THE SEISMIC PERFORMANCE OF FLOORING SYSTEMS EXECUTIVE SUMMARY. Dene Cook 1

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1 THE SEISMIC PERFORMANCE OF FLOORING SYSTEMS EXECUTIVE SUMMARY Dene Cook 1 This report has been prepared by the Technical advisory group on precast flooring systems. The group includes representatives from: Universities of Canterbury and Auckland The NZ Society of Earthquake Engineering The Society of Structural Engineers The Concrete Society Precast NZ Inc Precast floor manufacturers Consulting Engineers Cement and Concrete Association of NZ The group has been formed to disseminate the results from recent research to the industry, and provide input into the direction for future testing. The fundamental messages the group wishes to take to the industry are: The preferred seating arrangement for hollow core units supported on concrete beams is shown below. It is considered that using this seating detail will ensure improved seismic performance above that of the commonly used detail of providing plastic cut-offs in the cores to prevent infiltration of the topping concrete. The proposed detail has no cost penalty over the existing practise. Hollowcore units should not be positioned parallel and immediately adjacent to beams. They should be located a distance away ( mm) and linked to the beams by the concrete topping only. Exterior columns should be tied back into the structure either by transverse beams, or by ductile reinforcement in the floor slab. The reinforcement shall be capable of resisting a force equal to 5% of the gravity axial load in the column. The following sections provide information on: The reasoning behind the above recommendations Interpretation of recently completed research on flooring systems Aids to interpreting the results for structures with a different structural form to those tested. The direction of future testing. Preferred seating detail for Hollow core 1 Chairman of the Technical Advisory Group on Flooring Systems

2 INTRODUCTION Groundbreaking research being conducted at the Universities of Canterbury and Auckland is illustrating the need for additional research effort into understanding the seismic performance of flooring systems in multi-storey structures. The research has illustrated that excellent performance can be expected from walls beams, columns and beam-column joint zones of ductile frames designed to NZS However, for the arrangements tested, the performance of the floors did not match that of the frames. An interim summary of the results of the ongoing research being conducted by Jeff Matthews, Des Bull, and John Mander is down loadable from the Cement and Concrete Association website, under publications. (CCANZ website is ). Also available on the website is a paper by Lau, Fenwick and Davidson, summarising the results of testing conducted at the University of Auckland on frames with floor slabs containing prestressed units. In both cases the research relates to perimeter frames where the prestressed floor units have spans which are greater than the beams in the perimeter frames. core units that were adjacent to the primary beams, which are the top and bottom beams shown in the plan in Figure 1. Prior to failure (refer to summary paper on web for loading history), this cracking appeared to be limited to the two exterior hollowcore units and the ends of the internal units. It is believed that the incompatible vertical displacement between the beams and the hollow core units was largely responsible for the extensive web cracking in these units, see Figure 2 (a). 3. Localised vertical splitting cracks formed close to the seats of the hollow core units, as illustrated in Figure 2 (b). These cracks first became visible early in the loading history when the interstorey drifts were of the order of 0.3%. After one cycle to +2.5% and 2% drift, some of the floor units had dropped mm. The relative importance of ductility, drift, and beam/floor deformation incompatibility is different for each of the damage categories. The following section provides some guidance on interpreting the results for structures with a different configuration to that of the test unit. This paper has been prepared to assist in interpreting the results of testing recently completed, and complements the papers available at the CCANZ website. This paper also identifies the direction of proposed future research. CATEGORISATION OF DAMAGE FROM UNIVERSITY OF CANTERBURY TEST 300 series hollowcore units 750x750 columns 450x750 beams 250x750 tie beam 450x750 beams Figure 1 illustrates the unit tested at the University of Canterbury. In this test the floor was constructed from 300mm hollowcore units with a 75mm topping. (a) Plan 400x750 beams During the test three principle modes of damage were observed in the floor slab, as briefly outlined below. 1. Due to the elongation of the beam plastic hinge zones, the central column translated outwards, taking with it the first hollowcore unit. This occurred at a ductility factor of 3 and a drift of 1.9%. The result was a wide crack forming between the first and second hollowcore units, with the mesh (non ductile) in the topping concrete failing at the crack. 2. Longitudinal cracks developed in the webs of the hollowcore units as illustrated in Figure 2 (a). Extensive web cracking developed in the hollow Double acting roller bearings (b) Front elevation 6100 Pinned fixed column (c) Side elevation Figure 1. Plan and Elevation of the test specimen

