Structural Tests of Concrete Composite-Cross-Laminated Timber Floors

Final Report

Report No. 17-01

Christopher Higgins, Ph.D., P.E. (NY)

Andre R. Barbosa, Ph.D.

and

Curtis Blank, M.S.

12/04/2017

School of Civil and Construction Engineering College of Engineering Oregon State University

Corvallis, OR 97331

1

TABLE OF CONTENTS ACKNOWLEDGEMENTS ...................................................................................................... 2

ABSTRACT ........................................................................................................................... 3

Introduction ...................................................................................................................... 4

Experimental Program...................................................................................................... 5

2.1 Overview ..................................................................................................................... 5

2.2 Comparative One-Way Bending (CB) Tests .............................................................. 6

Specimen Descriptions........................................................................................ 6

Reinforcing Steel .............................................................................................. 13

Concrete Materials ............................................................................................ 15

Experimental Setup for CB specimens ............................................................. 15

Instrumentation Plan and Data Reduction Methods for CB Specimens ........... 17

2.3 Orthotropic Stiffness (OS) Tests ............................................................................... 18

Specimen Descriptions...................................................................................... 18

Experimental Setup for OS specimens ............................................................. 22

Instrumentation Plan and Data Reduction Methods for OS Specimens ........... 27

2.4 Full Scale Specimen (FS).......................................................................................... 29

Specimen Description ....................................................................................... 29

Instrumentation Plan and Data Reduction Methods ......................................... 34

Experimental Setup and Testing Protocol......................................................... 37

2.5 Long-term Creep (LT) Tests ..................................................................................... 39

Specimen Descriptions...................................................................................... 39

Instrumentation Plan and Testing Protocol LS tests ......................................... 45

Experimental Results...................................................................................................... 48

3.1 CB Test Results......................................................................................................... 48

3.2 OS Test Results ......................................................................................................... 53

3.3 FS Test Results ......................................................................................................... 60

3.4 LT Test Results ......................................................................................................... 67

LT- Specimen.................................................................................................... 67

LT+ Specimen ................................................................................................... 68

Conclusions .................................................................................................................... 69

References ...................................................................................................................... 72

Appendix A - CONCRETE SPECIFICATIONS .................................................................... 73

2

ACKNOWLEDGEMENTS

Financial support for this research was provided by the Softwood Lumber Board (SLB)

through Skidmore, Owings, and Merrill LLC (SOM). The findings and conclusions of this work

are solely those of the authors.

3

ABSTRACT

Experimental tests of a composite concrete-cross-laminated timber (CLT) floor system were

conducted. The floor system was constructed with 5-ply CLT panels (6.75 in. thick) made

composite with a 2.25 in. thick reinforced concrete topping slab. Four series of tests were

performed using different specimen configurations and laboratory testing methods. Tests included:

(1) Comparative one-way bending tests (CB) to evaluate the performance of alternative shear

connectors used to join the concrete slab to the CLT panel; (2) Orthotropic stiffness and strength

tests (OS) to evaluate the elastic orthotropic stiffness of the deck system and provide strength

results for weak-axis bending and negative moment strength; (3) Full-scale system performance

tests (FS) of a continuous floor span to establish strength at realistic span lengths and the influence

of continuity; and (4) Long-term deformation tests (LT) to investigate creep deflections of the

composite concrete-CLT floor system considering positive and negative bending influences.

Results include overall strength, elastic stiffness values, deformation capacity, slip

deformations along the concrete-CLT interface, predicted neutral axis locations in the composite

concrete-CLT systems, and connection deformations.

4

Introduction

Skidmore, Owings & Merrill (SOM) recently developed a structural engineering system for a

prototype 42-story building consisting of mass-timber structural elements. The approach is

detailed in two reports (SOM 2013, SOM 2014) that address the potential for a high-rise timber

building. In the first report, design solutions for several specialties including structural,

architectural, interior architecture, and building services were presented. The concepts discussed

for the gravity load resisting system articulated in the first report were further detailed in the second

report, in which SOM proposed the concept of “Concrete Jointed Timber Frame” (CJTF) system.

This gravity framing system includes composite mass-timber floor panels, reinforced concrete

spandrels, and reinforced concrete joints that serve to connect the vertical mass-timber elements.

The SOM CJTF makes use of glulam columns and cross-laminated timber (CLT) walls for the

gravity load system. Columns and walls are connected with interior and perimeter precast concrete

beams and spandrels that support composite concrete-CLT floors, which carry the floor loads.

While the structural behavior of glulam columns is reasonably well understood, the SOM CJTF

has several innovative connection and composite material solutions that require physical testing to

facilitate adoption into practice. Structural tests provide insight into the behavior, produce data to

allow engineers to predict performance of the structural elements, and enable development of

rational design approaches for the system, members, and connections. This report details

experimental tests of a composite concrete-CLT floor system that serves an integral role in SOM’s

CJTF concept.

5

Experimental Program

2.1 Overview

Several experimental tests were conducted to evaluate the structural performance of a new type

of composite-concrete CLT floor system for tall building design. The following specimen types

were investigated:

1) Comparative one-way bending tests (CB)

2) Orthotropic stiffness and strength tests (OS)

3) Full-scale system performance tests (FS)

4) Long-term deformation tests (LT)

All specimens were constructed with 5-ply CLT panels (6.75 in. thick) made composite with

a 2.25 in. thick reinforced concrete topping slab. The CLT used in the specimens was CrossLam®

produced by StructurLam of British Columbia, Canada, APA certified with a V2 grade according

to ANSI/PRG 320. The timber in the panels consisted of spruce-pine-fir V2 grade for all

laminations, a nonvisual grade unfinished top surface (at the concrete interface surface), and a

finished bottom surface. A water repellent was applied to all top surfaces and end grains in the

shop. The CLT panels were fabricated to the required finished dimensions prior to shipping to the

Structural Engineering Research Laboratory at Oregon State University. In the laboratory,

different shear connectors were installed as prescribed, formwork was attached to edges of the

panels, reinforcing steel was fabricated and tied, and then concrete was placed, consolidated,

finished, and cured. Specimens were then placed in different laboratory setups and tested. Details

of the specimens and experimental methods are described in this chapter.

6

2.2 Comparative One-Way Bending (CB) Tests

Specimen Descriptions

Eight (8) one-way bending specimens were designed, constructed, and tested to assess and

compare the behavior and strength of 5-ply CLT panels joined to a concrete topping slab using

alternative shear connectors at the interface. The width of the specimens was 24 in. and the span

length was 10 ft. The overall panel length was 10 ft – 8 in. to provide a 4 in. overhang past the

centerline of the supports. The test matrix is shown in Table 2.1. Two specimens consisted of 5-

ply CLT panels without a topping slab for reference. The other six specimens consisted of CLT

panels with a 2.25 in. thick normal-weight concrete slab. The total height of the CLT with concrete

topping slab was 9 in. Four alternative shear connections were investigated: tightly spaced (12 in.)

diagonally inclined self-tapping screws (STS), widely spaced (24 in.) diagonally inclined STS,

HBV steel mesh connectors, and Nelson headed studs welded to a plate attached to the CLT with

STS. Two replicate specimens were tested for each of the STS specimens. All specimens were 24

in. wide by 128 in. long. Specimens with STS were designated as SC1, SC2, SW1, and SW2, with

C representing the closely spaced (12 in.) screw spacing and the W representing the widely spaced

(24 in.) screw spacing. The HBV specimen was designated as HBV and the headed stud specimen

was designated as HS. The panels without concrete topping were designated as CLT1 and CLT2.

Table 2.1 - Test matrix for one-way bending comparative tests with alternative connectors.

Specimen ID Specimen Type

Shear Connection Type Concrete Batch Average Moisture Content (%)

CB-CLT1 1 No Concrete NA 12.1 CB-CLT2 1 No Concrete NA 12.0 CB-SC1 2 Closely Spaced STS 1 12.5 CB-SC2 2 Closely Spaced STS 1 12.8 CB-SW1 3 Widely Spaced STS 1 11.6 CB-SW2 3 Widely Spaced STS 1 12.0 CB-HBV 4 Steel Mesh with Epoxy 1 12.8 CB-NS 5 Studs on Plates with STS 2 13.7

7

CB-STS Specimens

Two different specimen configurations were constructed using STS: designated specimens SC

and SW. These specimens were constructed using ASSY plus VG screws having a cylindrical head

as seen in Fig. 2.1. The steel screws were zinc coated and had a self-drilling point. The screw sizes

were 5/16 in. diameter and 8 5/8 in. long. The screw holes were partially predrilled to a depth of

approximately 1 in. to assure the screw geometry. The screws were installed at a 45-degree angle

to the surface of the CLT using a drilling guide to ensure the proper placement angle. The depth

of penetration was 5.25 in. (vertical projection) placing the tip approximately at the center of the

4th layer measured from the top surface of the CLT panel. The vertical projection of the shank in

the concrete slab was approximately 1-3/8”. Screws were inclined in the direction of the shear flow

so that extraction would dominate the loading condition on the screws. These four specimens were

constructed from the first batch of concrete. Details for the CLT panels with STS connectors are

shown in Figs. 2.2a and 2.2b and 2-3a and 2-3b for the CB-SC and CB-SW configurations,

respectively.

