Effect of Bearing Direction and Mounting Techniques on Cross-Lami- nated Timber Elements in the Field

Erik NILSSON 1 , Sylvain MÉNARD 2 , Delphine BARD HAGBERG 3 , Klas HAGBERG 4

1,2 University of Quebec at Chicoutimi, Canada

3 Lund University, Sweden 1,4 Acouwood AB, Sweden

ABSTRACT Vibration reduction index ( 𝐾𝐾 𝑖𝑖𝑖𝑖 ) measurements in the field have some challenges compared to labor- atory measurements. Firstly, the measurement requires access to a construction site during the short time span when the cross-laminated timber (CLT) elements are apparent. Secondly, building con- tractors are often on a tight time schedule. Therefore, it is important to find a solution that minimizes the measurement time on site. Moreover, 𝐾𝐾 𝑖𝑖𝑖𝑖 measurements in the field include several types of junc- tions with different bearing directions which may be of importance. This paper aims to evaluate two different mounting techniques with accelerometers on CLT elements and to discuss how the bearing direction could affect the vibration level difference of junctions. Measurement data indicate few de- viations between mounting techniques with bee wax or double-sided adhesive tape when accelerom- eters are attached to CLT elements. Furthermore, field measurements indicate that the vibration level will decrease with increased lamellas over the same CLT element. Double-sided adhesive tape is an adequate substitute for bee wax in the field for mounting accelerometers on CLT elements, with some limitations at high frequencies. Measurement data concludes that the bearing direction of CLT ele- ments can influence the vibration reduction index of a junction. Keywords: Vibration reduction index, Cross-Laminated Timber, Bearing Direction, Mounting Tech- nique

1. INTRODUCTION

Flanking sound transmission paths play an important role when the airborne sound reduction is meas- ured in the field since these values will be lower than those obtained in the laboratory. The flanking paths can be measure with the vibration reduction index 𝐾𝐾 𝑖𝑖𝑖𝑖 . However, 𝐾𝐾 𝑖𝑖𝑖𝑖 measurements on cross- laminated timber (CLT) in the field have some complications since they require access to a construc- tion site during the short time span when the cross-laminated timber elements are apparent. Therefore, it is important to find a solution that minimizes the measurement time on site.

1 Email: Erik.Nilsson1@uqac.ca

2 Email: Sylvain_Menard@uqac.ca

3 Email: Delphine.Bard@construction.lth.se

4 Email: Klas.Hagberg@acouwood.com

inter.noise 2-24 august scornsuevearcanmus )())D

Several studies have investigated the impact of flanking transmission paths for concrete and ma- sonry structures [1-5]. The standard ISO 12354-1 specifies calculation models for direct and indirect flanking transmission paths to estimate the airborne sound insulation between adjacent rooms [6]. The indirect paths are mainly described with empirical formulas, based on the mass difference of connected elements, characterized by the vibration reduction index, 𝐾𝐾 𝑖𝑖𝑖𝑖 [6]. Many empirical formulas in ISO 12354-1 describe heavy structures such as concrete and masonry. The standard also provides empirical formulas of junctions with framed lightweight constructions and CLT elements [6].

CLT is increasing in popularity in several countries as a building element [7]. The product has a good environmental profile and can compete with other traditional structure materials since it has excellent strength and stiffness properties [8]. Several studies have measured the vibration reduction index of CLT elements and mainly in the laboratory (mock-ups), but more data is needed [9-13]. Ref. [9] have compared measurements of 𝐾𝐾 𝑖𝑖𝑖𝑖 against the empirical values in ISO 12354-1 [6] and results show that measured values in the laboratory deviate from the values in the standard. ISO 12354-1 should be used with special consideration when estimating the impact of different flanking sound transmission paths for CLT elements. More measurements and studies are required for different junc- tion types to achieve high precision calculations. The vibration reduction index for CLT elements is typically measured in the laboratory, which may not reflect the field results. To decide whether there is a difference, vibration reduction index measurements on CLT elements in the field are needed.