3 A Prying action associated with displacement between beam and hollowcore unit In-situ concrete Web crack Hollowcore unit A Figure 2a. Web cracking in Hollowcore floor units Development of strands at the end of the unit are ineffective in the short distance available Tension force transferred through mortar Supporting beam rotates with interstorey drift Figure 2b. Splitting failure at the end of the hollowcore units AIDS TO INTERPRETING THE RESEARCH The technical Advisory Group (TAG) on flooring systems has been set up to assist in the dissemination of the information from the research conducted to date, and provide direction for future research. This group comprises of representatives from- Universities of Canterbury and Auckland The NZ Society of Earthquake Engineering The Society of Structural Engineers The Concrete Society Precast NZ Inc Precast floor manufacturers Consulting Engineers Cement and Concrete Association of NZ There has been a call from the industry to provide some interim clarification and guidance on the issues involved. Previous information presented on this research focused only on providing a factual summary of the testing. Interpretation was left to the reader. This has resulted in considerably different interpretations of the results. This is understandable as interpretation involves grappling with issues such as those listed below. (a) Are there drifts below which acceptable performance as defined in the building code is expected? (b) Is a thinner floor (i.e. 200 series Hollowcore) likely to perform better and do other forms of precast components sustain similar damage?

4 (c) If the flooring system and the perimeter beams span the same distance, does this improve the performance of the floor? (d) If the units are seated on a bearing pads, or debonded from the supporting beams does this give improved seismic performance? (e) The structure tested was ductile, meaning that plastic hinges formed in the beams. Would the floor performance have been superior if the frames had been designed using a limited ductility, or an elastic design philosophy? (f) Would the performance of the floor be enhanced if the hollowcore unit adjacent and parallel to the primary beam (the alpha slab) was placed say mm away from the beam? (g) How would the floor have performed within a ductile, limited ductile or elastically responding structural wall structure? (h) What would be the effect if the perimeter frames did not have corner columns? The honest answer to the above is that we will not know for certain, but further research should give guidance on these aspects. However, the TAG is optimistic that with minor modifications to detailing the performance of the floors can be enhanced to match that of the supporting structure. The first stage of proposed future research is summarised in section 4. The following comments have been provided to the above questions. The comments represent the best estimates of the TAG with the very limited information that is currently available. Further research is required, and as this is completed the comments will be refined. (a) Drift By adopting the recommendations presented in the executive summary, it is perceived that much-improved performance of the flooring system can be achieved. However, for the assistance of designers, some guidance of drift limits for the tested arrangement is required. The interpretation of the results was the subject of considerable debate within the technical advisory group. Two schools of thought developed for determining a drift limit above which potentially impaired structural performance may occur. The first, would suggest that the seating of the floor units were damaged, and mm downward movement of the floor occurred, at a drift of 1.8%. Advocates of this school would suggest that this represents a peak drift limit above which some concern exists for the integrity of the floor system. The argument would be that the facts that the floor was not subjected to gravity loads, vertical accelerations, or biaxial seismic attack is balanced by the fact that actual buildings in real earthquakes often perform better than would be suggested by laboratory testing. The other viewpoint is that, there may be potential for at least partial failure of the floor when the peak inter-storey drift reaches 1.2%. It being argued that this value is determined by taking due allowance for the absence of gravity load, vertical accelerations, biaxial loading, and the fact that the limit should be based upon the tail of the probability distribution function. As the research is still in progress, and there is only one test, it is not known how conservative or un-conservative these limits are. It also needs to be acknowledged that these drift limits are specific to the arrangement tested. The following sections give some speculative discussion on how other structural arrangements may perform. It should be noted that the inter-storey drift is intended to be a peak value, and not the ultimate-limit state design inter-storey drift calculated by the method defined in the current Loadings Standard (NZS ). To calculate the peak inter-storey drift from a code based design approach allowance has to be made for the factors outlined below. The ultimate limit state inter-storey drift calculated by the method outlined in clause of NZS corresponds to a design value for a 475 year return earthquake. To translate this into a peak value it should be divided by the structural performance factor (S p ). These limits are based upon safeguarding people from injury caused by potential structural failure; the drift limit associated with economic loss would be lower. (b) Floor thickness and type of floor units Whether a thinner flooring unit (200 series Hollowcore) would allow an increase in the above drift precaution is at present unknown. Failure of the seating in the test unit appeared to