8

Fig. 2.1: Diagonally-installed STS screw in CLT panel (shown with #3 rebar tied to provide required cover).

Fig. 2.2a: Plan view of details for CB-SC specimens with closely spaced STS

9

Fig. 2.2b: Elevation view of details for CB-SC specimens with closely spaced STS.

Fig. 2-3a: Plan view of details for CB-SW specimens with widely spaced STS.

Fig. 2.3b: Elevation view of details for CB-SW specimens with widely spaced STS.

10

CB-HBV Specimen

The CB-HBV specimen was constructed using ten (10) HBV steel mesh connectors as seen in

Figs. 2-4a and b. The connector plates were 19.75 in. long, and 2 9/16 in. tall. The connector plates

were embedded 1.25 in. into 1/8 in. wide continuous grooves that were saw cut into the top surface

of the CLT panel. Proprietary epoxy manufactured specifically for installation of the HBV

connectors was used to bond the steel mesh connectors in the grooves of the CLT panel. To

maintain the top cover over the transverse reinforcing bars, a ¾ in. deep notch was cut in the center

of each HBV plate where it coincided with the transverse reinforcing steel as seen in Fig. 2.5. HBV

connector plates along the edges were spaced 30 in. on-center in the direction of span and 16 in.

on-center in the transverse direction as seen in Fig. 2.4a. This specimen was constructed using the

first batch of concrete.

Fig. 2.4a: Plan view of details for CB-HBV specimen.

11

Fig. 2.4b: Elevation view of details for CB-HBV specimen.

Fig. 2.5: Details of notch in HBV connector to maintain cover over #3 reinforcing steel.

CB-Headed Nelson Stud Specimen

The CB-HS specimen was constructed using eight (8) specially made steel plates fastened to

the CLT with STS screws as seen in Fig. 2.6a and 2.6b. The A36 steel plate was 1/4 in. thick, 4 in.

wide, and 13 in. long. Four (4) Nelson studs, 3/8 in. diameter by 1-3/8 in. long, were arc welded

to each plate. The studs were space at 3 in. on-center. Each plate was fastened to the CLT panel

using ten (10) STS screws located around the perimeter of the plate. The STS screws were 5/6 in.

diameter and 3-1/8 in. long ASSY 3.0 SK screws. The screw holes in the steel plate were 3/8 in.

12

diameter and had an edge distance of ½ in. This is the only specimen in this test series that was

constructed using the second batch of concrete. A drawing and a digital image of a plate with

welded headed studs attached to the CLT panel surface with STS and reinforcing steel prior to

concrete placement is shown in Fig. 2.7a and b, respectively.

Fig. 2.6a: Plan view of details for CB-HS headed stud specimen.

Fig. 2.6b: Elevation view of details for CB-HS headed stud specimen.

13

Fig. 2.7a: Top view of plate with welded headed studs attached to CLT panel with STS.

Fig. 2.7b: Dimensions of steel plate with welded headed studs (hatched circles) attached to CLT panel with STS (open circles).

Reinforcing Steel

The reinforcing steel consisted of ASTM A615 Grade 60 #3 bars that were fabricated by a

local reinforcing steel fabricator. Standard 90o hooks were fabricated at the ends of the bars and

1.5 in. side cover was provided. Reinforcing bars were placed on the CTL panels and positioned

to provide ¾ in. top clear cover. They were held in place by attaching them to the relevant shear

14

connectors with ductile steel tie wire (see Fig. 2.1). Reinforcing bars were spaced 6 in. on-center

in the direction of the span and 12 in. on-center transverse to the direction of the span as seen in

Fig. 2.8. Material properties were established according to ASTM E8 (2015) from reinforcing steel

samples taken from the same heat of steel. Strain was measured using a 2 in. gage length. Five (5)

replicate samples were tested. The measured material properties are reported in Table 2.2.

Fig. 2.8: Reinforcing details and placement for one-way bending specimens (CB-SC specimen shown prior to placement of edge forms).

Table 2.2: Reinforcing steel material properties

Reinforcing Bar Size

Yield Stress (ksi)

Ultimate Stress (ksi)

Elongation (%)

#3 Average COV Average COV Average COV 66.1 8.0% 103.9 5.3% 19.3 16.8%

15

Concrete Materials

Specimens in this test phase were constructed from two different batches of concrete using the

same concrete mix design. The mix proportions are shown in Appendix A. The mix used a

relatively low water/cement ratio of 0.45, 3/8 in. maximum aggregate size, and was designed to

achieve 5,000 psi compressive strength at 28 days. Concrete was provided by a local ready-mix

supplier and placed into the forms of the CLT panels. The concrete was placed and then

consolidated using a vibrating roller screed. Power and hand troweling were used to achieve a

smooth and dense finish. After finishing, a water-based curing and sealing compound was applied

to help retain moisture in the concrete. Specimens were cured for at least seven days before moving.

Cylinders, 4 in. diameter by 8 in. long, were produced at the time of concrete placement and field

cured. Compressive strength was established according to ASTM C39 (2012) flexural tensile

strength (MOR) was established according to ASTM C78 (2012). The concrete compressive and

flexural strengths for each of the concrete placements are reported in Table 2.3.

Table 2.3: Measured concrete mechanical properties.

Placement ID

CB- Specimens

Age (days)

Avg. Compressive

Strength (psi)

St. Dev. Compressive

Strength (psi)

Average MOR (psi)

St. Dev. MOR (psi)

Cast 1 SC1, SC2, SW1, SW2

HBV

28 5435 219 - - 110 5772 630 387 13 138 5840 389 393 -

Cast2 HS 28 4989 213 - - 56 6475 195 346 15 101 6331 213 349 13

Experimental Setup for CB specimens

All one-way bending specimens were tested in a simply supported configuration on a 120

in. span length with a line load applied at midspan. This loading produced three-point bending of

16

the specimens. The simple supports consisted of 2 in. diameter steel rollers in contact with a 4 in.

wide, 1 in. thick plate under the CLT panel to prevent perpendicular to grain crushing. The loading

frame with a typical specimen is shown schematically in Fig. 2.9a and a digital image of the setup

is shown in Fig. 2.9b. A W8x31 steel beam was connected to the load cell to spread the load

uniformly across the specimens and prevent transverse curvature. Under the W8x31 was a 2 in.

diameter steel roller that rested on a 1/8 in. thick aluminum strip to provide uniform contact across

the concrete surface at the loading location. The specimens were subjected to increasing amplitude

monotonically applied load at a constant displacement rate of 0.01 in/sec until failure. Prior to

testing, the moisture content was measured and recorded. The average measured moisture content

for each specimen at the time of test was 11.4, 11.7, and 13.0% for specimens OS-1, OS-2, and

OS-3, respectively.

N

Load Cell

2.25 in. Concrete slab

10 ft

2 in. roller on 4 in. bearing Plate

W8x31Strong Floor2 in. roller

5 ply CLT

Reaction FrameServo-hydraulic Actuator

Fig. 2.9: Schematic of test setup with typical CB specimen (strong-direction positive bending).

17

Fig. 2.10: Images of the experimental setup for the comparative one-way bending specimens.

Instrumentation Plan and Data Reduction Methods for CB Specimens

Twenty two (22) sensors were used to monitor the behavior of each specimen during tests.

This included eight (8) displacement transducers to measure the vertical displacement of the

specimen relative to the laboratory floor, ten (10) displacement transducers to measure slip across

the interface between the concrete and CLT, two (2) sensors to measure longitudinal strain in the

top and bottom layers of the CLT panel, a load cell to measure the applied actuator force and lastly

a displacement transducer to measure the actuator displacement. The instrument locations and

channel reference labels are shown in Fig. 2.11.

