Test report on cyclic behaviour of replicated timber diaphragms representing a detached house

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Test report on cyclic behaviour of replicated timber diaphragms representing a detached

house

Ravenshorst, Geert; Mirra, Michele

Publication date

2017

Document Version

Final published version

Citation (APA)

Ravenshorst, G., & Mirra, M. (2017). Test report on cyclic behaviour of replicated timber diaphragms

representing a detached house. Delft University of Technology.

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To cite this publication, please use the final published version (if applicable).

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(g.j.p.ravenshorst@tudelft.nl)

TU Delft Large-scale testing campaign 2016 – WP4

TEST REPORT ON CYCLIC BEHAVIOUR OF

REPLICATED TIMBER DIAPHRAGMS

REPRESENTING A DETACHED HOUSE

Authors: Geert J.P. Ravenshorst, Michele Mirra

Cite as: Ravenshorst, G. J. P., Mirra, M. Test report on cyclic behaviour of replicated timber diaphragms representing a detached house. Report no. C31B67WP4-14, 30th_{December 2017. Delft University of} Technology.

This document is made available via the website ‘Structural Response to Earthquakes’ and the TU Delft repository. While citing, please verify if there are recent updates of this research in the form of scientific papers.

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system of any nature, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of TU Delft.

TU Delft and those who have contributed to this publication did exercise the greatest care in putting together this publication. This report will be available as-is, and TU Delft makes no representations of warranties of any kind concerning this Report. This includes, without limitation, fitness for a particular purpose, non-infringement, absence of latent or other defects, accuracy, or the presence or absence of errors, whether or not discoverable. Except to the extent required by applicable law, in no event will TU Delft be liable for on any legal theory for any special, incidental consequential, punitive or exemplary damages arising out of the use of this report.

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Version 1 – Final 30/12/2017

1 Introduction

The in-plane strength and stiffness as well as the hysteretic behaviour of timber diaphragms can be investigated by performing quasi-static cyclic tests. In this report the behaviour of traditional timber floor and roof configurations was studied representing a detached masonry house with timber floors and roofs. The configurations are based on a case study from a detached Groningen house that was demolished. Before the demolition samples from approximately 1.5 m by 1.5 m could be extracted from floors and the roof.

Characteristics of the demolished house (Rengersweg 11 Godlinze) were: - Detached house, built in 1920

- At ground storey double wythe clay masonry, at first storey single leaf gables of clay masonry - Timber floors built-up with joists and planks nailed on top of the joists.

- Timber roof with vertical rafters spanning from wall plate to top beam, small horizontal purlins on rafters with vertical planks nailed to the purlins.

Based on the extracted samples the configurations and material properties could be determined. Based on these findings timber diaphragms (floors and roof) were replicated, built from new timber material and fasteners, with the same material properties as found in the extracted samples.

Replicated specimens of approximately 4.0 m by 2.75 m were prepared and tested. After the testing of the replicated specimens representing the situation in practice (non-strengthened), the specimens were strengthened with plywood panels screwed on the timber planks.

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2 Test specimens

2.1 Overview of test specimens

In this report the tests on 5 diaphragms are reported representing 4 floor specimens and 1 sample representing a roof.

In table 1 an overview of the test specimens and the performed tests are given. Table 1: Tested specimens

Specimen

code description Strengthening Test as-built strengthening Test after DFpar-1 Floor sample loaded parallel to the joists Plywood panels Y Y DFpar-2 Floor sample loaded parallel to the joists Plywood panels Y Y DFper-3a Floor sample loaded perpendicular to the joists Plywood panels Y Y DFper-3b Floor sample loaded perpendicular to the joists _{and wood blocks}Plywood panels Y Y DRpar-4 Roof sample loaded parallel to the purlins _{and steel angles}Plywood panels Y Y

2.2 Background for the determination of the configuration of the

test specimen

In figure 1 the principle for the determination of the configuration of the test diaphragm samples representing the traditional floors of detached houses is shown. Based on the expected deformation of the floors under horizontal loading it was decided to test the diaphragms in a cantilever test set-up, representing half of the floor. In this way economic use of the existing in-plane set-up at the TU Delft could be used, and the requested information can be retrieved from the tests. The diaphragm test size that could be tested was approximately 4.0 m by 2.75 m.

In figure 2 the principle for the determination of the configuration of the test diaphragm sample representing a roof in a detached house is shown. The roof consists of two clamped diaphragms, fastened into the facade wall. The test specimen therefore represents one clamped roof diaphragm (pitch).

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Version 1 – Final 30/12/2017 Figure 2 – Principle for the configuration of the tested samples representing roof diaphragms

2.3 Description of the test specimen

2.3.1 Specimen DFpar-1

The configuration of specimen DFpar-1 before and after strengthening is given in figure 3.

The specimen is made up of spruce floorplanks 18 mm x 165 mm, joists 60 mm x 130 mm, connected with nails with a diameter of 3 mm. At the top 2 joists are used, to give enough room for the planks to rotate. The planks were connected to the joists with two nails on every joist. The two nails on a plank had a spacing of approximately 100 mm. This guidance has been given to the carpenter. The position of the nails had not be exactly pointed out on the planks, but will placed on the carpenters judgement, to obtain the scatter in spacing observed in practice. The planks have tongue and grooves: to represent the observed situations in reality, the tongues were not fully pushed to the side of the next plank, but a gap of approximately 2 mm was left.

At the bottom, on both sides of the planks, 2 layers of plywood panels were glued to ensure the clamping of the planks themselves. The bottom parts of these plywood panels were glued to a steel beam on which the diaphragm was positioned for testing.

After the test of the non-strengthened configuration, the diaphragm was strengthened with plywood panels of approximately 600 mm x 1200 mm x 18 mm. This size represents panels that in practice can be easily handled inside houses. The panels are cut with the intention that at the end they overlap half of a plank. In that way the force transfer from one panel to the other is done within a plank. However, for this diaphragm the panels were sometimes overlapping the entire plank, where the screws become positioned in the tongue of a plank.