5 be caused by a combination of prying action and horizontal shear forces caused by rotation of the beams. Making the flooring unit thinner may reduce the prying forces generated by the differential displacement between the hollowcore unit and the adjacent beam. However, the thinner unit is also likely to be shorter and no more flexible. Proposed further testing should assist in providing further guidance on this issue (refer section 4). Tests conducted at Auckland University on a perimeter frame and floor slab built up from 150mm deep prestressed ribs did not show any web cracking or vertical splitting cracks near the support zones. However, further tests are required to check the sensitivity of different types of precast units to the failure modes observed with the hollowcore units. (c) Absence of transverse beam at central column The unit adjacent and parallel to the primary beams (alpha slab) in the Canterbury test, would probably have performed better than was observed if a beam supporting the floor had been present at the central column. There are two distinctly different reasons for this supposition. Firstly the alpha slab deteriorated due to deformation incompatibility between the stiff hollowcore floor and the adjacent beams. If the floors and beams spanned similar lengths, the level of deformation incompatibility should be smaller, and hence less damage would have been expected in the floor. Secondly, if a transverse beam had been provided at the central column, it is probable that the separation crack that occurred in the topping between the Hollowcore units would not have developed, as the central column would be constrained against outward translation. (d) Alternative support details for the floor units Seating the Hollowcore units on a bearing pad, and providing a compressible material on the end face of the unit, is considered likely to improve the performance of units beyond that observed in the Canterbury test. This is likely to be cost neutral compared to existing practise. However, it does alter the fixity at the end of the units and therefore the vibration characteristics of the floor are changed. In filling some of the core and providing a hair clip detail may also improve seismic performance. Further testing is planned to evaluate these details (refer section 4). (e) The significance of ductility The structure was designed using capacity design techniques to ensure that plastic hinging occurred in the beams. Hinging in the beams results in elongation of the plastic hinges, and this was believed to be a significant contributing factor in the outward translation of the central exterior column. If the frame had been designed elastically, some small beam elongation would still have occurred however, it is probable that the central column would not have translated meaning that the longitudinal tearing of the topping between the first and second floor units would probably not have occurred. (f) The damage sustained by the alpha hollowcore unit appears to arise from two conditions. Firstly the differential vertical deflection between the beams and the hollowcore units gave rise to prying shear forces in the topping concrete. These in turn led to splitting stresses in the webs, see Figure 2 (a). Secondly, the incompatible rotation between the end of the hollowcore unit and the supporting beams led to the formation of the vertical splitting cracks in the hollowcore units close to the support locations, see Figure 2 (b). Both the vertical differential displacement and the incompatible rotation are a function of drift. Hence, the type of failure that was observed in the alpha slab may not be significantly influenced by ductility. The significance of hinging in the beams on the observed seating failure of the flooring units beyond the alpha slab is less clear. These units are not subjected to the same level of deformation incompatibility along their lengths as the alpha slab was. Damage to these units is dominated by the mechanism shown if figure 2(b). Plastic elongation of the beams may place the seating of the flooring units under tensile stresses that do not exist to such an extent in an elastically responding frame. How the tensile, prying, and shear forces interact to produce failure at the floor support is unclear. Some increase in the precautionary drift limit maybe acceptable for elastically responding structure, however, the magnitude of any relaxation is not known until further research is completed Locating the first hollowcore unit a distance away form the adjacent primary beam Moving the hollowcore units away from the parallel beam could be expected to improve the