Both the east (E) and west (W) faces of the specimens were instrumented to measure the

vertical deformations of the specimens relative to the laboratory floor at the supports and at

midspan. The average specimen displacements along the span were taken as the average of the

east and west sensors at the coincident span locations. The rigid body motion of the specimen (due

to such things as non conservative contact bearing at supports) was removed to provide the

absolute specimen displacement by subtracting the average support deformations. Sensors placed

to measure relative slip between the concrete and top CLT layer were spaced at 12 in. on-center

18

along the span with the first and last sensors located above the simple supports. The slip sensors

were used to compute the slip strains along the panel by taking the change in slip deformations

between adjacent sensors and dividing by the 12 in. sensors spacing taken as the gage length. The

slip deformation at the load point (midspan of the specimens) was assumed to be zero.

N2

N

NWS0, NES0SWS0, SES0

N3N4S4S3S2S1S0 N1 N0TS

BS

NEQSEQ

12 in.

Load, ActDisp

CLE, CLW30 in.

30 in.

Fig. 2.11: Typical instrumentation layout for CB specimens.

2.3 Orthotropic Stiffness (OS) Tests

Specimen Descriptions

The OS specimens were constructed to establish the elastic orthotropic stiffness properties

and orthotropic strength characteristics for a concrete composite with CLT panels using the closely

spaced STS as shear connectors. The configuration of the STS for the OS specimens corresponded

to those from the CB-CS1 and CB-CS2 specimens described in the previous section. However, to

establish the orthotropic stiffness properties representative of a continuum for a typical building

floor system, wider specimens were developed for this test series. To facilitate measurement of the

twisting stiffness, a square panel was desirable. The selected specimen dimension was 8 ft square

that allowed a span length of 7 ft-4 in. with 4 in. overhang at support locations. Three (3) specimens

were constructed.

19

The specimens were fabricated with the same CLT and concrete topping slab as described

previously. ASSY plus VG screws with 5/16 in. diameter and 8 5/8 in. length were installed in the

specimens at a 45 degree angle to the surface of the CLT using the same drilling guide as described

previously. The screw holes were also partially predrilled to a depth of approximately 1 in. to

assure the screw geometry. The same depth of penetration of 5.25 in. (vertical projection) was

employed providing a vertical projection of the shank in the concrete slab of approximately 1-

3/8”in. The screws are all aligned in the strong-direction of the panel. All three specimens were

constructed from the first batch of concrete. The concrete was placed, finished, and cured using

the procedures detailed previously in Section 2.2.3. Reinforcing steel was #3 bars from the same

heat described previously in Section 2.2.2. The bars were fabricated by a local reinforcing steel

fabricator with standard 90o hooks at the ends. Side cover of 1.5 in. and ¾ in. top clear cover was

provided. The bars were held in place by attaching them to the relevant shear connectors with

ductile steel tie wire and spaced 12 in. on-center in both directions. Details for the OS specimens

are shown in Figs. 2.12a and 2.12b. A digital image of an OS specimen prior to placement of the

concrete is shown in Fig. 2.13.

20

Fig. 2.12a: Plan view of details for OS specimens.

21

Fig. 2.12b: Elevation view of details for OS specimens.

Fig. 2.13 Top View of OS specimens with reinforcing steel and side forms prior to placement of concrete.

All three OS specimens were tested in each configuration to provide replicates for

determining the elastic orthotropic stiffness properties. Twelve (12) elastic tests were performed.

The following four (4) test configurations were investigated: strong-direction positive bending

22

(concrete in flexural compression) designated as OS-Dx+, strong-direction negative bending

(concrete in flexural tension) designated as OS-Dx-, weak-direction positive bending (concrete in

flexural compression) designated as OS-Dy+, weak-direction negative bending (concrete in

flexural tension) designated as OS-Dy-, twisting positive bending (concrete in compression)

designated as OS-Dxy+, twisting negative bending (concrete in flexural tension) designated as OS-

Dxy-. After completion of the elastic orthotropic stiffness tests, three (3) failure tests were

conducted: OS-Dx-, OS-Dy+ and OS-Dy-. The strong-direction positive bending strength was not

investigated in this series, as the previous CB-CS1 and CB-CS2 specimens could be used as

comparisons. To conduct the negative bending and twisting tests, the specimens were carefully

inverted to place the concrete on the bottom of the specimens prior to conducting the relevant tests.

Experimental Setup for OS specimens

Two different setups were required for the three loading conditions (strong-direction

bending, weak-direction bending, and twisting). The strong and weak-direction bending

configurations used the same setup with a servo-hydraulic actuator and stiff loading beam at

midspan to enforce one-way bending in the specimens as illustrated in Figs. 2.14a and 2.14b. The

span length was 7 ft-4 in. with a 4 in. overhang of the specimens past the lines of support. A digital

image of a specimen in the test setup is shown in Fig. 2.15. Only the specimen orientation had to

be changed (rotated 90o on the supports) to conduct the bending tests in the two orthogonal

directions.

The maximum load applied to the OS bending specimens were estimated based on results from

the CB specimens. The maximum loads were established to keep the response in the linear elastic

23

range. The maximum applied load was 43 kips for the strong-axis and 33 kips for the weak-axis

orientations. The observed response was indicative of elastic behavior in all cases.

The orthotropic bending stiffness properties, Dx and Dy (kip-in2/in.) are computed as:

𝐷𝐷𝑦𝑦 = 𝑃𝑃∆𝐶𝐶𝐶𝐶

𝐿𝐿3

48𝑤𝑤; 𝐷𝐷𝑥𝑥 = 𝑃𝑃

∆𝐶𝐶𝐶𝐶

𝐿𝐿3

48𝑤𝑤 [1a and 1b]

where L (in) is the span length (88 in. for present span), w (in) is the width of the panel (96 in. for

present specimens), P (kips) is the applied load, and ∆CL (in) is the midspan displacement. In each

test configuration, the specimens were loaded and unloaded 3 times and a best-fit line of the load-

average midspan displacement (corrected for support motions) was generated for each loading and

unloading cycle. To compute Dx and Dy in the present work, P/∆CL was taken as the best-fit average

of the three loading and unloading cycles for each specimen.

West

12 in.

88 in.

4 in.2.25 in. Concrete slab

2 in. roller

5 ply CLT

W30x311

W12x120

W14x145

Strong Floor

Servo-hydraulic Actuator

Fig. 2.14a: Elevation view of experimental setup for OS bending tests (looking north).

24

N

2.25 in. Concrete slab

8 ftStrong Floor

2 in. roller

5 ply CLT

W30x311

Servo-hydraulic Actuator

W14x176

W14x145

Fig. 2.14b: Elevation view of experimental setup for OS bending tests (looking west).

Fig. 2.15: Digital photograph of OS specimen in strong-axis negative bending.

25

The torsional stiffness test setup required supporting two (2) opposite corners of the

specimens on rollers placed on the strong floor, anchoring one corner to the strong floor to prevent

it from uplifting, and then placing pre-measured static weights on the remaining free corner as

illustrated in Fig. 2.16. This support and loading configuration produces twisting of the specimen.

The opposite corners were supported 2 in. diameter steel rollers, which were 10 in. long. Steel

plates 4 x 12 in. were placed at the loaded corner and the held-down corner to provide a uniform

contact area at precisely known locations on the panel. Three (3) steel weights were incrementally

placed on the specimen as illustrated in Fig. 2.17. The first weighed 166 lbs, the second weighed

172 lbs, and third weighed 224 lbs. The maximum load applied to the corner of the specimens was

562 lbs. The load sequence was placed and removed 3 times. This provided step loading and

unloading functions on the specimen as seen in Fig. 2.18. The first loading and unloading sequence

was used to ensure the specimens were well seated in the setup and not used to determine the

stiffness properties. Only the second and third loading and unloading sequences were used to

compute the twisting stiffness. The torsional stiffness parameter Dxy can be computed for the given

loading setup as:

𝐷𝐷𝑥𝑥𝑦𝑦 = 𝑃𝑃∆𝐶𝐶

𝐿𝐿2

4 [2]

where P is the applied load, L is the edge distance of the panel from the roller support to the steel

contact plates on the specimen (88 in. for present case), and ∆L (in) is the displacement of the

specimen under the loading point. Each specimen was loaded and unloaded 3 times (with three

step intervals) and a best-fit curve of the load-corner displacement (corrected for support motions)

was generated for the second and third loading and unloading cycles. To compute Dxy in the present

work, P/∆L was taken from the best-fit average of the three loading and unloading cycles.

26

Fig. 2.16: Schematic of support and loading conditions for OS specimen to determine torsional stiffness.

Fig. 2.17: Digital photograph of OS specimen in torsional stiffness test (concrete is in compression).

∆L

27

Fig. 2.18: Step loading function from incremental steel weights applied to the corner of twisting OS specimens.

