TESTING SUPPORT CONNECTIONS OF RIB AND TIMBER INFILL PRECAST CONCRETE FLOORS

S R CORNEY1, R S HENRY1, J M INGHAM1

1 Department of Civil and Environmental Engineering, University of Auckland SUMMARY It has been found from previous research that support connections for precast concrete floor units are subjected to critical earthquake induced deformations, including a relative rotation between the unit and support beam and a pull-off effect caused by beam elongation. Traditional support connection details used for rib and timber infill (RT) floors consisted of the ends of the ribs being cast into their supporting beam. Damage to the precast ribs can occur when the end of the rib is trapped in the support beam, causing a vertical flexural crack in the rib that may compromise the vertical load carrying capacity of the floor. Such cracks were observed during the 2010/2011 Canterbury earthquakes and in response to this, SESOC have provided recommended rib seating details that enable the rib to slide and rotate without sustaining damage. An experimental programme was initiated to investigate the seismic performance of both the existing and recommended support connection details for RT floors. A test setup was developed to subject the connection to deformations simulating seismic actions in a typical structure incorporating 175 mm deep ribs in addition to gravitational loading. The first three tests have been performed and it was found that the SESOC recommended support connection details successfully enabled the rib to deform without becoming trapped in the support beam or sustaining damage. INTRODUCTION Reinforced concrete structures in New Zealand commonly consist of precast concrete components, and precast concrete flooring systems are almost exclusively used rather than cast-in-place concrete floors. Typical precast concrete floor systems include prestressed hollow-core, double-tee, ribs with timber infill, and flat slab units, all with a thin insitu concrete topping. It has become economical within the New Zealand construction market to install these precast concrete floor systems when considering factors such as time and ease of construction and the benefits of reducing on-site formwork. Floors are required to behave as a diaphragm and transfer seismic induced inertia and transfer forces into the lateral load resisting elements during an earthquake. It is essential that the support connection details of the precast floor units are capable of providing this diaphragm action (Fenwick et al. 2010). Rib and timber floors are a common form of precast concrete flooring in New Zealand and contain intermediary prestressed precast ribs spaced with timber infill strips that act as permanent formwork as an insitu concrete topping is placed. RT floors may suffer potentially brittle failure of the ribs when subjected to positive bending actions at the supports (Fenwick et al. 2011). Additionally, it has been common practice to cast the ribs into the support beam which induces a weak section to form at the face of the support and compromises the shear strength of the rib (Fenwick et al. 2011). An experimental testing programme has been initiated to examine the seismic performance of both traditionally produced ribs and support connection

details, as well as those consistent with current best practice. The results of the first three tests are reported, with further tests ongoing. DAMAGING MECHANISMS Floor diaphragms in multi-story buildings have the dual functions of supporting gravity loads in addition to providing diaphragm action to distribute inertial and transfer forces to lateral load resisting elements during an earthquake. The thin topping (65 mm or greater) is typically relied upon to connect the precast units together and provide the required diaphragm action. The precast unit support connections are vulnerable to damage and, once damaged, the ability of the connection to transfer seismic inertia forces to the lateral load resisting system is greatly diminished (Matthews 2004). There are two actions in particular that have been identified from previous research to cause the most damage to the precast floor unit support connections (Matthews 2004; Jensen 2006; Lau 2007). As a ductile multi-story building deforms when subjected to lateral loading, the beams rotate relative to the floors which remain horizontal. The relative rotation induced between the precast unit and its support causes stresses to be developed in the continuity reinforcement placed in the insitu topping (PCFOG committee 2009). The supporting ledge can also be made to spall due to the prying action applied by the unit (Jensen et al. 2007). Additionally, in ductile frames the plastic hinges that form in the beam elongate during cyclic loading (Lau 2001). The result of this elongation in a beam oriented parallel to a precast unit is that the distance between supports will increase, reducing the width of beam on which the unit is supported (PCFOG committee 2009). As well as breaking down diaphragm action, these factors can lead to a unit losing its support, as shown in Figure 1.