The panels were screwed to the planks around the edges of the panels with screws with a diameter of 4.5 mm x 40 mm length. The plywood panels were predrilled with a hole of 3.5 mm before application of the screws. Only at the top the panels were screwed through the planks to the top joist with a diameter of 5 mm x 70 mm length.

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Version 1 – Final 30/12/2017 3.87 0. 65 0. 65 0. 65 0. 35 0. 35 2. 76 _{D 3.5 x 65} 0. 35 2. 76 1. 20 1. 10 0. 60 1. 20 0. 50 0.59 0.67 0.67 0.67 0.67 0.59

Figure 3 – Configuration of specimen DFpar-1: non-strengthened (above) and after strengthening (below)

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Version 1 – Final 30/12/2017 2.3.2 Specimen DFpar-2

The configuration of specimen DFpar-2 before and after strengthening is given in figure 5: this floor was tested with the same configuration of the previous DFpar-1, but with some slight differences in the dimensions.

The specimen was made up of spruce floorplanks 24 mm x 165 mm, joists 60 mm x 130 mm, connected with nails with a diameter of 3 mm. At the top 2 joists are used, to give enough room for the planks to rotate.

The planks were connected to the joists with two nails on every joist; like in the previous case the two nails on a plank had a spacing of approximately 100 mm and a gap of approximately 2 mm was left between the tongue and groove of each plank.

At the bottom, on both sides of the planks, two layers of plywood panels were glued to ensure the clamping of the planks. The bottom parts of these plywood panels were glued to a steel beam on which the diaphragm was positioned for testing.

After the test of the non-strengthened diaphragm, also in this case the strengthening intervention was done with plywood panels of approximately 600 mm x 1200 mm x 18 mm. The panels are cut with the intention that at the end they overlap half of a plank. In that way the force transfer from one panel to the other is done within a plank. In this case this detail was correctly executed.

The panels were screwed to the planks around the edges of the panels with screws with a diameter of 5 mm x 60 mm length. In this case self-tapping screws were used without predrilling in the plywood panels. Only at the top the panels were screwed through the planks to the top joist with a diameter of 5 mm x 70 mm length.

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Version 1 – Final 30/12/2017 3.87 0. 65 0. 65 0. 65 0. 35 0. 35 2. 76 _{D 3.5 x 65} 0. 35 2. 76 1. 20 1. 10 0. 60 1. 20 0. 50 0.59 0.67 0.67 0.67 0.67 0.59

Figure 1 – Configuration of specimen DFpar-2: non-strengthened (above) and after strengthening (below)

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Version 1 – Final 30/12/2017 2.3.3 Specimen DFper-3a

The configuration of specimen DFper-3a before and after strengthening is given in figure 7: this floor was tested with another configuration with respect to the previous ones: in this case the load was applied perpendicular to the joists.

The specimen was made of spruce floorplanks 18 mm x 165 mm, joists 50 mm x 110 mm, connected with nails with a diameter of 3 mm. At the top the contact between joist and masonry was considered to be hinged: the blocks were > shaped on the left and < shaped on the right side of the joist.

The planks were connected to the joists with two nails on every joist; like in the previous cases the two nails on each plank had a spacing of approximately 100 mm and a gap of approximately 2 mm was left between the tongue and groove.

At the bottom, on both sides of the joists, 2 layers of plywood panels were glued to ensure the clamping. The bottom parts of these plywood panels were glued to a steel beam on which the diaphragm was positioned for testing.

After the test of the non-strengthened diaphragm, also in this case it was strengthened with plywood panels of approximately 800 mm x 1200 mm x 18 mm.

The plywood panels were screwed to the planks with screws with a diameter of 5 mm x 60 mm length without predrilling around the circumference of the panels. The plywood panels were applied in the same way as for DFpar-1 and DFpar-2 overlapping half of a plank at the sides.

In figure 8 the replicated specimen before and after strengthening is shown. 2.3.4 Specimen DFper-3b

This sample was identical to specimen DFper-3a: the only two differences were in the top connection of the joists and in the strengthening technique.

In this case, the top connection was considered as clamped by masonry bricks and mortar, even if this effect proved to be quite limited.

As for the strengthening technique, in addition to the overlay of plywood panels, timber blocks were applied on top of the floor in between each couple of joists (see again Figure 8). This was done to achieve not only an improvement in floor stiffness and shear transfer capacity, but also to simulate an example of a possible strengthened diffused connection between the floor and the masonry walls.

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Version 1 – Final 30/12/2017 Figure 3 – Configuration of specimen DFper-3: non-strengthened (above) and after strengthening (below)

Figure 4 – Specimen DFper-3 non-strengthened, seen from the back side (left) and after strengthening (right), from the front side.

Dfper-3a Dfper-3b

Only for Dfper-3b strengthened: wood blocks between joists

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Version 1 – Final 30/12/2017 2.3.5 Specimen DRpar-4

The configuration of specimen DFpar-4 before and after strengthening is given in figure 9: this roof diaphragm was tested with the load parallel to the purlins, i.e. perpendicular to the rafters.

The specimen was made up of spruce floorplanks 18 mm x 165 mm, rafters 50 mm x 105 mm and purlins 35 x 60 mm. Planks and purlins are connected with 2 nails with a diameter of 3 mm at their intersection, while purlins and rafters are connected with 1 nail with a diameter of 5 mm at the intersection. At the top the rafters were directly connected with one nail to the timber girder, again with a diameter of 5 mm. The planks were connected to the purlins with two nails with a spacing of approximately 100 mm, and a gap of about 2 mm was left between the tongues and grooves.

At the bottom, the timber element representing the wall plate was directly glued to the steel beam supporting the specimen.