Instrumentation Plan and Data Reduction Methods for OS Specimens

Two different instrumentation plans were required for the OS specimens. The bending tests

required one set and the torsional test required a separate set of instrumentation as described in the

following sections.

OS Strong-axis and Weak-axis Bending Tests

A total of 19 channels were used for the OS bending tests. Three (3) displacement sensors

were placed above the east (E) and west (W) supports to measure support motion relative to the

strong floor. At midspan, attached to the underside of the specimens, three (3) displacement

sensors were placed to measure the specimen displacement relative to the strong floor. The average

rigid body support deformations were removed from the average midspan displacements.

Displacement sensors were installed to measure the relative displacement between the concrete

topping slab and upper lamination of the CLT panel. Slip sensors were placed over the roller

support locations and spaced 12 in. toward the center of the panel on both sides of loading point.

28

Slip strains were computed using the method described for the CB specimens. Two additional

displacement sensors were used to measure strains in the upper and lower laminations of the CLT

panel. These sensors were placed 30 in. from the west roller support end and had a gage length of

5.5 in. Load was measured by a load cell mounted in series with the servo-hydraulic actuator. The

instrumentation plan for the orthotropic bending tests are shown in Fig. 2.19.

W2

West

ENS,ECLS,ESS

W1 W012 in.

Load, ActDisp

CLN, CLM, CLS

44 in.

E3E2E1E0 TS

BS

W3

WNS,WCLS,WSS

12 in.

30 in.

5.5 in. gage

Fig. 2.19: Instrumentation plan for OS specimens subject to strong and weak-axis bending (positive bending strong-axis shown).

OS Torsional Test

For the OS twisting test, only four (4) displacement sensors were required. Sensors were placed

at each corner of the specimen to measure the vertical movement of the panel relative to the strong

floor as illustrated in Fig. 2.20. Three (3) of the four sensors were used to measure the rigid body

movements of the specimen. Rigid body motions were removed from the measured free end

29

deformation by subtracting the average settlement of the specimen at the roller-supported corners

and removing the rotation about the roller supports as measured at the corner opposite the load

point.

SE

SW

NW

NEFree corner (load applied )

Held-down corner

Fig. 2.20: Instrumentation plan for OS specimens subject to twisting (concrete in tension shown).

2.4 Full Scale Specimen (FS)

Specimen Description

The overall length of the FS specimen was 37.5 ft and consisted of a 24 ft main span with 6 ft-

9 in. cantilever overhangs on the north and south ends as illustrated in Figs. 2.21a and 2.21b. The

center of reaction on the cantilever overhangs were 6 ft from the centerlines of bearing for the

main span. The cantilevered ends were held down to the strong floor to represent the negative

moment produced in a continuous floor system and the reaction point represents approximately

the point of inflection on the span for the uniform dead load case.

30

The specimen was constructed using 5 different individual 5 ply CLT panels that were the

same as that described for the CB and OS specimens. There were two 6 ft long cantilever end

panels, two 3 ft wide beam ledge panels to support the main and cantilever CLT panels and to form

the integral concrete beam, and the 22.5 ft long main span panel (strong direction of the ledge

beam was orthogonal to the main and cantilever spans). The cantilever and main span CLT panels

were 8 ft wide, while the beam ledge CLT panel was 8 ft long. The cantilever and main span panels

rested on the beam ledge CLT panel over a 9 in. bearing length and were fastened together with

CSK screws as illustrated in Fig 2.22. Concrete composite shear connectors were the same ASSY

STS used in the CB and OS specimens and used the same details and installation methods. The

spacing of the ASSY STS screws varied over the CLT panels as illustrated in Fig. 2.21b. A 2.25

in. thick concrete topping slab was placed on the CLT panels. The concrete material was the same

as that detailed previously for the CB and OS specimens and was placed, consolidated, and cured

in the same manner. The reinforcing spacing was similar to the OS specimens with the #3

reinforcing bars spaced at 12 in. on-center in the longitudinal and transverse directions. There was

a lap splice detail located at midspan of the main span with a lap length of 24 in. The reinforcing

details within the transverse concrete beams is shown in Fig. 2.23 and included two (2) #3 bars on

the bottom and three (3) #3 bars on the bottom transverse to the floor system span as well as #3

closed stirrups spaced at approximately 6 in. on-center. The specimen was constructed in the setup

are of the laboratory and then moved as a monolithic element onto the laboratory strong floor for

testing. Support conditions during specimen movement were designed to represent the test support

conditions to minimize transport induced self-weight tensile stresses as shown in Fig. 2.24.

Fig. 2.21a: Plan view of full-scale system specimen.

32

Fig. 2.21b: Elevation view of FS specimen.

Fig. 2.22: Elevation view of interior support connection between CLT panels and CLT ledge beam for FS specimen.

Fig. 2.23: View of interior support reinforcing cage (#3 stirrups and #3 transverse bars).

34

Fig. 2.24: Lifting and moving specimen from construction location onto strong floor.

Instrumentation Plan and Data Reduction Methods

To measure the response of the FS specimen, 40 different sensors were deployed as illustrated

in Figs. 2.25a and 2.25b. There were 9 displacement sensors located underneath the main span to

measure the vertical deformations relative to the strong floor (3 across midspan, 3 across at ¼ span,

and 3 across at ¾ span). The three displacement sensors at each span location were averaged to

report the average specimen deformation at the location. Two displacement sensors positioned at

the tip of each cantilever and located transversely at 1/3 points were used to measure the cantilever

tip motion relative to the strong floor (4 total). These were averaged to give the average cantilever

top deformation. Above each roller support, two displacement sensors were placed to measure

support deformations relative to the strong floor. These sensors were used to determine the average

support motions (due to such things as fiber crushing, support settlement, etc.) relative to the strong

35

floor and permitted rigid body movements to be removed from the other vertical displacement

sensors placed on the strong floor.

Bending strains in the extreme CLT lamella of the main span were measured with two

displacement sensors located 3 in. away from midspan. The gauge length used for the measurement

was 5.5 in. Slip deformations between the concrete slab and uppermost CLT lamella were

measured with displacement sensors located on the main span and at the tips of the cantilevers.

Slip sensors on the main span were spaced as shown in Fig. 2.25a.

Fig. 2.25a: Elevation view of instrumentation plan for FS specimen (upper is north side, lower is south side).

36

Fig. 2.25b: Plan view of instrumentation plan for FS specimen (upper is north side, lower is south side).

The ledge beam connections to the main and cantilever spans were instrumented with

displacement sensors to determine the relative motions taking place between the different

connected elements as shown in Figs. 2.26a and 2.26b.

SEJTop

SEJBottom

Note: SE = SoutheastSW=SouthwestNE=NortheastNW=Northwest

SECantV SEMainV

Fig. 2.26a: Locations of displacement sensors to measure motions at ledge beam connection.

37

Fig. 2.26b: Digital image of sensor locations at ledge beam connections (SE connection shown).

Experimental Setup and Testing Protocol

The FS specimen was placed onto W33x291 reaction beams placed on the laboratory strong

floor at a spacing of 24 ft on-center as seen in Fig. 2.27a and 2.27b. A 2 in. diameter steel roller

and 4 in. wide by 1.5 in. thick bearing plate were placed between the steel reaction beam and

specimen to provide a continuous support reaction along the transverse ledge beams. The

cantilever ends were anchored using MC12x50 sections anchored down to the strong floor with

1.25 in. diameter A193-B7 threaded rods. The channels and anchorages restricted uplift of the

cantilever ends and produced negative moment in the specimen over the reaction beams when load

was applied to the main span. The threaded rods were hand tightened during assembly to bring

surfaces into contact but not to produce initial negative bending.

The specimen was loaded with two line loads spaced 6 ft – 6 in. apart and centered over the

main span. The shear span from the line load to simple supports was 8 ft 9 in. The load was

distributed to the concrete surface through two stacked 4 in. wide by 1 in. thick steel bearing plates

that were 8 ft long. Stiff loading beams (W14x145) attached to a transfer beam (W14x176) were

connected to a servo-hydraulic actuator and the loading setup produced one-way bending in the

38

specimen with a constant moment between the line loads. Prior to testing, the moisture content of

the specimen was measured. The average moisture content of the FS specimen was 13.1%. The

specimen was tested in displacement control at a continuous rate of 0.01 in/sec and several loading

and unloading intervals were performed. The actuator load was applied to several load levels as

shown in Table 2.4. After reaching each load level, the load was reduced by approximately 80%

to reduce creep effects and to permit observations to take place. After achieving the 50 kip load

level, the load was reduced to the design level of 10 kips, and then reloaded to assess the hysteresis

of the floor system.

Fig. 2.27a: Elevation view of FS experimental setup (northern portion shown looking east, symmetrical about actuator).