Figure 1. Precast unit losing its support (after Jensen et al (2007))

Furthermore, there are concerns regarding the seismic performance of the support connections for rib and timber floors. Connection details that have been commonly used in the past can result in the rib being trapped in the support beam and the formation of rib cracking under positive rotational deformations. The failure mechanism is illustrated in Figure 2a where it can be seen that the rib damage has resulted in loss of support for the rib. Cracks consistent with this failure mechanism were observed in some structures during the 2010/2011 Canterbury earthquakes as shown in Figure 2b (Corney et al. 2014). The vulnerability of the floor as a result of these cracks is further increased by the effects of beam elongation as the crack widths are made to increase, resulting in the loss of shear transfer via aggregate interlock and dowel action of the strands more readily (Fenwick et al. 2011). Two primary construction practices have been identified to potentially cause these cracks to be induced:

Casting the ends of the ribs into the supporting beam (old practice) rather than letting the rib slide without sustaining damage by seating it on a low-friction bearing strip (new practice).

The previous practice of haunching the first timber infill down to the top of the support ledge can result in over-strengthening this region, causing the weak section to develop further along the rib and away from the support.

(a) Schematic (after Fenwick et al. (2011))

. (b) Damage during the Canterbury

earthquakes

Figure 2. Positive moment cracking in ribs

RECOMMENDED RIB AND TIMBER SUPPORT DETAILS The Structural Engineering Society (SESOC) provided recommendations for rib and timber infill floors in the interim design guidance published following the Canterbury Earthquakes (2011). These changes have also been proposed to the Concrete Structures Standard, NZS 3101:2006 for use in buildings with significant lateral drift demands. The recommended support detail is indicated in Figure 3, and included the following features:

Ribs seated on low friction bearing strip.

Haunching avoided and a vertical from to be placed under the first timber infill out from the support instead.

15 mm chamfer (in addition to seating length) to help prevent ledge spalling.

Closed stirrups at 150 mm max centres within the rib.

Support ledge tied back into beam with adequate reinforcement detailing.

Figure 3. Recommended rib support detail (SESOC (2011))

EXPERIMENTAL TESTING To investigate the seismic performance of rib and timber support connection details, a total of eight specimens will be subjected to simulated seismic loading along with gravitational loading. The testing regime is outlined below in Table 1 and it is intended that the test series will comprehensively evaluate the seismic performance of rib and timber infill systems, both of those currently being produced and of those in existing building stock. The main parameters tested include:

Test RT1 consists of all SESOC recommendations being followed, both support conditions and rib detailing and it is intended to act as a benchmark for the other currently produced details.

Tests RT2 – RT3 are intended to verify the performance of rib details currently being produced whilst seating details remain consistent with SESOC recommendations.

Tests RT4, RT6 and RT8 are intended to test the haunched connection detail with both formerly produced (open stirrups), currently recommended (closed stirrups) and currently produced (helix stirrups) ribs respectively. The vulnerability of the connection detail will be examined along with potential serviceability advantages of the older detail.

Tests RT5 and RT7 will determine the gravity load carrying capacity of older details where any rib damage was observed during tests RT4 and RT6 respectively and the floor is considered to be vulnerable to collapse.

Table 1. Testing plan

Test Stirrup type

Support connection type

Loading Status

RT1 Closed SESOC Seismic to ±4.5% Completed

RT2 Open SESOC Seismic to ±4.5% Completed

RT3 Helix SESOC Seismic to ±4.5% Completed

RT4 Open Haunched Seismic to ±4.5% To be completed

RT5 Open Haunched Seismic then vertical load to failure To be completed

RT6 Closed Haunched Seismic to ±4.5% To be completed

RT7 Closed Haunched Seismic then vertical load to failure To be completed

RT8 Helix Haunched Seismic to ±4.5% To be completed

To date only tests RT1, RT2 and RT3 have been completed. The design and testing of these three tests is reported here and the remaining tests are ongoing.