6 performance of the alpha slab zone for the reasons set out below. The topping spanning between the floor and hollowcore unit is flexible and it should be able to accommodate some of the incompatibility vertical deflections between the beam and alpha hollowcore unit. The alpha unit will not be seated on potential plastic hinges in the beams in the transverse direction. Moving the hollowcore unit away from the beam reduces the contribution the unit makes to the moment capacity of the beam and it limits this contribution to the shear capacity of the linking slab. The research conducted by Lau, Fenwick, and Davidson, using a precast rib and infill floor system, supports this supposition but also reemphasises the importance of considering deformation incompatibility between beams and the adjacent floor (refer question (h) for more detail). (g) Performance of shear wall structures. Although the test was conducted on a framed structure, some guidance is possible on the likely performance of shear walled structures. For a shear wall structure, the following is likely- For the situation where the shear wall is shorter than the flooring unit running adjacent and parallel to the wall, the floor may sustain damage due to the prying action caused by incompatible displacements. For example a structural wall with a length of 4000mm, sustaining an inter-story drift of 1.5 percent would induce incompatible displacements of at least 25mm. In addition, in ductile walls, elongation of a wall can induce vertical displacements relative to surrounding columns, which may add to the incompatible rotation between supporting beams and the hollow core units. It is acknowledged however, that many shear wall structures are deigned using an elastic design philosophy and with interstorey drifts of only 0.5-1%, so the deformation incompatibility would be less than indicated in this example. Where the wall and adjacent floor unit are the same length, little damage to the floor is expected. Longitudinal tearing between units, as observed in the Canterbury test, is unlikely to occur. The damage to the seating of the hollowcore units, resulting in vertical splitting cracks due to incompatible rotations may still develop. Hence the drift limits previously suggested should also be applied for this form of structure. (h) What would be the effect if the perimeter frames had not had a corner column This question has been addressed by the research conducted by Lau et al at Auckland. A paper on this research is available on the CCANZ website. The research identified the importance of considering the potential deformation incompatibility between the floor and the adjacent beams. In these tests, damage occurred adjacent to the column next to the cantilevered section due to differential deflection between the beams and the prestressed flooring units. This structural arrangement significantly increases the potential differential deflection between the precast flooring units and the beams associated with seismic sway of the structure. With the prestressed stem type slabs used in the Auckland test this issue was not one of life safety, as the differential deflection was taken up by vertical bending, and finally by shear failure of the insitu slab linking the beam to the first prestressed stem beam. However, if hollowcore units had been used, that were not offset from the beams, the increased differential deflection could be expected to hasten the web cracking failure mode observed in the Canterbury test. FUTURE RESEARCH Research into improving the performance of the seating detail of hollowcore is proposed using subassemblages comprising supporting beams and hollowcore units. This research is focused on evaluating the performance of alternative seating arrangements. Questions regarding deformation incompatibility cannot be fully addressed by component testing, but will be evaluated by future use of the Jeff Matthews rig. Figure 3 illustrates the four seating arrangements that it is planned to test in the near future. Planning for the tests is underway and is scheduled completion date is in December The aim of the testing is to provide initial recommended drift ratings for various seating arrangements.

7 6.2.3 of AS/NZS 1170 Part 0 - the new loading standard, requires that parts are tied back into the structure with a capacity of at least 5% of the axial load carried by the member. 2. Allow for deformation incompatibility between beams and flooring units that run parallel to them by offsetting the flooring units away from the beams and connecting them with a topping slab only. Permanent timber formwork may be used for this purpose. 3. Particularly in buildings where the predicted peak inter-storey drifts are greater than 1.2%, use details illustrated as test #3 and #4 in figure 3 to seat the hollowcore units. For the details in test #3, the designer should however, consider the implications on the vibration characteristics of the floor by converting the seating detail from partially fixed to simply supported. MORE INFORMATION More information on the proposed testing, or the interpretation of this paper can be obtained by contacting Dene Cook, Chairman of the Technical Advisory Group on Flooring Systems, , Dene.Cook@cca.org.nz Figure 3 Proposed seating arrangements that will be tested. INTERIM RECOMMENDATIONS Disclaimer This interim report has been produced to assist in interpreting the results of a research project that is still in progress. The information represents the best estimate of the advisory group using the information available in July Further research programmed in the near future may result in modifications to the recommendations. Although the authors have exercised due care in writing this paper, no responsibility can be taken in its application by the authors, their employers, or the sponsoring organisation. With the completion of further research work, firmer recommendations may be made regarding some of the questions that were posed. However, there is a demand from the industry to provide some interim guidance on the detailing of flooring systems. The following suggestions are provided on the basis of the group s best estimate at the present. Please refer to the disclaimer at the end of this document to appreciate the conditions under which this guidance is given. 1. In ductile frames ensure that all columns are tied back into the floor either by beams or by reinforcement placed within the slab. Clause