39

W

8ftStrongfloor

2in.Steel roller

W33x291

Servo-hydraulicActuator

W14x145

2.25 in.Concreteslab5plyCLTfloorpanel

1in.Steel bearingplate

2-1 in. Steel bearingplates

W14x176

5plyCLTledgebeam

Fig. 2.27b: Elevation view of FS experimental setup (looking north).

Table 2.4: Load levels for FS specimen.

Load Level Load (kips) 1 10 2 15 3 25 4 40 5 50 6 60 7 80

2.5 Long-term Creep (LT) Tests

Specimen Descriptions

Two long-term creep deformation specimens were constructed for this test program. One

specimen was designed to assess the contributions of positive moment to long-term deformations

40

(designated as LT+) and the other specimen was designed to assess the contributions of negative

moment to long-term deformations (designated as LT-).

The LS+ specimen was 20 ft – 8 in. long and 4 ft wide as shown in Fig. 2.28a and 2.28b. It

was tested on a 20 ft simple span, allowing 4 in. overhang past the centerline of supports. The 20

ft span length corresponds approximately to the positive moment region between dead load points

of inflection on a 24 ft multi-span continuous floor system. The concrete, reinforcing steel, and

CLT were the same materials as those used in the previously described specimens. The

construction practices were also the same. The shear connectors used for the LT specimens to

develop composite action were the same STSs as described previously. The placement of the STSs

was similar to that used in the FS specimen with spacing of 12 in. on-center from the support to 5

ft - 4 in. into the span and then spaced 24 in. on-center to midspan. The reinforcing steel was

spaced 6 in. on-center in the direction of the span and 12 in. on-center in the transverse direction.

The roller support consisted of a 2 in. diameter Schedule 40 steel pipe supported on a Douglas Fir

4x4 placed on the laboratory floor as shown in Fig. 2.29.

Fig. 2.28a: Plan view of LT+ specimen overall geometry, reinforcing steel, and STS placement.

Fig. 2.28b: Elevation view of LT+ specimen overall geometry, reinforcing steel, and STS placement.

Fig. 2.29: Steel pipe roller support for LT+ specimen.

The LT- specimen was 13 ft – 6 in. long and 4 ft wide as shown in Fig 2.30a and 2.30b. It was

tested as two cantilever spans that balance on a continuously supported modified transverse ledge

beam. The 6 ft-9 in. distance from the centerline of support to the tip of each cantilever corresponds

approximately to the negative moment region between dead load points of inflection on a 24 ft

multi-span continuous floor system. The concrete, reinforcing steel, and CLT were the same

materials as those used in the previously described specimens and the construction practices were

the same. The shear connectors used for the LT specimens to develop composite action were the

same STSs as described previously. The STS placement was similar to that of the FS specimen

with spacing of 12 in. on-center from the support to 52 in. into the cantilever span and then spaced

18 in. on center to the tip of the cantilever. The reinforcing steel in the concrete slab was spaced

6 in. on-center in the direction of the span and 12 in. on-center in the transverse direction. The

concrete beam section over the interior support for the LT- specimen was reinforced with two (2)

#3 bars on the top and bottom transverse to the floor system span as well as #3 closed stirrups

spaced at approximately 6 in. on-center as shown in Fig. 2.31. The modified ledge beam consisted

of solid-sawn Douglas Fir with finished dimensions of 1.5 in. thick and 11.125 in. wide laid flat to

43

form the concrete beam with the CLT panels overlapping 1.75 in. on each side as seen in Fig. 2.32.

The modified ledge beam was supported on two pieces of W14x158 that were 17 in. long and

placed on the laboratory floor. The W14 supports required the modified ledge beam to span only

14 in.

Fig. 2.30a: Plan view of LT- specimen overall geometry, reinforcing steel, and STS placement.

Fig. 2.30b: Elevation view of LT- specimen overall geometry, reinforcing steel, and STS placement.

44

Fig. 2.31: Reinforcing details in the concrete in-fill beam over support location for LT- specimen.

Fig. 2.32: Modified ledge beam support for LT- specimen (hairline cracks observed in topping slab).

45

Instrumentation Plan and Testing Protocol LS tests

After the concrete on the LT specimens reached 28-day strength, the formwork was removed

and displacement sensors replaced on the specimens. The displacement sensors selected were

manual dial gages (Fig. 2.33) which are less susceptible to thermal drift and had a resolution of

1/1000 in. Three sensors were placed on the LT+ specimen to measure vertical displacement of

the simple span at the ¼, midspan, and ¾ span locations. Two displacement sensors were placed

on the LT- specimen, one on each cantilever end.

After installation of the sensors, 5 in. thick, 48 in. wide by 24 in. long concrete blocks were

placed on the top of the specimens. The concrete blocks were sized to provide a target additional

superimposed uniform load of approximately 60 psf. This load magnitude was chosen to represent

additional dead loads as well as a fraction of the live load that act on the floor system over the

long-term. The 24 in. dimension was oriented in the direction of the specimen span and spaces of

approximately 0.5 in. were used between blocks to prevent bridging of the load. Each block was

weighed prior to placement on the specimens and the average block weight was 466.8 lbs that

corresponds to a uniform load of 58.4 psf. The concrete blocks covered the entire surface area of

the specimens. To ensure a more uniform pressure loading across the specimen surface, rubber

mats were placed between the concrete slab and concrete blocks as shown in Fig. 2.34. The LT-

specimen was supported at the cantilever ends during casting and curing of the concrete topping

slab. Next, the concrete block weights were applied at an age of 33 days. After all the concrete

blocks were installed, the jacks were slowly released so that the floor system alone balanced and

supported the weights. Once the panel had the cantilever end supports removed, there was some

rigid body rotation, which resulted in one sensor showing negative values (uplift), and the other

positive values (downward). The rigid body rotation was removed by taking the average initial

46

values so that the initial cantilever tip deflections were the same in both spans and downward

(0.0255 in.).

Once the weights were in place, the area around the specimens was cordoned off to prevent

accidental contact with the specimens. Data from the vertical displacement sensors for both

specimens were manually collected daily for a period of 130 days.

Fig. 2.33: Dial gage to measure long-term vertical deformations of LT specimens (LT- shown).

47

Fig. 2.34: Rubber mats to provide more uniform distribution of load from concrete block static weights (LT+ in foreground).

48

Experimental Results

The key results are reported here for all of the experiments conducted in this research program.

Results are reported in separate sections for each specimen configuration and comprehensive data

sets can be made available in electronic format.

3.1 CB Test Results

All CB specimens failed due to tension fracture of the lowest lamination of the CLT panel.

Concrete crushing of the concrete was not observed when the maximum load was achieved. The

overall applied load-midspan displacement responses for all CB specimens are shown in Fig. 3.1.

As seen here, the stiffness of the HBV specimen was over twice that of the plain CLT specimens

and the strength was over 1.5 times that of the plain CLT specimens. The HS specimens had a

similarly stiff elastic loading response, but significant slip occurs early in the loading history (12.5

kips) which reduced the stiffness and limited the strength. The plain CLT specimens exhibited the

lowest stiffness and strength and clearly show the contribution of the alternative connectors to the

structural response.

The loading branch between and applied load of 5 and 15 kips was used to compute the best-

fit line to describe the elastic stiffness (load/average midspan displacement with rigid body support

motions removed). This range of load is sufficiently away from initial seating and load stiffening,

but prior to significant slip of the shear connectors. All specimens with the exception of CB-HS

were elastic within this loading range. The elastic stiffness for specimens CB-HS was determined

from 5 kips to 12.5 kips (when first slip was observed). The best-fit elastic stiffness values were

used to compute Dx for the 24 in. wide specimens according to Eqn. 1. The best-fit elastic stiffness,

Dx, maximum load, displacement at maximum load, and load occurring at abrupt first slip are

reported in Table 3.1. The average strength, maximum deformation capacity, and Dx of specimens

49

with different shear connectors are compared to the average results from the CB-CLT specimens

in Table 3.2. The results show that the addition of composite action increases the strength and

stiffness in all cases, but reduces the deformation capacity.

The two CB-SC specimens exhibited very similar responses. One of the CB-SW specimens

exhibited load-deformation response similar to the CB-SC specimens, but more slip was observed

at similar load and the strength was reduced. The two CB-CLT specimens exhibited different

stiffness and strength, which indicates quite a lot of dispersion and the need for additional samples

to statistically quantify CLT properties generally as well as those of composite concrete-CLT

specimens.

Fig. 3.1: Overall load-midspan displacement response for all CB specimens.

50

Table 3.1: Key response parameters for CB test specimens.