After the test of the non-strengthened diaphragm, it was strengthened with plywood panels (18 mm thick) inserted between the purlins. At the bottom the wall plate was extended with glued timber infill pieces, on which the horizontal part of the steel angles was screwed. The vertical part of the steel angle was then connected to the lowest panel with screws.

The panels were screwed to the planks around the edges of the panels with screws with a diameter of 4,5 mm x 40 mm length, without predrilling, while the steel angles were fixed with screws 6 mm x 70 mm. In figure 10 the replicated specimen before and after strengthening is shown.

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Version 1 – Final 30/12/2017 Figure 5 – Configuration of specimen DFpar-4: non-strengthened (above) and after strengthening (below)

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Version 1 – Final 30/12/2017 Figure 6 – specimen DFpar-4 non-strengthened (left) and after strengthening (right)

2.4 Material properties of original and replicated diaphragms

The density, dynamic modulus of elasticity and moisture content of the planks, joists and plywood panels were measured before the construction of the replicated specimens. In table 2 the mean values of these properties are given.

Table 2: Material properties of timber elements used in the replicated diaphragms

Element Wood species Density _(kg/m3₎ Modulus of elasticity _(MPa) Moisture content _(%)

Planks Spruce 462 11210 11,5

Joists Spruce 453 13120 11,2

Plywood panels

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3 Test set-up and test protocol

3.1 Test set-up

Figure 11 shows the in-plane set-up for the diaphragm testing with specimen DFpar-1 placed inside it. The test diaphragm is glued to a bottom HEB 300 steel beam, which is bolted to the part of the test set-up connected to the lab floor. The diaphragms are tested with a minimum vertical load: to diminish the influence of the weight of the set-up on the specimen, the top beam that introduces the horizontal load is a timber I-beam made of LVL and plywood. In that way the top weight is only 2 kN. During testing no influence of this top weight was observed. The application of the horizontal loading is given in figure 11 as well. To transfer the horizontal load to the diaphragm, the bottom flange of the I-beams is fastened to the top joist with screws with a diameter of 10 mm spaced 150 mm.

The steel columns on the left and on the right of the specimen in figure 11 are used for transportation and to measure the vertical displacement of the timber I-beam, they are not connected to the diaphragm. Lateral displacement of the timber I-beam during the test is prevented. This is done by applying vertical steel elements as shown in the cross section. At this position two plywood plates will be mounted to the LVL I-beam. Between the plywood plates and steel beams Teflon will be positioned to allow vertical sliding and frictionless movement of the timber girder.

The measurement plan is given in figure 12. The overview of the applied sensors is given in table 3. Depending on the configuration of the test specimens, the position of the sensor was in some cases slightly adapted for different tests.

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Version 1 – Final 30/12/2017 Figure 12 – Measurement plan.

Table 3 - Overview of the measuring points and sensor types used in the in-plane tests.

No. Description Sensor Type Stroke (mm)

1 Vertical displacement between top and bottom steel beam _{(front-right side)} Linear potentiometer +/-100 2 Vertical displacement between top and bottom steel beam _{(front-right side)} Linear potentiometer +/-100 3 Diagonal displacement between steel beams (front side) Linear potentiometer +/-50 4 Diagonal displacement between steel beams (front side) Linear potentiometer +/-50 5 Vertical displacement between bottom steel beam and plank _{(back side)} Linear potentiometer +/-10 6 Vertical displacement between bottom steel beam and plank _{(back side)} Linear potentiometer +/-10 7 Vertical displacement between bottom steel beam and plank _{(back side)} Linear potentiometer +/-10 8 Vertical displacement between bottom steel beam and plank _{(back side)} Linear potentiometer +/-10 9 Vertical displacement between top joist and plank (back _side) Linear potentiometer +/-25 10 Vertical displacement between top joist and plank (back _side) Linear potentiometer +/-25 11 Vertical displacement between two planks (back side) Linear potentiometer +/-25 12 Vertical displacement between two planks (back side) Linear potentiometer +/-25 13 Vertical displacement between two planks (back side) Linear potentiometer +/-25 14 Horizontal displacement between plank and top joist (back _side) Linear potentiometer +/-25 15 Horizontal displacement between top joist and bottom _{flange of LVL-I-beam} Linear potentiometer +/-50

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Version 1 – Final 30/12/2017 17 Out-of-plane displacement at the top of the wall (back side) Laser +/-100 18 Out-of-plane displacement at the top of the wall (back side) Laser +/-100 19 Horizontal displacement top steel beam Linear potentiometer +/-100 20 Horizontal displacement steel column on bottom beam in _{test frame to external frame} Linear potentiometer +/-100 21 Horizontal displacement bottom steel beam to external _frame. Linear potentiometer +/-50 22 Horizontal displacement top plywood panel to steel column _{at bottom beam.} Linear potentiometer +/-50 23 Horizontal displacement top diaphragm to steel column at _{bottom beam.} Linear potentiometer +/-100 24 Horizontal actuator 400 kN – load and displacement for DFpar-1. Horizontal actuator 100 kN for DFpar-2, DFper-3

and DRpar-4. Load cell and HBM LVDT +/-100

FR1-FR4 Vertical displacement of the bottom steel beam with respect _{to the floor} Linear potentiometer +/-10 FR5-FR6 Vertical displacement between top beam and columns Linear potentiometer +/-100

3.2 Test protocol

The test scheme for cyclic loading according to ISO 21581 [1] will be used. According to ISO 21581 the test schedule shall produce:

• data that sufficiently describe the elastic and inelastic cyclic properties of the specimen; • demands representative of those imposed by earthquakes.

The cyclic displacement schedule given in figure 13 was followed with a rate of displacement to achieve the ultimate displacement between 1 and 30 minutes according to ISO 21581.