Vibration measurements on an element shall be performed with accelerometers that are mounted directly on the surface according to ISO 10848-1 [14]. The accelerometers must have sufficient effi- ciency and low noise to obtain an adequate signal-to-noise ratio. Furthermore, the mass of the accel- erometers should be small enough to minimize the effect of mass loading [14]. ISO 10848-1 mentions that the fixing of accelerometers should be stiff in the normal direction of the surface and the standard suggests the use of bee wax or petroleum wax, with some caution regarding weak fixing which could cause measurement errors at high frequencies [14].

Bee wax as a mounting technique implies some difficulties in the field when measuring the vibra- tion reduction index on CLT elements. Since bee wax does not adhere well to the surface without a little preparation, mounting each accelerometer requires some time, and the bee wax will not stick if the accelerometer is moved. Additionally, bee wax requires a certain temperature to function properly based on measurements conducted both during winter and summer on CLT elements in the field. The temperature is usually very low inside the buildings when measurements take place during the winter period. The primary reason is that the climate shell is often built simultaneously with indoor con- struction and vibration measurements cannot usually take place after the work has started indoors. A laboratory or mock-up is not affected by this problem because the climate is controlled, unlike a field situation. Due to its ability to work with temperature changes, double-sided tape proves to be a suit- able option. Moreover, mounting time decreases since the tape can be left on the CLT elements with- out losing its effectiveness during the measurement period. However, double-sided tape is not men- tioned in the ISO 10848-1 [14], and therefore, the mounting technique requires evaluation before it is used in practice.

CLT elements in the field typically vary significantly more compared to the ones tested in the laboratory in size, various openings, and bearing directions. A corner room in an apartment with four CLT walls will have two façades and two internal walls with multiple floor elements connected. The bearing direction of the floor in one room is usually oriented the same way and if the bearing direction has an impact on the vibration reduction index, then the room will have a minimum of four different junction types. The junctions in that room will consist of two T-junctions (façade) with the bearing direction of the floor parallel and perpendicular to the junction, and two X-junctions (internal walls), also with different bearing directions in relation to the junctions. If the bearing direction has an impact on the sound reduction index, and if it is not considered during calculation, then the measurements will not match the calculations to a certain degree.

The purpose of this paper is to evaluate two different mounting techniques with accelerometers on CLT elements (bee wax and double-sided tape) and to discuss how the bearing direction could affect the vibration level difference of junctions.

2. CROSS-LAMINATED TIMBER

Cross-laminated timber (CLT) is an engineered wood product made of different layers of kiln-dried dimension lumber boards stacked in alternating directions (crosswise 90°) and glued into place. CLT elements always consist of an odd number of layers, usually between three to seven, and the orientation of lamellas provides improved dimensional stability [7]. The panels are prefabricated, and they can be cut with high precision with CNC (Computer Numerical Controlled) routers. The finished product is stiff, strong, and stable which makes it suitable for several applications including floors, walls, and roofs [8]. Wood has three principal axes with respect to grain direction and growth rings. Lumber boards, therefore, have three different moduli of elasticity depending on the axis (longitudinal, radial, and tangential) [15]. CLT panels will, therefore, also have different mod- uli of elasticity depending on the global axes. Moreover, due to alternative direction of the lumber boards, CLT elements will have different moduli of elasticity depending on the bearing direction [8]. A T-junction with CLT walls and a CLT floor is illustrated in Figure 1 with different bearing directions in relation to the walls (perpendicular and parallel to the junction). Most often, the strongest load-bearing direction is parallel to the boards on the outer layer [8] which is illustrated in Figure 1.

(a) Bearing direction perpendicular to the junction. (b) Bearing direction parallel to the junction.