Figure 4. Support connection details

Specimen Design The RT support connections for tests RT1, RT2 and RT3 were designed to be consistent with the recommendations provided by SESOC (with the exception of the type of rib stirrup used). The support detail is shown in Figure 4 and rib stirrup arrangements used for each test are shown in Figure 5. The main features of the support connection design include:

A 1250 mm width of floor incorporating two ribs positioned at 900 mm centres (shown in Figure 5)

Four D12 bars positioned at 300 mm centres extend 1200 mm into the insitu topping as continuity reinforcement over the entire strand transfer length. These bars lap onto SE62 ductile mesh in the topping.

The ribs seated on a low-friction bearing strip on top of a 75 mm width of support.

A 15 mm chamfer at the edge of the ledge.

a) RT1 - Closed loop stirrups

b) RT2 - Open leg stirrups

c) RT3 – Helix stirrups

Figure 5. Rib details and specimen cross section

Test Setup The test setup used a self-reacting frame that was previously used for precast floor tests performed by Fowler (2012). The test setup was similar to sub-assembly tests developed at the University of Canterbury for previous research on precast flooring connections (Bull and Matthews 2003), except that the support beam was rotated instead of the precast unit. The mechanisms used to apply both seismic and gravitational actions are described in detail below. Application of Seismic Actions Figure 6 shows the method used to subject the RT support connection to simulated earthquake induced displacements. Rotation was achieved using a double acting jack that rotated the support beam about a cast in steel pin. Elongation was achieved using a jack located at the far end of the specimen, positioned such that the applied load acted directly through the pin. By allowing the floor to remain horizontal throughout testing it is believed that the actions induced during an earthquake are applied realistically to the support connection. Figure 7a shows the steel frame assembly used to contain the test unit and provide a self-reacting arrangement to prevent the need for a strong floor. The jacks used to apply the rotation and elongation are shown in Figure 7b and Figure 7c respectively.

Application of Gravitational Actions During all tests a gravitational load was positioned on the specimen to replicate the loading likely to be present at the support connection of a structure with a 175RT floor. Figure 6 shows the positioning of a 2.6 t coil of prestressing strand. The coil was positioned such that the reaction force due to this additional load is equivalent to a 3 kPa live load over the floor surface for the prototype structure being tested. During tests RT5 and RT7 a vertical load will be applied to the top of the RT floor after simulated seismic actions have been applied. This vertical load will be applied by an additional jack located above the unit and positioned such that the load is applied at a distance of 2.5 times the depth of the overall floor section from the support, as shown in Figure 6. The position of the vertical load relative to the support ledge is based on the distance used during FIP hollow-core loading tests (Federation Internationale de la Precontrainte 1992) (2.5 times the depth of the floor). The purpose of tests RT5 and RT7 is to determine the residual gravity load carrying capacity of both the formerly used and currently produced ribs where vulnerabilities appeared during the corresponding seismic loading test (RT4 and RT6 respectively).

a) Steel frame

b) Rotation jack

c) Elongation jack

Figure 7. Test setup and steel frame assembly

Vertical load applied 690 mm from support during RT5 and RT7

Pin allows for support beam rotation

Elongation applied to far end of floor by a 100 t jack

RHS stub connected to beam

Positive and negative drifts applied by a 50 t jack acting through RHS stub

Roller support 4300 mm from connection

2.6 t strand coil positioned on floor to simulate live load at support connection

Figure 6. Loading mechanism

Figure 8. Seismic loading protocol

Seismic Loading Protocol To simulate the beam elongation and support beam rotation, a loading protocol previously developed by Jensen (2006) was adapted to suit a typical 175RT structure. The prototype building represented an office building with inter-frame spans of approximately 7 m and 750 mm deep reinforced concrete beams spanning parallel to the precast ribs. The perimeter frame of such a building was considered to contain a single beam spanning the length of the precast ribs and so it could be considered that one plastic hinge would contribute to the pull-off effect observed at each end of the rib. The loading protocol developed for this prototype structure is shown in Figure 8 with the positive drift direction representing positive bending moments at the support connection. Several assumptions were used to develop this loading protocol that are based on observations from large scale precast flooring seismic tests performed at the University of Canterbury (Lindsay 2004; Matthews 2004; MacPherson 2005). These assumptions include the elastic and plastic lever arms that are taken as percentages of the parallel beam depth. Instrumentation A series of gauges were used to measure the forces and deformation of the test specimen, including:

Load cells installed on both jacks to measure the force applied.