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Version 1 – Final 30/12/2017 The ultimate displacement is advised to be obtained from a monotonic test. However, that would double the number of test specimens in this testing program.

The maximum ultimate displacement is determined by the stroke of the horizontal actuator, which is +/- 100 mm. An lu of 30 mm will be assumed, starting with a displacement of 0,375 mm in the first cycle, to ensure enough information in the elastic phase. The amplitude given in table 3 is the amplitude imposed on the actuator. Due to play in the system the actual displacement of the diaphragm at the top might be slightly different. Therefore, when the deformation is reported, this is the actual deformation at the top of the diaphragm.

In table 4 the loading scheme for the diaphragms are given: a run is defined as the path where a deformation loop is performed with one maximum positive and one maximum negative displacement. A cycle is the defined as a number of runs having the same maximum displacement. This means that in this test a cycle consist of 3 runs.

Table 4 – Loading scheme for the trial floor.

Cycle Number of runs Amplitude _{% of l} Uact Rate Duration

u mm mm/s s

1

3

_1.25

_0.375

_0.05

₃₀

2

3

_2.5

_0.75

_0.1

₃₀

3

320

96

12.8

90 3.3 Testing of non-strengthened specimen

The non-strengthened specimens were tested to a displacement where failure did not occur in the timber elements, to ensure that after the test strengthening could be applied. Slightly plastic behaviour in the connections could occur. The maximum displacement that was applied was 60 mm or 66 mm.

3.4 Testing of strengthened specimen

The strengthened specimens were tested to failure, or if no failure would occur to a maximum displacement of +/- 96 mm.

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4 Test results

4.1 Specimen DFpar-1

4.1.1 DFpar-1 non-strengthened

In figure 14 the hysteretic behaviour of DFpar-1 non-strengthened is shown. When the test was stopped no failure in the timber was observed. The relatively small energy dissipation that can be observed is caused by the energy dissipated in the plank-joist connections.

Figure 14 – Hysteretic graph of non-strengthened diaphragm DFpar-1. Horizontal force at the top against the horizontal top displacement.

4.1.2 DFpar-1 strengthened

In figure 15 the hysteretic behaviour of DFpar-1 strengthened is shown. The test was stopped after failure in the top row of screws connecting the plywood panel to the top joist. No failure in the timber or plywood panels was observed.

-30 -25 -20 -15 -10 -5 0 5 10 15 20 25 -80 -60 -40 -20 0 20 40 60 80

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Version 1 – Final 30/12/2017 Figure 15 – Hysteretic graph of strengthened diaphragm DFpar-1. Horizontal force at the top against the horizontal top displacement.

Figure 16 below shows the failure in the top row of screws that connect the plywood panels with the top joist. After a large deformation these screws broke off, as can be seen in the red circles. Although for these row of screws a larger diameter of the screws was used, they were governing because they had to transfer the force from the plywood panels through the planks to the top joist.

Figure 16 – In the red circles: broken screws connecting the plywood panel with the top joist -100 -80 -60 -40 -20 0 20 40 60 80 100 -80 -60 -40 -20 0 20 40 60 80

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Version 1 – Final 30/12/2017 For a possible explanation of lower stiffness compared to strengthened DFpar-2 see figure 17 below. The red circles highlights the screws connecting the plywood panels to the planks that are (not intended) in the plank but in the tongue. The strengthening procedure was adapted to prevent this to occur in the next strengthened diaphragms.

Figure 17 – In the red circles the screws connecting the plywood panel with the planks. The screws are accidentally positioned in the tongue of the plank.

4.1.3 DFpar-1 comparison strengthened and non-strengthened

In figures 18 and 19 the hysteretic plots of the strengthened and non-strengthened diaphragm are combined, in figure 18 for the entire test and in figure 19 for the first 4 cycles, representing the elastic phase.

Graph 18 shows much more energy dissipation for the strengthened diaphragm than for the non-strengthened diaphragm. This is due to the larger amount of screws used to connect the plywood panels and the fact that their position is more effective to take up forces than only the plank-joist connections.

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Version 1 – Final 30/12/2017 Figure 18 – Hysteretic graph of non-strengthened and strengthened diaphragm DFpar-1 for the entire test. Horizontal force at the top against the horizontal top displacement.

Figure 19 – Hysteretic graph of non-strengthened and strengthened diaphragm DFpar-1 for the first 4 cycles. Horizontal force at the top against the horizontal top displacement.

-100 -80 -60 -40 -20 0 20 40 60 80 100 -80 -60 -40 -20 0 20 40 60 80

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-10 -5 0 5 10 15 -3 -2 -1 0 1 2 3 4

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Version 1 – Final 30/12/2017 4.2.1 DFpar-1 non-strengthened

Figure 20 – Hysteretic graph of non-strengthened diaphragm DFpar-2. Horizontal force at the top against the horizontal top displacement.

4.2.2 DFpar-2 strengthened

In figure 21 the hysteretic behaviour of DFpar-2 strengthened is shown. The test was stopped after failure in the glue connecting the timber diaphragm with the bottom steel beam, that connects the specimen to the test set-up. The maximum load which the diaphragm could resist was therefore not reached during the test.

-40 -30 -20 -10 0 10 20 30 40 -80 -60 -40 -20 0 20 40 60 80

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Version 1 – Final 30/12/2017 Figure 21 – Hysteretic graph of strengthened diaphragm DFpar-2. Horizontal force at the top against the horizontal top displacement.

Figure 22 below shows the failure of the glue line of the specimen with bottom steel beam. No failure in the timber diaphragm was observed then. This unexpected failure was probably due to a not perfect cleaned surface of the steel plates that were bolted to the steel beam. However, the load that was reached was at the same level as DFpar-1.

Figure 7 – Failure of the glue at the bottom of the floor.