Figure 1: T-junctions of CLT elements with two different bearing directions. 3. VIBRATION REDUCTION INDEX

Vibration measurements should be executed according to ISO 10848-1 [14] to calculate the vibration reduction index. Vibrations can be measured both with acceleration and velocity. However, acceler- ation is preferred prior to velocity when measuring the structural reverberation time to avoid signal processing affecting the decay curve. The averaged velocity level can be calculated according to Equation 1:

𝐿𝐿 𝑣𝑣 = 10 ∙log 10 ቆ

𝑣 𝑣 0 2 ቇ , (1)

1 𝑇 𝑇 𝑚 𝑚 ∫ 𝑣 𝑣 2 (𝑡 𝑡 )𝑑 𝑑 𝑑 𝑇 𝑇 𝑚 0

where 𝑣𝑣 is the velocity over time and 𝑣𝑣 0 is the reference velocity level [14].

Flanking transmission paths between two elements, 𝑖𝑖 and 𝑗𝑗 , can be quantified with the vibration reduction index in Equation 2 known as 𝐾𝐾 𝑖𝑖𝑖𝑖 [14] which is expressed in decibels:

𝐾𝐾 𝑖𝑖𝑖𝑖 = 𝐷𝐷 𝑣𝑣,ij തതതതത+ 10 ∙log 10 ൬ 𝑙 𝑙 𝑖 𝑖 𝑖

ඥ 𝑎 𝑎 𝑖 𝑖 𝑎 𝑎 𝑗 𝑗 ൰ . (2)

𝐷𝐷 𝑣𝑣,ij തതതതത in Equation 2 is described as the direction-averaged velocity level difference, which is calcu- lated as the mean value between the velocity level difference 𝐷𝐷 𝑣𝑣,ij (when element 𝑖𝑖 is excited) and 𝐷𝐷 𝑣𝑣,ji (when element 𝑗𝑗 is excited) [14] according to Equation 3:

𝐷𝐷 𝑣𝑣,ij തതതതത= 1

2 ∙൫𝐷𝐷 𝑣𝑣,ij + 𝐷𝐷 𝑣𝑣,ji ൯ . (3)

The vibration reduction index in Equation 2 is also dependent on the common junction length, 𝑙𝑙 𝑖𝑖𝑖𝑖 , and the equivalent sound absorption length for each element, 𝑎𝑎 𝑖𝑖 and 𝑎𝑎 𝑗𝑗 according to Equation 4:

𝑎𝑎 𝑗𝑗 = 2.2∙𝜋 𝜋 2 ∙𝑆 𝑆 𝑗

𝑇 𝑇 𝑠 𝑠 ,𝑗 𝑗 ∙𝑐 𝑐 0 ∙ඨ 𝑓 𝑓 𝑓 𝑟 𝑟 𝑟 𝑟 𝑟

. (4)

𝑆𝑆 𝑗𝑗 is the surface area of the element, 𝑇𝑇 𝑠𝑠,𝑗𝑗 is the structural reverberation time of the element (de- pendent on frequency), 𝑐𝑐 0 is the speed of sound in air, 𝑓𝑓 is the frequency and 𝑓𝑓 𝑟𝑟𝑟𝑟𝑟𝑟 is the reference frequency, 𝑓𝑓 𝑟𝑟𝑟𝑟𝑟𝑟 = 1000 Hz [14]. 4. MEASUREMENT SETUP 4.1. Mounting Technique The impact of different mounting techniques (bee wax and double-sided tape) was measured on a CLT wall with several configurations. The test configuration mainly consisted of four accelerometers and a transient excitation source with multiple impacts of a hammer. Accelerometers were mounted at different positions on a CLT wall according to Figure 2.

Pos 3

90°

Source

Pos 2

Reference

Pos 1

45°

0°

Tape

Recieving

Tape

Reference

Recieving

Bee wax

Bee wax

Figure 2: Measurement setup on the CLT wall.