Specimen ID Best Fit Elastic

Stiffness (kip/in)

Dx (kip-in2/in)

Load at First

Abrupt Slip** (kips)

Peak Load (kips)

Displacement @ Peak Load (in.)

CB-CLT1 13.71 20,569 - 21.9 1.72 CB-CLT2 16.05 24,070 - 26.2 1.76 CB-SC1 24.08 36,121 - 30.7 1.50 CB-SC2 25.30 37,947 - 31.1 1.53 CB-SW1 21.35 32,024 - 27.2 1.52 CB-SW2 24.57 36,849 - 26.6 1.19 CB-HBV 34.87 52,299 35.0 36.9 1.48 CB-HS 34.65 51,980 12.5 31.6 1.72

Table 3.2: Average strength and stiffness properties for CB test specimens relative to CB-CLT specimens.

Parameter Specimen Type CLT SC SW HBV HS

Dx (kip-in2/in) 22,320 37,034 34,437 52,299 51,980 Dx/DxCLT 1.00 1.66 1.54 2.34 2.33

Maximum Moment(kip-ft/ft) 30.06 38.63 33.63 46.13 39.50

Pmax/PmaxCLT 1.00 1.28 1.12 1.53 1.31 ∆/∆CLT 1.00 0.87 0.78 0.85 0.99

The slip deformations along the concrete-CLT interface showed typical nonlinear distributions

for all specimen types. The largest slip deformations occur over at the ends of the specimens (over

roller supports) and decrease toward the load point. The theoretical slip displacement at the load

point is zero. Typical applied load-slip displacements along the span are shown in Fig. 3.2a to d

for the four specimen types (SC, SW, HBV, and HS). The CB-HBV exhibited much smaller slip

51

deformations than the other specimens. The CB-SC and CB-SW exhibited slip deformations

between the concrete and steel from the initiation of loading. The SW specimen showed similar

slip displacements within 3 ft of the support (sensor measurements all approximately the same)

that is indicative of very small slip strains in this region and ineffective shear transfer.

a) b)

c) d)

Fig. 3.2: Slip displacement at concrete-CLT interface along span, a) is CB-SC, b) CB-SW, c) CB-HS, and d) CB-HBV (all scales the same).

The neutral axis was identified from two displacement sensors using a 5,5 in gage length to

estimate the strains in the uppermost and lowermost laminations of the CLT panel. The strain

profile through the panel was assumed to be linear (no slip between laminations). Based on the

distance separating the sensors, the neutral axis location from the lowest laminate (tension face)

was estimated. The neutral axis locations were computed when the applied load was greater than

52

5 kips until the maximum load was achieved. Loads below 5 kips were not used, as the calculations

are sensitive to noise at low signal levels. The neutral axis locations for all specimens are shown

in Fig. 3.3. As seen here, all the alternative shear connectors produced a neutral axis above that of

the bare CLT panel specimens, although there is significant variability in the results. The HBV

connector produced the highest neutral axis and maintained it to higher load levels than the other

alternatives. The two different STS specimens (CB-SC and CB-SW) had similar neutral axis

positions with CB-SC1 actually exhibiting lower composite action than the CB-SW specimens. It

should be noted that the neutral axis location was estimated on only one shear span of the specimen,

and may thus may not have captured the side that exhibited the greater loss of composite action.

Fig. 3.3: Predicted neutral axis location for CB specimens from extreme tension face of CLT panel.

53

3.2 OS Test Results

All three OS specimens were used to establish the orthotropic stiffness properties of the

composite concrete-CLT panels. The specimens were tested in the strong and weak bending

directions as well as twisting with the specimens flipped over so that the concrete slab was located

on the tension and compression faces. Each specimen was tested for each of the loading

configurations (6 elastic tests per specimen). The load was applied and removed three times and

best-fit curves of the midspan load-deformation response (corrected for support settlements) were

produced for each branch of loading and unloading (3 loading and 3 unloading). Typical load-

deformation responses in each of the four bending configurations are shown in Fig. 3.4. The

computed best-fit elastic stiffness for each loading and unloading cycle are shown in Table 3.3.

The average values were used to compute the Dx and Dy values according to Eqn. 1 using the full

8 ft. width of the panel and 88 in. span length. As seen in Table 3.4, the average Dx value for the

OS specimens was 32,881 kip-in2/in, which was 12.6% smaller than the average observed for the

CB-SC specimens and was also smaller than the average observed for the CB-SW specimens

(4.5% smaller). The weak-direction bending had higher variability than the strong-direction tests.

a) b)

54

c) d)

Fig. 3.4: Example elastic loading and unloading responses of OS2 specimen for a) strong direction positive bending (concrete in flexural compression) b) strong direction negative

bending (concrete in flexural tension), c) weak direction positive bending (concrete in flexural compression), and d) weak direction negative bending (concrete in flexural tension).

Table 3.3: Best-fit linear stiffness values for each OS specimen in each bending condition.

Specimen Test Arrangement

First Load

(kip/in)

First Unload (kip/in)

Second Load

(kip/in)

Second Load

(kip/in)

Third Load

(kip/in)

Third Load

(kip/in)

Avg. (kip/in)

Std. Dev.

(kip/in) OS1 Strong Axis

Positive Bending

223.7 229 222.4 231.8 220.9 229.2 226.2 4.40

OS1 Weak Axis Positive Bending

71.2 69.2 69.8 68.3 69.3 67.2 69.2 1.35

OS1 Strong Axis Negative Bending

165.1 170.8 180.3 174.8 180.3 175.3 174.4 5.83

OS1 Weak Axis Negative Bending

72.6 72.2 74.9 73.1 74.7 72.3 73.3 1.20

OS2 Strong Axis Positive Bending

214.7 227.7 221.0 220.4 219.5 228.4 221.8 5.23

OS2 Weak Axis Positive Bending

71.7 70.0 70.1 67.27 69.3 65.4 69.0 2.26

OS2 Strong Axis Negative Bending

170 178.3 176.1 172.3 177 176.2 175.0 3.16

OS2 Weak Axis Negative Bending

67.4 75.9 71.3 72.5 71.0 72.6 71.8 2.76

OS3 Strong Axis Positive Bending

225.5 215.7 227.1 210.8 226.6 207.8 219.0 8.60

OS3 Weak Axis Positive Bending

60.5 58.3 59.9 59.9 60.0 57.4 59.3 1.20

OS3 Strong Axis Negative Bending

162.9 167.0 173.6 169.2 165.3 170.6 168.1 3.84

OS3 Weak Axis Negative Bending

60.5 58.9 60.1 58.1 60.2 57.5 59.2 1.24

56

Table 3.4: Average orthotropic bending stiffness properties for OS specimens.

Orthotropic Stiffness

Parameter (kip-in2/in)

Specimen ID

Average Std. Dev. COV OS1 OS2 OS3

Dx M+ 33,452 32,802 32,388 32,881 536.8 1.6%

Dx M- 25,792 25,881 24,860 25,511 565.3 2.2%

Dy M+ 10,234 10,204 8,770 9,736 836.9 8.6%

Dy M- 10,840 10,618 8,755 10,071 1145.3 11.4%

After completion of the elastic stiffness tests, each of the three OS specimens was loaded to

failure in a different loading orientation (OS1 in weak-direction negative bending, OS2 in strong-

direction negative bending, and OS3 in weak-direction negative bending). These three tests

provide strength results for the bending conditions not investigated in the CB test series with

closely spaced STS (CB-SC1 and CB-SC2).

The overall load-midspan deformation responses are shown in Figs. 3.5a to 3.5c for the three

cases. The second axis in these figures shows the applied moment at midspan. The peak forces,

moments, and midspan displacement at peak load, for each test are shown in Table 3.5. The slip

deformations between the concrete and CLT panel along the span are shown in Figs. 3.6a to 3.6c.

The estimated neutral axis location relative to the extreme CLT tension face is shown in Fig. 3.7a

to 3.7c. As seen in these figures, the responses were elastic well above the elastic stiffness tests

conducted previously. The weak-direction slip deformations along the concrete-CLT interface

(OS1 and OS3) were twice that observed for the strong-direction specimen (OS2). This is because

the STS are oriented orthogonal to the shear flow for the weak-direction loading cases.

57

Table 3.5: Peak responses for strength tests of OS specimens.

Specimen Peak Load (kips)

Midspan Displacement at

Peak Load (in)

Maximum Moment (kip-ft)

Max. Moment/ft (kip-ft/ft)

OS1 90.1 1.64 165.2 20.6 OS2 153.5 1.03 281.4 35.2 OS3 86.7 1.63 159.0 19.9

Fig. 3.5a: OS1 specimen overall response for weak direction positive bending.