-100 -80 -60 -40 -20 0 20 40 60 80 100 -30 -20 -10 0 10 20 30 40 FH Ho riz ont al lo ad a t t he to p of di aphr ag m (kN )

δh horizontal displacement at the top of diaphragm (mm)

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Version 1 – Final 30/12/2017 Figures 23 and 24 combine the hysteretic plots of the strengthened and non-strengthened diaphragm, in figure 23 for the entire test and in figure 24 for the first 4 cycles, representing the elastic phase.

Figure 23 – Hysteretic graph of non-strengthened and strengthened diaphragm DFpar-2. Horizontal force at the top against the horizontal top displacement.

Figure 24 – Hysteretic graph of non-strengthened and strengthened diaphragm DFpar-2. Horizontal force at the top against the horizontal top displacement.

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-20 -15 -10 -5 0 5 10 15 20 -2,5 -2 -1,5 -1 -0,5 0 0,5 1 1,5 2 2,5 3

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4.3 Specimen DFper-3a

4.3.1 DFper-3a non-strengthened

In figure 25 the hysteretic behaviour of DFper-3a non-strengthened is shown. When the test was stopped no failure in the timber was observed. The relatively small energy dissipation that can be observed is caused by the energy dissipated in the plank-joist connections.

Figure 25 – Hysteretic graph of non-strengthened diaphragm DFper-3a. Horizontal force at the top against the horizontal top displacement.

4.3.2 DFper-3a strengthened

In figure 26 the hysteretic behaviour of DFper-3a strengthened is shown. The test was stopped after failure of the screws connecting the outer left and outer right joist with the plywood panels (the other joists were not directly connected with the plywood panels). Also cracking in the outer left and right joist was observed. In the first cycles strength and stiffness of the floor significantly improved with respect to the non-strengthened version. However, the full resistance of the floor could not be exploited because after 24-30 mm cycles a softening phase took place. As observed, this behaviour was caused by three failure mechanisms:

• Pull out or tension failure of the top row of nails connecting the planks to the joist;

• Pull out or tension failure of the screws used for the strengthening and failure of the wood joist in the meantime, starting with the screw on top of the joist and then with the one below and so on; • Mostly due to the previous two, the movement of the joists became more and more independent of

the planks and panels (that functioned as a single panel), with large bending on top of the outer joists until complete crack of them happened.

-5 -4 -3 -2 -1 0 1 2 3 4 -80 -60 -40 -20 0 20 40 60 80

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Version 1 – Final 30/12/2017 Figure 26 – Hysteretic graph of strengthened diaphragm DFper-3a. Horizontal force at the top against the horizontal top displacement.

Therefore, after the peak, the floor is not able to work as a whole anymore: the fundamental difference between this specimen and the two floor diaphragms loaded parallel to the joists is related to how the load is transferred to the diaphragm from the top.

The principle of the strengthening is that the planks and panels are connected and ensure that the specimen acts as a diaphragm. At the edges, the load has to be transferred to the masonry walls. In this case the load from the diaphragm is transferred to the masonry by the joist that are in a pocket (in this case simulated by wooden blocks). However, in this test, the panels are screwed only on the outer left and right joist. In between the panels are only screwed to the planks. Therefore, the load from the diaphragm (planks and panels) had to be transferred by the vertical row of screws in the left and outer joist. The force is thereby concentrated in the top screw, which failed first.

This failure could have been prevented by applying timber beams perpendicular to the joists at the top. Then, a horizontal row of screws could have connected the panels with these timber beams. These beams would then have transferred the load by contact compression to the joists, close to the support. This solution was tested with sample DFper-3b.

Figure 27 shows all the observed failure for this strengthened floor.

Figure 8 – Failure modes for DFper-3a diaphragm. -30 -20 -10 0 10 20 30 -100 -80 -60 -40 -20 0 20 40 60 80 100 FH Ho riz ont al lo ad a t t he to p of di aphr ag m (kN )

DFper-3 strenghtened

Pull out failure of nails and screws on top

Bending failure of the outer joists

(29)

Version 1 – Final 30/12/2017 4.3.3 DFper-3a comparison strengthened and non-strengthened

Figures 28 and 29 combine the hysteretic plots of the strengthened and non-strengthened diaphragm, in figure 28 for the entire test and in figure 29 for the first 4 cycles, representing the elastic phase.

Figure 28 – Hysteretic graph of non-strengthened and strengthened diaphragm DFper-3a. Horizontal force at the top against the horizontal top displacement.

Figure 29 – Hysteretic graph of non-strengthened and strengthened diaphragm DFper-3a. Horizontal force at the top against the horizontal top displacement.

-30 -20 -10 0 10 20 30 40 -100 -80 -60 -40 -20 0 20 40 60 80 100

F

H

Ho

riz

ont

al

lo

ad

a

t t

he

to

p

of

di

aphr

ag

m

(kN

)

δ

h

horizontal displacement at the top of diaphragm (mm)

DFper-3 unstrengthened

DFper-3 strenghtened

-4 -3 -2 -1 0 1 2 3 4 5 -2 -1,5 -1 -0,5 0 0,5 1 1,5 2 2,5

F

H

Ho

riz

ont

al

lo

ad

a

t t

he

to

p

of

di

aphr

ag

m

(kN

)

δ

h

horizontal displacement at the top of diaphragm (mm)

DFper-3 unstrengthened

DFper-3 strenghtened

(30)

Version 1 – Final 30/12/2017 4.4.1 DFper-3b non-strengthened

In figure 25 the hysteretic behaviour of DFper-3b non-strengthened is shown. When the test was stopped no failure in the timber was observed. The relatively small energy dissipation that can be observed is caused by the energy dissipated in the plank-joist connections.

Figure 30 – Hysteretic graph of non-strengthened diaphragm DFper-3b. Horizontal force at the top against the horizontal top displacement.