Displacement gauges to determine the pull off deformation at the far end of the floor and the rotation of the beam.

Deformation at the rib-beam interface was measured by gauges placed along the topping along with gauges spanning between the ribs and beam.

Rib cracking was measured by gauges positioned along the rib.

Gauges were placed on the underside of the ribs and beam to measure any vertical deformation.

Specimen Construction and Material Properties The construction of the testing components and test setup was conducted at Stresscrete in Papakura. The 175 mm deep ribs were produced at Formstress, at a length of 4.75 m to fit the test rig. The ribs were produced with a design mix of 45 MPa, standard for Formstress ribs and each rib had an RB20 bar cast into its far end. An SHS was fastened to the RB20 bars and spanned between the two ribs such that the elongation jack could apply load directly through the ribs. The lower portion of the support beam was precast and placed in the test rig after which the ribs were positioned on the lower portion of the beam and the connection reinforcement and infill strips were placed. The topping concrete and remainder of the support beam concrete was poured with a steel sleeve cast into the support beam to allow for the placement of the rotational pin. The concrete strengths measured for the support beam, topping and ribs of test specimens RT1, RT2 and RT3 are shown in Table 2. A design mix of 35 MPa was used for the topping

concrete to enable testing after ten days of casting at a target strength of 25–30 MPa, a common strength for insitu toppings. A lower strength than specified was generally reached at the time of testing and the influence of these topping strengths on the performance of the support connection are discussed in the results.

Table 2. Component concrete strengths

Concrete Unit Target Strength (MPa) Actual Strength (MPa)

RT1 - Support Beam 40 55 RT1 - Topping Concrete RT1 - Ribs

30 45

18 75

RT2 - Support Beam RT2 - Topping Concrete RT2 - Ribs

40 30 45

60 15 71

RT3 - Support Beam RT3 - Topping Concrete RT3 - Ribs

40 30 45

46 22 57

Preliminary Results Loading was applied to specimens RT1, RT2 and RT3 to achieve the target rotation and elongation at the support connection to the full extent of the protocol without causing loss of vertical support for the floor. During each test a crack formed at the rib-beam interface early into the first cycle to +0.5% drift and before any significant load had been sustained, highlighting the low positive moment capacity of the connection, as shown in Figure 9a. As further deformation was applied throughout the duration of the test, the response was dominated by the opening of this crack at the rib to beam interface, which would readily result in the breakdown of diaphragm action for the floor at relatively low drift levels. The connection details were sufficient to prevent the undesirable failure mechanism discussed earlier in each test with the ribs free to rotate on the support beam and no positive moment rib cracking was observed, as shown in Figure 9b.

RT1 RT2 RT3

a) Damage concentration

b) Rib sliding

c) Rib entrapment

d) Ledge spalling

Figure 9. Testing observations RT1 – RT3

During the later cycles of each test, however, it was observed that a minor degree of spalling took place at the bottom corner of at least one of the ribs and this is shown in Figure 9c. The level of spalling was typically minor with a supported width of approximately 10 mm being lost from the end of the rib and the fact that the insitu topping partially flowed around the back of the vertical form between the ribs is thought to have contributed to this entrapment. The