Fig. 3.5b: OS2 specimen overall response for strong direction negative bending.

58

Fig. 3.5c: OS3 specimen overall response for weak direction negative bending.

a) b)

c) Fig. 3.6: Slip between concrete and CLT panel along span for strength tests a) OS1, b) OS2, and

c) OS3.

59

a) b)

c) Fig.3.7: Computed neutral axis location relative to extreme tensile fiber of CLT panel for

strength tests a) OS1, b) OS2, and c) OS3.

The torsional stiffness parameter, Dxy, was computed according to Eqn. 2. The average

stiffness values measured for the three steps of loading and unloading for the second and third

loading sequences are shown in Table 3.6. As seen here the torsional stiffness is very low compared

with the flexural stiffness values. The twisting stiffness was consistently approximately 20%

higher for the orientation when the concrete was in tension (concrete side up).

60

Table 3.6: Torsional orthotropic stiffness values for OS specimens.

Specimen

Concrete in Tension (concrete side up) Load

Cycle 2 Dxy

(kip-in^2/in)

Unload Cycle 2

Dxy (kip-

in^2/in)

Load Cycle 3

Dxy (kip-

in^2/in)

Unload Cycle 3

Dxy (kip-

in^2/in)

Avg. Load Dxy

(kip-in^2/in)

Avg. Unload

Dxy (kip-

in^2/in)

Avg. Dxy

(kip-in^2/in)

COV Dxy (%)

OS1 5217 5371 5248 5264 5232 5317 5275 12.0% OS2 4650 4780 4797 4872 4724 4826 4775 13.8% OS3 5521 5349 5417 5311 5469 5330 5399 15.2%

Avg. 5150 13.6% Concrete in Compression (concrete side down)

OS1 4070 4196 4122 4155 4096 4175 4136 11.6% OS2 4656 4765 4672 4766 4664 4766 4715 11.7% OS3 4018 4146 4003 4073 4010 4110 4060 13.7%

Avg. 4304 12.3%

3.3 FS Test Results

The FS specimen was used to establish the structural performance of the composite concrete

CLT system in a representative building span configuration that includes continuity (negative

moment) at the interior supports. The FS specimen was tested in the strong bending direction with

stiff line loads to produce one-way bending in the specimen. The load was applied and removed

several times. The applied load-average vertical displacement relative to the strong floor (with

rigid body support settlements removed) is shown in Fig. 3.8. Failure occurred at a maximum

applied actuator force was 82.4 kips with an average midspan displacement of 6.17 in. The tension

surface of the CLT panel ruptured at failure in the constant moment region just inside one of the

line loads as shown in Fig, 3.9. Negative values in Fig. 3.8 correspond to uplift, which was

observed for the cantilever ends.

The forces generated at the cantilever ends was estimated based on the elastic stiffness values

(Dx) from the OS specimens. First, the midspan deflection in the main span and the cantilever tip

61

displacements were estimated for the applied line loads. Then the hogging displacement at

midspan of the main span and the cantilever tip displacements were estimated from the forces

applied to the cantilever ends as illustrated in Fig. 3.10. The hogging stiffness was taken as Dx for

the negative moment case (25,511 kip-in2/in) and the simple span bending stiffness was taken from

the Dx for positive bending (32,881 kip-in2/in). For actuator load levels of 20, 30, and 40 kips,

where the structural response is elastic, the end cantilever force (P’) was computed by averaging

the load required to balance the cantilever tip and main span midspan displacements. The average

cantilever forces for the three actuator applied load levels were 5.7, 8.53, and 10.75 kips,

respectively. As seen in Fig. 3.8, the cantilever vertical deformation response remained linear with

the applied main span load. Thus, the cantilever force-tip deformation response was estimated

from a best-fit line of the estimated cantilever forces computed above at the respective cantilever

displacements. The resulting cantilever force-deformation stiffness was estimated as 17.0 kip/in

of cantilever uplift. At the smaller of the two cantilever uplift displacements (1.13 in.) observed in

the experiment, the cantilever force was estimated as 19.2 kips. This produced a maximum

negative moment over the interior support of 14.4 kip-ft/ft. The negative moment capacity from

specimen OS-2 was 35.2 kip-ft/ft, which is more than twice the negative moment estimated in the

FS specimen. The estimated maximum positive moment was 30.7 kip-ft/ft in the main span. If the

dead load moments from the self-weight of the concrete (estimated at 27 psf) and CLT panel

(estimated at 35 psf) are added to the negative (1.0 kip-ft/ft ) and positive moments (2.1 kip-ft/ft)

from the actuator applied load, the total negative moment is 15.4 kip-ft/ft and the positive moment

is 32.8 kip-ft/ft at failure. The average positive moment capacity from four (4) CB specimens

containing STS was 36.1 kip-ft/ft, which was larger (10% higher) than that observed for the FS

specimen. The positive moment strength of the FS specimen was 9% larger than the average CB-

62

CLT specimens (5-ply CLT panels without topping slab) and shows additional strength gain due

to the composite concrete slab, although not to the same degree exhibited by the CB-SC and CB-

SW specimens.

If the Dx value for positive moment was taken closer to that observed on average for the CB

specimens (35,700 kip-in2/in), the above analysis would produce negative bending moment

(including self-weight) of 14.1 kip-ft/ft and the total positive moment at midspan (including self-

weight) would be 34.1 kip-ft/ft. This is closer (within about 6%) to that observed on average for

the CB specimens.

If the floor system was located in a continuous framing system with three equal spans and the

uniform load was patterned to produce the maximum positive moment in the end span, the ratio of

maximum positive to maximum negative moment would be approximately 2:1. This corresponds

to the ratio of the maximum positive and negative moments produced in the laboratory loading

conditions and shows that the test conditions reasonably represented the magnitudes and

distributions for producing the controlling positive moment load effects.

The relative slip measured between the concrete slab and upper lamina of the CLT panel are

shown in Fig. 3.11a for the north half and Fig. 3.11b for the south half of the FS specimen. The

very similar magnitude slip deformations in the main shear span (especially on the south half)

showed limited composite action between the concrete slab and CLT panel. The magnitude of the

slip deformations at failure were approximately twice those observed for the CB specimens with

STS shear connectors.

The predicted neutral axis location from the two displacement sensors placed in the uppermost

and lowermost CLT layers over a 5.5 in. gage length near midspan is shown in Fig. 3.12. The

neutral axis was about 4.4 in. from the extreme tension fiber until it begins to shift downward at

63

approximately 50 kips. This shift would indicate reduction in the composite action. The joint

motions are shown in Fig. 3.14. As seen, here all four joints exhibited similar opening at the top

with about 0.2 in. opening at ultimate.

Fig. 3.8: Overall load-average vertical displacements (corrected for support deformations) along span of FS specimen.

64

Fig. 3.9: Failure of the flexural tension surface of the CLT panel at ultimate.

∆+

∆−

P P

P’ P’

∆ Experiment

Fig. 10: Superposition of mainspan and cantilever loads to find P’ to match measured deflection at midspan of main span and tips of cantilevers in FS specimen.

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Fig. 3.11a: Relative slip displacements between concrete slab and CLT along north half of span

for FS specimen.

Fig. 3.11b: Relative slip displacements between concrete slab and CLT along south half of span

for FS specimen.

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Fig. 3.12: Predicted neutral axis location relative to extreme tensile fiber of CLT panel for FS

specimen.

Fig. 3.13: Support motions at ledge beam connections for FS specimen.

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3.4 LT Test Results

LT- Specimen

The measured long-term displacements for specimen LT- are shown in Fig. 3.14. Some

variations were observed for the displacement response due to changing ambient temperatures

within the laboratory. The deformations increased more rapidly over the first two-week period and

then slowed. However, even after 100 days, the long-term deformations were still increasing.

Overall, the change in displacement over the course of 132 days was 0.1470 in. and 0.1920 in. for

two cantilever ends of the specimen. The average long-term deflection was 0.1695 in.

Fig. 3.14: Long-term vertical displacements of LT- specimen.

LT+ Specimen

The measured vertical deflections along the LT+ specimen over the first 136 days are shown

in Fig. 3.15. The data collection began when the concrete topping slab was placed. The CLT panel

exhibited an initial deflection (0.22 in. at midspan) caused by the application of the fluid concrete

onto the CLT panel surface at day zero. The deflection reduced in the days after concrete placement

due to the evaporation of water within the fresh concrete. At 28 days, additional concrete weights

were applied to the concrete slab, which produced another increment of deflection (0.376 in. at

midspan). Then the weigths were left in place and additional creep deflections were measured.