4.4.2 DFper-3b strengthened

In figure 31 the hysteretic behaviour of DFper-3b strengthened is shown. The presence of the timber blocks strongly improved the strength and the stiffness of the diaphragm, much more compared to sample DFper-3a. The failure was caused by the progressive development of cracks in the planks in which the screws connecting the plywood panels to them were placed (Figure 32).

Besides, the widespread plasticization of the fasteners caused a very high level of energy dissipation. -6 -4 -2 0 2 4 6 -80 -60 -40 -20 0 20 40 60 80 Ho riz on ta l f or ce a t t he to p o f d ia ph ra gm ( kN )

(31)

Version 1 – Final 30/12/2017 Figure 31 – Hysteretic graph of strengthened diaphragm DFper-3. Horizontal force at the top against the horizontal top displacement.

Figure 32 – Failure modes for DFper-3b diaphragm. -80 -60 -40 -20 0 20 40 60 80 -100 -80 -60 -40 -20 0 20 40 60 80 100 Ho riz ont al f or ce o n t op o f di aphr ag m ( kN )

(32)

Version 1 – Final 30/12/2017 Figures 33 and 34 combine the hysteretic plots of the strengthened and non-strengthened diaphragm, in figure 33 for the entire test and in figure 34 for the first 4 cycles, representing the elastic phase.

Figure 33 – Hysteretic graph of non-strengthened and strengthened diaphragm DFper-3b. Horizontal force at the top against the horizontal top displacement.

Figure 34 – Hysteretic graph of non-strengthened and strengthened diaphragm DFper-3b. Horizontal force at the top against the horizontal top displacement.

-80 -60 -40 -20 0 20 40 60 80 -100 -80 -60 -40 -20 0 20 40 60 80 100 Ho riz ont al f or ce o n t op o f di aphr ag m ( kN )

Horizontal displacement on top of diaphragm (mm)

Dfper-3b non-strengthened Dfper-3b strengthened

-8 -6 -4 -2 0 2 4 6 8 10 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 Ho riz ont al f or ce o n t op o f di aphr ag m ( kN )

Horizontal displacement on top of diaphragm (mm) Dfper-3b non-strengthened Dfper-3b strengthened

(33)

4.5 Specimen DRpar-4

4.5.1 DRpar-4 non-strengthened

Figure 35 – Hysteretic graph of non-strengthened diaphragm DRpar-4. Horizontal force at the top against the horizontal top displacement.

4.5.2 DRpar-4 strengthened

In figure 36 the hysteretic behaviour of DFpar-4 strengthened is shown. The test was stopped after failure of the nails connecting the planks with the top purlin. Also at the strengthened bottom pull out failure and tension failure of screws connecting the bottom wall plate with steel angles could be observed. So the top capacity at the clamped bottom was probably reached and in the post-peak phase the upper nails pulled out. No failure in the timber elements or plywood was observed.

The reinforcement appears to be very effective and very stiff compared to the non-strengthened specimen. In this case the panels were applied between the purlins, so vertically not in direct contact. The force was transferred through the planks. An interesting fact is that the connections on the whole floor developed plastic behaviour: on top the nails developed two plastic hinges before pull out failure, while the screws connecting the steel profile and the wall plate caused strong wood embedment and developed at least one plastic hinge in the areas with the highest stresses. For two of these screws tension failure was observed, while for six of them a pull out failure occurred (see figure 37). The other screws of the steel plates were slightly bended, causing only wood embedment.

-4 -3 -2 -1 0 1 2 3 -80 -60 -40 -20 0 20 40 60 80

F

H

Ho

riz

ont

al

lo

ad

a

t t

he

to

p

of

di

aphr

ag

m

(kN

)

δ

h

horizontal displacement at the top of diaphragm (mm)

(34)

Version 1 – Final 30/12/2017 Figure 36 – Hysteretic graph of strengthened diaphragm DRpar-4. Horizontal force at the top against the

horizontal top displacement.

Figure 37 – Failure modes for specimen DRpar-4 -50 -40 -30 -20 -10 0 10 20 30 40 -100 -80 -60 -40 -20 0 20 40 60 80 100

F

H

Ho

riz

ont

al

lo

ad

a

t t

he

to

p

of

di

aphr

ag

m

(kN

)

δ

h

horizontal displacement at the top of diaphragm (mm)

DRpar-4 strengthened

Tension failure of bottom screws

Pull out failure of bottom screws

(35)

Version 1 – Final 30/12/2017 4.5.3 DRpar-4 comparison strengthened and non-strengthened

In figures 38 and 39 the hysteretic plots of the strengthened and non-strengthened diaphragm DRpar-4 are combined, in figure 38 for the entire test and in figure 39 for the first 4 cycles, representing the elastic phase.

Graph 38 shows much more energy dissipation for the strengthened diaphragm than for the non-strengthened one. This is due to the larger amount of screws used to connect the plywood panels and the fact that their position is more effective to take up forces than only the plank-joist connections. Also the bottom clamping of the plywood panels with the steel angles and the timber wall plate with the steel angles with screws has dissipative behaviour.

Figure 38 –Hysteretic graph of non-strengthened and strengthened diaphragm DRpar-4. Horizontal force at the top against the horizontal top displacement.

Figure 39 – Hysteretic graph of non-strengthened and strengthened diaphragm DRpar-4. Horizontal force at the top against the horizontal top displacement.