support ledge details were sufficient to prevent any spalling of the ledge from taking place during the testing of RT1 and RT3, as is indicated in Figure 9d. During the testing of RT2 on the other hand it was found that a large degree of ledge spalling took place underneath the ribs after the first cycle to 2% drift had been sustained. The spalling during test RT2 is thought to be the result of construction inconsistencies alone as variations between the three tests were limited to the types of rib stirrups used, as was outlined in Table 1. It can be seen that vertical support of the rib was still maintained despite the rib having been pulled off the ledge with virtually no residual seating remaining. The width of the interface crack during the final cycles of the loading protocol can be seen, at which point the continuity reinforcement in the topping remained intact. Figure 10 shows the drift and elongation deformations applied during each test. These deformations have been modified to exclude very high levels of positive rotation (as high as +14%) which were induced during the first few cycles, most notably in test RT1. Figure 9a highlights the result of these excessive deformations where concrete crushing was made to occur along the line of a secondary crack during test RT1. The damage did not appear to affect the subsequent behaviour of the connection throughout the remainder of the test. The high levels of rotation came about due to the very limited resistance of the connection to positive moments and as the jack pressure was released after applying negative rotation, the support beam is made to freely rotate under the combined weight of the floor, the coil and its own self weight. Due to limitations with the test setup positive elongation was applied in excess of that calculated for the prototype structure (shown in Figure 8) throughout the entire test. The testing arrangement used involves elongation being induced at the centre of rotation as positive rotations are applied and this is illustrated in Figure 10 by the skewed loading combinations. It can be seen in Figure 10 also that the loading protocol was carried out fully (to ±4.5% drift) in test RT1 while negative rotations in excess of 4.5% were applied in tests RT2 and RT3.

a) RT1

b) RT2

c) RT3

Figure 10. Applied loading

The measured moment-drift response for all three tests is shown in Figure 11. A low positive moment capacity was observed due to the rib support acting like a pin, while the negative moment capacity was much higher (approximately 40 kNm) due to the continuity reinforcement in the topping. The continuity reinforcement did not fracture in any test during the intended loading protocol to 4.5% drift. However, during test RT2 where a final cycle to -6% was applied fracture of the reinforcement occurred, resulting in a sharp drop in negative moment capacity at 5.5%, as shown in Figure 11b. During the testing of RT3 negative moments were applied up to 7% but these could not cause the rupture of the continuity reinforcement. The fact that the initial negative moment capacity of the stiff, uncracked section was higher than the capacity of the cracked section later in the test highlighted that the continuity reinforcement was insufficient to cause additional secondary cracks.

a) RT1

b) RT2

c) RT3

Figure 11. Moment rotation response

CONCLUSIONS An experimental testing program is currently in progress to investigate the seismic performance of both currently recommended and existing RT flooring support details. The first three tests have been completed and simulated earthquake loading was applied to the support connection of RT specimens with recommended detailing. The following preliminary conclusions are drawn from the results of the three tests that have been conducted to-date:

The SESOC recommended support connection details provide good seismic performance and robustness. The connection detail prevented the formation of positive moment cracks and enabled the rib to slide without becoming entrapped in the support connection to drifts in excess of ±4.5%.

As a result of the lack of rib damage, no significant influence of the rib stirrup configuration was observed during the tests.

Despite the connection details enabling the rib to slide, partial entrapment of bottom corner of ribs was found to occur at large drift levels.

Severe ledge spalling beneath the ribs was observed in some tests while the ledge appeared to remain undamaged in others.

Concentration of deformation to a single crack would result in the loss of diaphragm integrity at the damaged support.

ACKNOWLEDGEMENTS Financial support for this research was provided by the Natural Hazards Research Platform through contract C05X0907 in addition to Stresscrete Northern Ltd, Formstress Precast Ltd and the University of Auckland. The authors would like to thank Paul Cane, Thamer Lasso, Greg Johnston, Luis Daza, Tony Kempson, Tony Roberts and the team at Stresscrete for their assistance and providing their facilities for use throughout the duration of this project as well as advice provided by Precast NZ members, in particular Rod Fulford and John Marshall. The authors would also like to acknowledge the work of University of Auckland undergraduate students Aingaran Manokaran and Tony Fu during the first stage of testing. REFERENCES Bull, D. and J. Matthews (2003), “Proof of concept tests for hollow-core floor unit connections”, Precast NZ Report, C2003-1, University of Canterbury, New Zealand. Corney, S. R., R. S. Henry and J. M. Ingham (2014), "Performance of precast concrete floor systems during the 2010/2011 Canterbury earthquake series.", Magazine of Concrete Research, Vol. 66, No. 11, pp. 563-575. Federation Internationale de la Precontrainte (1992), “Quality assurance of hollow core slab floors”, London.

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