From the point just after the additional concrete blocks were applied to the last point recorded, the

LT+ specimen exhibited creep displacement of 0.570 in. at midspan, 0.492 in. at the ¼ point, and

0.499 inches at the ¾ point between the simple supports. The long-term displacemented still

exhibited an increasing trend at the end of the measurement period.

Fig. 3.15: Long-term vertical displacements of LT+ specimen.

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Conclusions

SOM has proposed a concept of “Concrete Jointed Timber Frame” (CJTF) system that

articulates several innovative connection and composite material solutions. One of the proposed

advances is the development of a composite concrete-CLT floor system that can economically

achieve all design performance requirements. Experimental validation was needed to advance the

floor system into practice. Experimental tests of a composite concrete-cross-laminated timber

(CLT) floor system were conducted in the Structural Engineering Research Laboratory at Oregon

State University. The floor system was constructed with 5-ply CLT panels (6.75 in. thick) made

composite with a 2.25 in. thick reinforced concrete topping slab. Four series of tests were

performed using different specimen configurations and laboratory testing methods:

1) Comparative one-way bending tests (CB) to evaluate the performance of alternative shear

connectors used to join the concrete slab to the CLT panel.

2) Orthotropic stiffness and strength tests (OS) to evaluate the elastic orthotropic stiffness of

the deck system and provide strength results for weak-axis bending and negative moment strength.

3) Full-scale system performance tests (FS) of a continuous floor span to establish strength at

realistic span lengths and the influence of continuity.

4) Long-term deformation tests (LT) to investigate creep deflections of the composite concrete-

CLT floor system considering positive and negative bending influences.

During the experiments sensors were placed on the specimens to collect data from which to

assess the overall strength and deformation capacity, elastic stiffness values, slip deformations

along the concrete-CLT interface, neutral axis locations in the composite concrete-CLT section,

and connection deformations. Based on these tests results, the following conclusions are presented:

70

• All the alternative shear connectors provided increased strength and stiffness compared

to the plain CLT specimens for the CB test series.

• The HBV shear connectors provided the largest stiffness and strength of the different

CB specimens. Very little relative slip was observed at the concrete-CLT interface until

near failure.

• One of the widely spaced STS specimens (CB-SW2) exhibited about the same stiffness

as one as both the closely spaced STS specimens (CB-SC1 and CS-SC2). The SC

specimens exhibited higher strength than the SW specimens did.

• The CB specimen with headed studs (CB-HS) exhibited abrupt slip at low load levels

when other specimens were elastic.

• The elastic orthotropic stiffness values were established for the composite concrete-

CLT system using STS. The average stiffness values from three (3) specimens

subjected to multiple elastic loading and unloading cycles were: for strong-direction

positive moment bending (Dx M+ ) = 32,881 kip-in2/in; strong-direction negative

moment bending (Dx M-) = 25,511 kip-in2/in, weak-direction positive moment bending

(Dy M+ ) = 9,736 kip-in2/in, weak-direction negative moment bending (Dy M-) =

10,071 kip-in2/in, twisting stiffness with the concrete in compression (Dxy+) = 4304

kip-in2/in, and twisting stiffness with the concrete in tension (Dxy-) = 5150 kip-in2/in.

• The relative stiffness between the strong- and weak-bending directions was about 3 and

the deck system is best characterized as orthotropic instead of isotropic.

• The OS specimens exhibited slightly lower strong-direction bending stiffness and

strength compared to the CB specimens.

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• The weak-direction bending strength was about 57% less than the strong direction

bending strength (either positive or negative bending).

• The FS specimen failed in positive bending in the constant moment region of the main

span. The predicted ultimate bending moment, including self-weight of the specimen

was in the range of 33 to 34 kip-ft/ft, depending on whether the OS or CB orthotropic

stiffness values are used. This greatly exceeds the required design strength.

• The negative moment was approximately half the positive moment and the magnitudes

and gradients reasonably reflect the in-situ loading conditions on a multi-span

continuous floor system under uniform load.

• The LT creep tests showed that long-term deformations were continuing to increase

even after 130 days.

• The negative moment region exhibited higher rates of deformation during the first 2

weeks, after which the rate decreased. The positive moment region exhibited about the

same rate of deformation over the period of monitoring.

References

APA–The Engineered Wood Association. (2012). “Standard for performance-rated cross

laminated timber.” ANSI/APA PRG-320, Tacoma,WA. ASTM C78, (2012). “Standard Test Method for Flexural Strength of Concrete, Using Simple

Beam with Third Point Loading,” ASTM International, West Conshohocken, PA, 2012, <http://compass.astm.org/EDIT/html_annot.cgi?C78+16>. Web. 12 July. 2016.

ASTM C39, (2012). “Standard Test Method for Compressive Strength of Cylindrical Concrete

Specimens,” ASTM International, West Conshohocken, PA, 2012, <http://compass.astm.org/EDIT/html_annot.cgi?C39+16b>. Web. 06 July. 2016.

ASTM E8, (2012). “Standard Test Method for Tension of Metallic Materials,” ASTM

International, West Conshohocken, PA, 2012, <http://compass.astm.org/EDIT/html _annot.cgi?E8+15a>. Web. 29 July. 2016.

Skidmore, Owings, Merrill, (2013). “Timber Tower Research Project.” SOM, Chicago, IL.

http://www.som.com/FILE/20378/timber-tower-final-report-and-sketches.pdf

Skidmore, Owings, Merrill, (2014). “Timber Tower Research Project. System Report #1, Gravity Framing Development of Concrete Jointed Timber Frame System” SOM, Chicago, IL http://www.som.com/FILE/22294/timber-report-gravity-system.pdf

73

Appendix A - CONCRETE SPECIFICATIONS

74

75

Mix ID Number: 031-7HN3R2S0

Concrete Mix Design

MIX DESIGN QUANT IT IESMaterial Produc t/ Sourc e SG W eight Volume Mass Volume

Cement CalPortland, Type I-II 3.15 630 lb 3.21 ft3 374 kg 0.119 m3

Fly Ash LaFarge, Class C 2.68 75 lb 0.45 ft3 44 kg 0.017 m3

Water(Total) Well/ Corvallis R-Mix Plant 1 1.00 300 lb 4.81 ft3 178 kg 0.178 m3

3/ 8-#8 PCC Grits ODOT Corvallis (Builders Supply) 2.51 * 1101 lb* 7.03 ft3 653 kg* 0.260 m3

PCC Sand Corvallis (Builders Supply) 2.58 * 1714 lb* 10.65 ft3 1017 kg* 0.394 m3

Admixtures Grace 1.00 5 lb 0.08 ft3 2.86 kg 0.003 m3

Total Mix Weight (Mass): 3825 lb 2269 kg

Air(Entrap/ Entrain) 3.0 % 0.81 ft3 0.030 m3

Total Mix Volume: 27.00 ft3 1.000 m3

ADMIXT URESProduc t Produc tName/ Type SG Dosage Rate Dosage (English) Dosage (Metric )

Water Reducer Grace WRDA-64 1.00 3.50 oz/ cwt** 24.7 oz/ cy** 954.5 mL/ m3**

High-Range Water Redu Grace Adva Flex 1.00 7.00 oz/ cwt** 49.4 oz/ cy** 1909.0 mL/ m3**

Add'l Fibers lb/ cy** 0.0lb/ cy** 0.0 kg/ m3**

MIX DESIGN PROPERT IESAggregate Properties ODOT# ^ SG Abs FM Unit W eight

3/ 8-#8 PCC Grits ODOT 2003-00000-GRITS-002 22-001-2 2.51 3.6 98.9 pcf 1584 kg/ m3 Dry Rodded

PCC Sand 2003-00000-0SAND-001 22-001-2 2.58 3.8 3.05

Plastic Properties: Slump: 7.0 + 1.0 inch 175 + 25 mm

Air Content: 3.0 + 1.5 %

Unit W eight (W et): 141.7 pcf 2269 kg/ m3

Design Properties: Required Strength (f 'c ): 5000 34

Total Cementit ious: 705 lb 7.5 Sack 418 kg

Fly Ash %: 10.6 %

w/ c Ratio***: 0.43 (incl Admix)

Projec t:

Contrac tor:

Comments:

Footnotes: *SSD Weights and SG. ** Admixture dosage rate will be adjusted according to manufacturer's

recommendations to accommodate varying field conditions. *** This is a design w/ c ratio and production

w/ c ratio may vary as recognized by industry standards such as ASTM C 94. ^ODOT Source #.

Submitted By: Date Submitted:

Designed By:

psi @ 28 days MPa @ 28 days

2/ 13/ 2017