-50 -40 -30 -20 -10 0 10 20 30 40 50 -100 -80 -60 -40 -20 0 20 40 60 80 100 FH Ho riz ont al lo ad a t t he to p of di aphr ag m (kN )

DRpar-4 unstrengthened

DRpar-4 strengthened

-8 -6 -4 -2 0 2 4 6 8 -2,5 -2 -1,5 -1 -0,5 0 0,5 1 1,5 2 2,5 FH Ho riz ont al lo ad a t t he to p of di aphr ag m (kN )

DRpar-4 unstrengthened

DRpar-4 strengthened

(36)

5 Determination of the apparent shear stiffness of the

tested diaphragms

5.1 Introduction

In NEN NPR 9998-2017 the concept of an apparent shear stiffness Gdiap' in KN/m is adopted. The meaning of this apparent shear stiffness is: Gdiap' is the shear stiffness of the diaphragm in KN/m2 multiplied by the thickness of the diaphragm in m. By the adoption of Gdiap' the information about the thickness is therefore implicitly incorporated in this property. The table of NPR 9998-2017 with values of Gdiap' also gives no information about the applied nails and the sizes and quality of the joists. It therefore gives a very rough indication of an apparent shear stiffness of traditional floors. Because a shear stiffness is given it assumes that the floor behaves as a shear beam which might be questionable. Furthermore it is unclear if this value can be used only in the initial phase or over the entire deformation range.

For a cantilever beam according to figure 40 the top displacement using Gdiap' in KN/m can be calculated with the following formula:

𝛿𝛿ℎ=_{𝐵𝐵 ∗ 𝐺𝐺}𝐹𝐹𝐻𝐻∗ 𝐿𝐿 𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑′

Figure 40 - Notation for the calculation of the displacement of a cantilever beam. With:

• FH is the horizontal top load (kN)

• δh is the horizontal deformation at the top of the cantilever (m) • B is the height of the diaphragm beam (m)

• L is the length over which the diaphragm beam is shearing off (m) For the tested diaphragms Gdiap' (in KN/m) can then be determined with:

𝐺𝐺𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑′=𝐹𝐹_{𝐵𝐵 ∗ 𝛿𝛿}𝐻𝐻∗ 𝐿𝐿

ℎ

The value of Gdiap' can be given for different values of the ratio FH/ δh to indicate non-linear behaviour. For that the values of FH and δh are determined from the backbone curves of the hysteresis plots.

In the next sections the values for Gdiap' are given for the tested diaphragms. It must be noticed that the found values for Gdiap' in this report relate to the dimensions of the tested diaphragms.

(37)

5.2 Specimen DFpar-1

In table 4 the values for Gdiap' for the non-strengthened and the strengthened diaphragm DFpar-1 are given. When δh is calculated taking only into account the bending stiffness of the planks, a corresponding Gdiap' of 225 kN/m is found. That means that for lower displacements the double nails at every plank-joist connections have a contribution to the stiffness that diminishes for larger displacements.

Table 4 – Values for Gdiap' for DFpar-1

δh (mm) Non-strenghtened Gdiap' (KN/m) Gdiap' Strenghtened (KN/m) 2

371 3337

5

309 1953

10

276 1454

15

266 1354

20

259 1214

30

246 1061

40

235

961

50

230

852

60

218 5.3 Specimen DFpar-2

In table 5 the values for Gdiap' for the non-strengthened and the strengthened diaphragm DFpar-2 are given. When δh is calculated taking only into account the bending stiffness of the planks, a corresponding Gdiap' of 301 kN/m is found. That means that for lower displacements the double nails at every plank-joist connections have a contribution to the stiffness that diminishes for larger displacements.

The difference of the stiffness between the non-strengthened and strengthened diaphragms of DFpar-1 and DFpar-2 can mainly be explained by the thickness of the floor planks (18 mm for DFpar-1 and 24 mm for DFpar-2). For the strengthened diaphragms the difference between DFpar-1 and DFpar-2 cannot be explained by the thickness of the planks because the stiffness is mainly governed by the panels. This could only have an effect in the elastic phase when the screws are not bent yet. As was observed for specimen DFpar-1 the screws connecting the panels sometimes were positioned in the tongue of a plank (and therefore being less effective). Another difference is that for DFpar-2 another type of screws was used (with also a thicker diameter), for which no predrilling was required and therefore not applied. For DFpar-1 predrilling was on the contrary applied.

Table 5 – Values for Gdiap' for DFpar-2

δh (mm) Non-strengthened Gdiap' (KN/m) Gdiap' Strengthened (KN/m) 2

₅₄₁

₄₂₀₂

5

430 2966

10

379 2363

15

346 2056

20

341 1881

30

314

40

296

50

303

60

291

(38)

Version 1 – Final 30/12/2017 In table 6 the values for Gdiap' for the non-strengthened and the strengthened diaphragm DFper-3 are given. When δh is calculated taking only into account the bending stiffness of the joists, a corresponding Gdiap' of 10 kN/m is found. That means that for lower displacements the double nails at every plank-joist connections and the friction between the different planks in the tongue and groove have a contribution to the stiffness that diminishes for larger displacements. The lower stiffness of the strengthened diaphragm compared to DFpar-1 and DFpar-2 can be explained from the way the load from the diaphragm was transferred to joists. When this is improved the stiffness of the strengthened diaphragm is higher, as it happens for version B.

Table 6 – Values for Gdiap' for DFper-3

δh (mm) Version A Version B Gdiap' Non-strengthened (KN/m) Gdiap' Strengthened (KN/m) Gdiap' Non-strengthened (KN/m) Gdiap' Strengthened (KN/m) 2 253 1421 160 2498

5

141 1174 137 2215

10

99 1012 114 1932

15

87 906 91 1649

20

71 775 67 1366

30

55

40

46

50

38

60

35

5.5 Specimen DRpar-4

In table 7 the values for Gdiap' for the non-strengthened and the strengthened diaphragm DRpar-4 are given. Because the support of the rafter at the top and bottom is hinged (connected with one nail), this means that the double nails at every plank-joist connections and the friction between the different planks in the tongue in groove determine the stiffness.

