it to the ratio between themaximum relative floor shear displacement (δf ) andthe height of the wall (H = 3 m), see box in Figure 11,the previous limits become: 0,23% for first cracking;0,30% for shear; 0,45% for maximum load; and 0,60%for rocking. It can be seen how the already poor capa-bility in redistributing the horizontal seismic forces isworsened by the loss of shear stiffness in correspon-dence of the damage zone of masonry walls, when theredistribution needs are very desirable to assure a goodseismic behaviour (Figure11).

Since the F2.M sample showed better performancesthan the F1.M one, its results will be considered forthe calibration of a FE numerical model, as describedin the following. Moreover, the boarding with tongue-and-groove connection will be used as basic type forthe planned strengthened cases.

4 NUMERICAL ANALYSIS

A simplified numerical model was developed in orderto identify the behaviour of the plain sample F2.M.

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Figure 12. Numerical model: (a) detail of connectionbetween board and beam, (b) general view of frame.

Figure 13. Comparison between experimental and numeri-cal data for the two small diagonal transducer 3 and 4 locatedon boarding side.

The beams were modelled with two-node beam ele-ments, while for the boards four-node shell elementswere used. The two nails fastening each board at eachbeam were modelled by two non-linear connection ele-ments with six DOF. These elements were joined to thebeam ones through rigid-link elements. The frictionaleffect between boards was modelled by means of non-linear contact points, working only in compression(Figure 12).

The beams and the boards were modelled byusing the same orthotropic elastic linear mate-rial, by assuming the properties of spruce wood:E1 = 11000 N/mm2 is the longitudinal elastic mod-ulus; E2, E3 = 367 N/mm2 are the transversal elas-tic moduli; G12 = 687 N/mm2 is the shear modulus;ν12,13 = 0,46, and ν21,31 = 0,03, are the Poisson coef-ficients in the combined directions 1,2,3 (see Fig. 12for reference).

The law derived from push-out tests (Figure 8) wasassigned to the two directions, as relative displacementof the connections elements between beam and boards.Finally, the steel frame was modelled as isotropic elas-tic (elastic modulus E = 210000 N/mm2, and Poissoncoefficient ν = 0,2).

The estimations of the model were compared withthe results of the experimental tests at different levels.First, the load-displacement curves obtained locally

Figure 14. Comparison between experimental and numeri-cal data for the two big diagonal transducer 7 and 8 locatedon the boarding side.

Figure 15. Comparison between experimental and numeri-cal data for the two big diagonal transducer 11 and 12 locatedon the back boarding side.

at every diagonal transducer (see Figure 3) were ana-lyzed: results are depicted through Figures 13 to 15.Then, the global drift of the whole specimen was con-sidered (Figure 16). It can be seen a good agreementat both levels, as the numerical modelled curves areable to reproduce the shear behaviour of the sample.

The model, sufficiently calibrated on experimentalbasis, was implemented for the analysis of the wholefloor sample, as given in Figure 17.

It will be used for predictive analyses on thestrengthened configurations, planned for the prosecu-tion of the research.

5 CONCLUSIONS

The preliminary results of an experimental campaignaimed at characterizing the mechanical behaviour oftimber floor specimens in different strengthened con-ditions have been presented. A new set-up has beenrealized, able to perform monotonic and cyclic testsby keeping the sample vertical, and to reduce severalproblems during the test execution.

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Figure 16. Comparison of experimental and numericalresults of shear behaviour of F2.M floor sample.

(a)

(b)

Figure 17. Principal stresses at 30 mm displacement:(a) tension, (b) compression.

Monotonic tests on two plain floor samples haveevidenced the better performances of tongue-and-grove connections among boards. The analysis ofresults pointed out the need of strengthening andstiffness of the horizontal structures, when floors areconsidered restrained as in common buildings.

A simplified numerical model, calibrated with theexperimental results, has shown a very good agree-ment with the experiments, both at local and globallevel, and will be considered for the design and thepredictive analysis on the further samples.

The research will prosecute with monotonic andcyclic tests on timber floor panels strengthened withdifferent methods (double boardings, metal diagonalstrips or diagonal board, etc.).

ACKNOWLEDGEMENTS

The authors wish to acknowledge the company “F.lliBozza Legnami” of Busa di Vigonza (Padova), forsupplying the wood and preparing the specimens.Thanks also to F. Mariga and M. Novello for the dataprocessing, and to the technical staff of the Labora-tory for Structural Material Testing at the Departmentof Structural and Transportation Engineering of theUniversity of Padova. This paper has been partiallysupported by RELUIS (Rete dei laboratori universi-tari di ingegneria sismica) in the framework of theNational executive project 2005–2008 (AttuazioneAccordo di Programma Quadro DPC-Reluis del15 Marzo 2005).

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