In the first position, four accelerometers were placed on the same board and at the same distance from the excitation source (see also Figure 3a from the field measurement). Two accelerometers were attached with bee wax and the remaining two were attached with double-sided tape. One accelerom- eter was also placed close to the source to measure the power input (not illustrated in Figure 2). One accelerometer of each mounting technique was moved from position 1 to positions 2 and 3 (see Figure 2), oriented 45 and 90 degrees from the reference position. In Figure 3b, receiving accelerometers are placed in position 3. Six excitations with the impact hammer were conducted on the CLT wall over a period of 30 seconds and the measured vibration was linearly averaged. The test was repeated so that two measurements were conducted for each position. The difference in velocity level of different mounting techniques was calculated as 𝐿𝐿 𝑣𝑣,𝑏𝑏𝑏𝑏𝑏𝑏 𝑤𝑤𝑤𝑤𝑤𝑤 −𝐿𝐿 𝑣𝑣,𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡 for each position and measurement. The two measurements for each position were then linearly averaged and compared with each other.

( a ) ( b )

Figure 3: Pictures on the CLT wall in the field: (a) Receiving accelerometers at position 1; (b) Receiving accelerometers at position 3. 4.2. Bearing Direction Two different test methods were conducted to investigate how the vibration level differs when the bearing direction was oriented perpendicular to the junction compared to parallel to the junction.

The first simplified test method was based on the setup in Figure 2, measurement pictures are presented in Figure 3. Two accelerometers were used as reference values at position 1 where one was fixed with bee wax and the other with tape. Two accelerometers were used to measure receiving values at positions 1-3 with different mounting techniques as described before. The idea of the test was to understand how the velocity level changes over the CLT element if the accelerometers are placed on the same outer lumber board (of the CLT wall’s five layers) as the one getting excited, or if they are moved around with the same distance from the source. In theory, the vibrations need to travel through more lumber boards to directly excite the board closest to the accelerometer. The meas- ured receiving values at positions 1-3 were subtracted from the reference values at position 1 for each position and each mounting technique.

The second test method was performed in accordance with ISO 10848-1 [14] where the vibration reduction index of a junction was determined. The measurements consisted of seven accelerometers, four on the source element and three on the receiving element, and an impact hammer. Several meas- urements were conducted for each flanking path of the junction with different positions of accelerom- eters and impact hammer. Two different junctions were tested in the same apartment with similar attributes, except that the bearing direction differed. The room, in which the measurements were conducted, is displayed in Figure 4 with markings on how the bearing direction was oriented. Meas- urement pictures are presented in Figure 5 for both junctions. The floor in Figure 4 is oriented parallel in relation to junction 1 and perpendicular in relation to junction 2. The vibration reduction index of both junctions was measured in two identical apartments and the difference of each junction was calculated for each flanking path according to ISO 10848-1 [14].

Figure 4: Plan drawing of two measured junctions in one apartment with different bearing direc- tions. The bearing direction of the floor is parallel in relation to junction 1 and perpendicular in re- lation to junction 2.

( a ) ( b )

( c ) ( d )

Figure 5: Pictures on the two measured junction types in the field: (a) Accelerometers placed on the receiving element for junction type 1; (b) Measurement setup; (c) Accelerometers placed on the source element for junction type 2; (d) Close-up on the used impact hammer.

4

5. MEASUREMENT RESULTS

Velocity level measurement results on the same CLT wall with tape and bee wax are presented in Figure 6 for different positions according to Figure 2. The result is calculated by subtracting the receiving velocity levels for each position and mounting technique from the reference velocity levels at position 1 for each mounting technique. The impact of both the bearing direction and mounting technique can be observed in Figure 6.

Figure 6: The impact of bearing direction and mounting technique for positions 1-3 on the CLT wall (see Figure 2). The green line, Pos 1: 0°, have both the reference and receiving accelerometers on the same outer lumber board, positioned close to each other, and the velocity level difference is therefore expected to be around zero. The result indicates that the velocity levels are reduced over the CLT wall when the accelerometers are placed on lumber boards that are not directly excited, with a few exceptions for certain frequency bands. Furthermore, Figure 6 also indicates that the difference is small between the two mounting techniques for frequencies below 2.5 kHz, again with some exceptions for certain frequency bands. 5.1. Mounting Technique The impact of different mounting techniques with bee wax and tape, with measurement setup accord- ing to Figure 2, can be calculated as the difference in measured velocity levels at each position. The receiving velocity levels for each position with tape are subtracted from the velocity level with bee wax and presented in Figure 7. The measurement result indicates that the mounting technique with bee wax has higher sensitivity than tape at high frequencies, meaning that weak fixing is more likely to occur for tape at higher frequencies.