Table 7 – Values for Gdiap' for DRpar-4

δh (mm) Non-strengthened Gdiap' (KN/m) Gdiap' Strengthened (KN/m) 2 158 2345

5

97 1652

10

69 1260

15

56 1059

20

47 910

30

38 735

40

32 634

50

27 511

60

25 410

(39)

6 Conclusions

In this reports the quasi-static cyclic pushover tests on 5 timber diaphragms are reported. The 4 timber diaphragms represent traditional floors and roofs that can be observed in detached masonry houses in Groningen. The hysteresis plots for the non-strengthened specimens are shown in figure 41. Specimens DFpar-1 and DFpar-2 were tested parallel to the joist. The difference is that for DFpar-1 the planks were 18 mm thick and for DFpar-2 the planks were 24 mm thick.

Specimen DFper-3a and 3b were loaded perpendicular to joists and were much more flexible than DFpar-1 and DFpar-2. Specimen DR-par-4 represented a roof diaphragm and showed a low racking stiffness.

Figure 41 – Hysteretic graphs of the non-strengthened diaphragms.

The hysteresis plots for the strengthened specimens are shown in figure 42. Specimen DFpar-2 is stiffer than DFpar-1, although the principle of strengthening was the same. However, in DFpar-1 the seams of the panels were not exactly aligned to the half of the planks, and for DFpar-2 other (self-tapping) screws were used.

Specimen DFper-3a showed lower strength and stiffness than DFpar-1 and DFpar-2. This was because the load was only directly transferred from the panels to the outer joists. By adding timber perpendicular beams between the joists (DFper-3b) the strength and stiffness is closer to that of DFpar-1 and DFpar-2.

The strengthening of specimen DR-par-4 showed to be very effective to create a clamping in the timber wall plate. -40 -30 -20 -10 0 10 20 30 40 -100 -80 -60 -40 -20 0 20 40 60 80 100 Hor iz on ta l f or ce on top of d ia ph ra gm (k N)

Horizontal displacement on top of diaphragm (mm)

(40)

Version 1 – Final 30/12/2017 Figure 42 – Hysteretic graphs of strengthened diaphragms.

In table 8 the values of Gdiap' as used in NPR 9998-2017 are given for a displacement of 2 mm and 20 mm. These values are valid for the sizes of the tested diaphragms, approximately 4.0 m by 2.75 m.

Table 8 – Values for Gdiap' for δh =2 mm and δh =20 mm

δh =2 mm δh =20 mm Gdiap' Non-strenghtened (KN/m) Gdiap' Strenghtened (KN/m) Gdiap' Non-strenghtened (KN/m) Gdiap' Strenghtened (KN/m) DFpar-1 371 3337 259 1214 DFpar-2 541 4202 341 1881 DFper-3a 253 1421 71 775 DFper-3b 160 2498 67 1366 DRpar-4 158 2345 47 910

Some general remarks regarding the tested diaphragms, representing traditional floors and roofs from detached houses:

• For loading parallel to the joists the stiffness is mainly governed by the bending stiffness of the planks, with some addition of the moment-resisting plank-joist connections and friction in the tongue and grooves of the planks.

• For loading perpendicular to the joist the stiffness is mainly governed by the stiffness of the joists, with some addition of the moment-resisting plank-joist connections and friction in the tongue and grooves of the planks.

• For the diaphragm representing the roof the stiffness is mainly governed by the stiffness of the moment-resisting plank-joist connections and friction in the tongue and grooves of the planks. • Strengthening the diaphragms with plywood panels gives a significant improvement in strength,

stiffness and energy dissipation compared to the non-strengthened diaphragms. The position of the seams of the plywood panels and the type of screws used can influence the level of improvement, and it is important to ensure an efficient transfer of shear forces.

• The creation of an effective clamping of the strengthened roof diaphragm to the wall plate can be established by steel angles connecting the roof diaphragm to the timber wall plate.

-100 -80 -60 -40 -20 0 20 40 60 80 -100 -80 -60 -40 -20 0 20 40 60 80 100 Hor izon tal for ce on top of d iap hr ag m (kN )

Horizontal displacement on top of diaphragm (mm)

(41)

7 References

[1] ISO 21581:2010. Timber structures- Static and cyclic lateral test load test methods for shear walls. International Organization for Standardization (ISO).

Test report on cyclic behaviour of replicated timber diaphragms representing a detached house

Test report on cyclic behaviour of replicated timber diaphragms representing a detached

house

Ravenshorst, Geert; Mirra, Michele

Publication date

2017

Document Version

Final published version

Citation (APA)

Ravenshorst, G., & Mirra, M. (2017). Test report on cyclic behaviour of replicated timber diaphragms

representing a detached house. Delft University of Technology.

Important note

To cite this publication, please use the final published version (if applicable).

Please check the document version above.

TU Delft Large-scale testing campaign 2016 – WP4

TEST REPORT ON CYCLIC BEHAVIOUR OF

REPLICATED TIMBER DIAPHRAGMS

REPRESENTING A DETACHED HOUSE

Authors: Geert J.P. Ravenshorst, Michele Mirra

Table of Contents

1 Introduction

2 Test specimens

2.1 Overview of test specimens

2.2 Background for the determination of the configuration of the

test specimen

2.3 Description of the test specimen

2.4 Material properties of original and replicated diaphragms

3 Test set-up and test protocol

3.1 Test set-up

3.2 Test protocol

1

1.25

0.375

0.05

30

2

2.5

0.75

0.1

30

3

5

1.5

0.2

30

4

7.5

2.25

0.3

30

5

10

3

0.4

30

6

20

6

0.8

90

7

40

12

1.6

90

8

60

18

2.4

90

9

80

24

3.2

90

10

100

30

4

90

_1.25

_0.375

_0.05

₃₀

_2.5

_0.75

_0.1

₃₀

₅

_1.5

_0.2

₃₀

_7.5

_2.25

_0.3

₃₀

₁₀

₃

_0.4

₃₀

₂₀

₆

_0.8

₉₀