Figure 7: The difference in mounting techniques with bee wax and double-sided tape for positions 1-3 on the CLT wall, according to Figure 2. 5.2. Bearing Direction The impact of bearing directions is displayed with several measurements and with two different test methods in Figure 8. Two curves, with yellow and red colors, illustrate test method 1. The measure- ments for method 1 are evaluated by subtracting the receiving velocity levels at positions 2 and 3 respectively from the reference velocity level at position 1 for each mounting technique. The mean value is thereafter calculated for the different mounting techniques and presented in Figure 8 (bee wax with yellow and double-sided tape with red). Values above zero indicate that the velocity level is higher when the bearing direction of the floor is oriented perpendicular in relation to the junction, compared to a parallel bearing direction in relation to the junction.

Furthermore, two grey curves are presented in Figure 8 which illustrates test method 2. The grey curves describe the difference in vibration reduction index between the two junctions for each apart- ment. The difference is calculated by subtracting the vibration reduction index of junction 2 (bearing direction perpendicular to the junction) from the vibration reduction index of junction 1 (bearing direction parallel to the junction), 𝐾𝐾 𝑖𝑖𝑖𝑖,𝑗𝑗𝑗𝑗𝑗𝑗𝑗𝑗𝑗𝑗𝑗𝑗𝑗𝑗𝑗𝑗1 −𝐾𝐾 𝑖𝑖𝑖𝑖,𝑗𝑗𝑗𝑗𝑗𝑗𝑗𝑗𝑗𝑗𝑗𝑗𝑗𝑗𝑗𝑗2 , for the flanking transmission paths that include the floor. The mean value of the different paths for each apartment is displayed with the grey curves in Figure 8. Once again, values above zero indicate that the vibration reduction index is higher when the bearing direction is oriented parallel to the junction, meaning that the measured receiving velocity levels are higher when the bearing direction is oriented perpendicular in relation to the junc- tion. Finally, the mean value of test method 2 is illustrated with a black curve.

The measurement result of both test methods has the same trend, and the values correspond with each other to a certain degree. Measurements from both test methods indicate that there is a difference between different bearing directions. Overall, the values presented in Figure 8 are positive, indicating that the favorable placement of a floor is when the bearing direction is parallel in relation to the junction.

Figure 8: Difference of bearing directions in relation to junctions. The left y-axis describes the ve- locity level difference with the two tested mounting techniques (yellow and red lines). The right y- axis describes the vibration reduction index difference between two junctions (grey and black curves) with different bearing directions in relation to the junction (perpendicular and parallel). 6. DISCUSSION

Field measurements on CLT elements imply several difficulties, including limited measurement time on the site since the contractors are often on a tight time schedule. It is therefore important to find optimal solutions that minimize the time spent on the site, while still making sure to record accurate results. ISO 10848-1 suggests the use of bee wax or petroleum wax as a mounting technique to fix the accelerometers on the elements. However, wax is mainly functional around room temperature and not during cold weather conditions, and the mounting time increases since wax requires some prepa- ration before usage compared to double-sided tape. Measurements were therefore performed on a CLT wall with two different mounting techniques, with bee wax and double-sided tape according to Figure 2. Results indicate small differences in receiving velocity levels at different positions on the CLT element with double-sided tape compared to bee wax for most frequency bands, see Figures 6,7. And the result is similar for all positions, with some exceptions at certain frequency bands. However, double-sided tape performs differently than bee wax at higher frequencies, above 2.5 kHz (Figure 7), with a difference of around 5 dB. ISO 10848-1 mentions that weak fixing could occur with bee wax which could cause measurement errors at high frequencies, and measurements show that this could also be the case with double-sided tape. Moreover, it seems to be more likely that double-sided tape ends up in measurement errors due to the weak fixing of the accelerometers, compared to bee wax, at higher frequencies since bee wax records higher receiving velocity levels.

Although double-sided tape induces more issues at high frequencies compared to bee wax, it is still a suitable option for field measurements. The frequency area that is of most interest for CLT constructions is from 50 Hz up to around 1 kHz, since that frequency range most often determines the sound reduction index of the constructions, concluded from several measurements on CLT build- ings and elements by the authors and by Ref. [16-18]. Therefore, measurement uncertainties at high frequencies are less relevant compared to the time saved when measuring the vibration reduction index in the field. Double-sided tape is therefore an adequate substitute for bee wax in the field since

the difference in velocity level is small between the two tested mounting techniques for frequencies below 2.5 kHz, for the frequency area of most importance. Moreover, vibration reduction index meas- urements are determined by the difference in velocity levels and not the actual velocity levels for each accelerometer position, suggesting that the uncertainties might even out the result. However, double-sided tape should be used with caution and the results and conclusions are mainly valid for CLT constructions. The same conclusion might not be valid for velocity level measurements on light- frame constructions.

Another interesting observation from the measurements is the difference in velocity levels for each mounting technique on the same lumber board at Pos 1: 0°. The velocity levels measured simultane- ously by the accelerometer on the same lumber board, at the same distance from the source, varies between 0-1 dB between 10 Hz to 2.5 kHz, and above 2.5 kHz, the difference is around 3-5 dB (see Figure 8). The difference in high frequencies could, once again, depend on weak fixing but it could also depend on where the accelerometers are placed on the outer lumber board since CLT and wood itself is not a homogenous construction and material [15].

The bearing direction of CLT floors typically varies depending on the layouts of different rooms in relation to the bearing CLT walls. A corner room with four CLT walls can have a minimum of four different junction types which could affect the sound reduction index between dwellings. Field meas- urements of the vibration reduction index were performed in a multi-family building with several stories and with different bearing directions in relation to the junction (perpendicular and parallel), described as test method 2. The result indicates that there is a difference in vibration reduction index depending on the orientation of the bearing direction. Vibration levels tend to be higher on the re- ceiving element when the bearing direction is perpendicular to the junction, meaning that the vibra- tion reduction index of a junction is higher when the bearing direction is parallel to the junction. The difference in vibration reduction index varies, on average, between 0 to 5 dB depending on the fre- quency with a mean value of 2.1 dB over the whole frequency range, 50 Hz to 5 kHz. Orientation of the bearing direction could therefore have an impact of several dB on the weighted sound reduction index between two apartments. These results are concluded in two apartments for different flanking transmission paths. In addition, measurements were performed on the wall where mounting tech- niques were evaluated, described as test method 1. This test method indicates similar results as test method 2, meaning that the receiving vibration levels are higher when the boards are oriented per- pendicular compared to parallel, in relation to the junctions.

The result from both test methods thereby explains, to a certain degree, why some rooms with similar geometry and structural build-up of walls and floor could have different sound insulation properties between apartments. However, the explanation is far more complex than just the bearing direction which probably represents a small contribution to the difference of the total sound insulation between apartments. 7. CONCLUSIONS

The purpose of the paper was to evaluate two different mounting techniques with accelerometers on CLT elements (bee wax and double-sided tape) and to discuss how the bearing direction could affect the vibration level difference of junctions.

Double-sided adhesive tape is an adequate substitute for bee wax in the field for mounting accel- erometers on CLT elements, with some limitations at high frequencies where it is more likely that weak fixing will occur.

Measurement data with two different test methods concludes that the bearing direction of CLT elements, in relation to junctions, can influence the vibration reduction index of different flanking transmission paths. The favorable placement of a floor is when the bearing direction is parallel in relation to the junction. Therefore, the orientation of the bearing direction, in relation to junctions, could affect the total measured sound insulation between apartments.

8. REFERENCES

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