structural safety and resistance of prefabricated … · prefabricated wall structures of...

2008 SLOVAK UNIVERSITY OF TECHNOLOGY34

DYNAMIC EFFECTS AND IMPACTS

Since 1997, increased seismic activity has been recorded mainly in West Bohemia. At the end of 2000, a strong shock reached a local magnitude of 3.4 and was felt by the inhabitants of the Cheb, Sokolov, Karlovy Vary and Tachov regions. A strong earthquake swarm has been registered since August 2001, the eight phases of which included over 1500 earthquakes, over 5% of which were also felt by local people (Fig. 1) [1].According to the map of seismic threats to the Czech Republic (annex to the national application document Eurocode 8 [3]), manifestations of natural earthquakes with a macroseismic intensity ranging between 6 and 6.5 degrees on the Richter scale may be

J. WITZANY, T. ČEJKA, R. ZIGLER

STRUCTURAL SAFETY AND RESISTANCE OF PREFABRICATED WALL SYSTEMS OF MULTI-STOREY BUILDINGS WITH REGARD TO THE EFFECTS OF A DYNAMIC LOAD CAUSED BY TECHNICAL SEISMICITY

KEY WORDS

• Technical seismicity,• prefabricated wall systems,• experimental research,• reliability.

ABSTRACT

The paper sums up the results of the experimental and theoretical analysis of the response of a model of a 7-storey prefabricated wall structure of a multi-storey building to the effects of technical seismicity. The research was part of the research plan MSM6840770001 „Reliability, optimization and durability of building materials and structures”.

prof. Ing. Jiří Witzany, DrSc.Dr.h.c., Ing. Tomáš Čejka Ph.D., Ing. Radek Zigler, Ph.D.

Department of Building Structures, Faculty of Civil Engineering, CTU in Prague, Thákurova 6, 166 29 Prague 6, [email protected] , [email protected] , [email protected]

Research fields: structural problems, analyses and designing of building structures, reconstruction and rehabilitation of buildings

2008/3 PAGES 34 – 41 RECEIVED 15. 4. 2008 ACCEPTED 10. 7. 2008

2008 SLOVAK UNIVERSITY OF TECHNOLOGY

Fig. 1 Macroseismic field of the most intensive earthquake of the seismic swarm in 1985/86 [1]

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35STRUCTURAL SAFETY AND RESISTANCE OF PREFABRICATED WALL SYSTEMS...

expected, while the expected values of a quasi-effective acceleration range from 0.06 to 0.4 g, i.e. 0.59 to 3.9 ms-2.These facts confirm the need for further investigation of issues related to the response of pre-cast panel structures to the effects of natural seismicity, potential consequences and effects on the residual structural safety of pre-cast panel buildings. An inseparable part of this activity must be continuous monitoring and investigation of selected representatives of pre-cast panel buildings with regard to the above-mentioned aspects, including evaluation and elaboration of designs and preventative measures aimed at securing the structural safety of pre-cast panel buildings located in seismically active areas in West Bohemia. Pre-cast panel buildings located in the vicinity of roads, motorway and railway corridors, near the routes of the underground and also close to intensive construction activity or industrial activity are exposed to so-called technical seismicity. In accordance with ČSN 73 0040, shocks caused by technical seismicity are evaluated as random long-term or short-term loads. The intensity and character of technical seismicity depend on the weight of the observed buildings, the design of their foundation structures and the geological conditions in the respective area. The intensity of shocks caused, e.g., by traffic further depends on the weight, speed and acceleration of moving vehicles and the surface and construction of the roadway or superstructure. The dominant frequencies of subsoil shocks due to road traffic usually range in an interval of 10 Hz to 80 Hz. The frequency spectrum of the seismic response and the value of the seismic load may be most accurately determined by experimental in-situ measurements.

CHARACTERISTICS OF MULTI-STOREY PREFABRICATED WALL SYSTEMS

Prefabricated wall structures of multi-storey buildings brought about a brand - new quality to building design, which required deeper theoretical knowledge, the replacement of empiric a knowledge by theory, and the substitution of idealized and considerably simplified models of the behaviour of structures and their parts by correct computational, physical (material) and loading models. The high rigidity of a prefabricated concrete wall structure and the resulting serious mechanical stress states caused particularly by the effects of volume changes (temperature, moisture content), the effects of the footing bottom changes in shape, etc., are the most frequent cause of failures, particularly failures of joints of units characterized by their insufficient yield and load-bearing capacity. Prefabricated wall systems are characterized by a deformation and failure mechanism, where wall units shift in joints disintegrated by cracks, i.e., so-called contact interfaces. In practical cases, it mostly

suffices to consider non-linearly elastic behaviour only in joints, considering the behaviour of units as linearly elastic as compression and shear stresses acting on units tend to be substantially lower than their ultimate strength at the proportional elastic limit (ultimate load-bearing capacity in the elastic domain). We may, therefore, presume that the limit state of the structure as a whole is preceded by joint failure, or that the structure passes from a linearly elastic behaviour to a non-linearly elastic to plastic behaviour, usually by exceeding the proportional elastic limit in the joints. The rigidity of joints exposed to repetitive loading varies and decreases in relation to the number of cycles. The specific feature of cyclic effects is the fact that the failure of joints or the structure occurs as a result of even such loads which do not reach the ultimate strength, as a result of practically any magnitude of load in individual loading cycles if the proportional elasticity limit of the linearly elastic behaviour of the joint (structure) defined by the “load x deformation” relationship was exceeded at least in one of these cycles. The failure of the joint (structure) occurs by reaching the limit deformation, taking the form of a so-called incremental collapse (Fig. 2). The growing deformation of the joint in each successive loading cycle, i.e. on-going joint plastification – as a consequence of the repetitive loading effect by the shear force Top – results in a gradual decrease in the rigidity (effectiveness) of the

Fig. 2 a) Working diagram of a vertical joint for loading by a monotonously growing shifting force and for low-cyclic loading by shifting force Top < Tm [4], b) a non-reinforced joint, Top = 123 kN, failure during 84th cycle (σx,m = -0,087 MPa)

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joint down to a value lim Kop,i, which is lower than the joint rigidity at reaching the limit load under the monotonously rising load Ku,m; and it holds true that Kop,1>Kop,2>…>Kop,i<Ku,m.The number of cycles nop of repetitive loading by the shear force Top depends on the magnitude of the force Top (Fig. 3). It shows a falling trend with the growing magnitude of the shear force Top. Experimental tests have manifested that a theoretical estimate of the number of cycles nop until failure may be based on the assumption that the magnitude of the limit joint deformation δm described by components δy,m a δx,m is independent of the loading history. The

number of cycles of repetitive loading grows with the growing joint ductility, i.e., the range of the interval (δy,el - δy,m) [4].In order make a rough estimate of the number of cycles nop preceding the failure of joints, the following relations may be used [7]:

for Top=Tu,el, To=0

A=(µσKT-σx)

The assessment of the residual structural safety of mainly older types of precast-panel houses erected until 1974, which show some serious design defects (horizontal bracing at the floor slab level – reinforcement imbedded into joints of floor units), requires determination of the residual rigidities of the joints disintegrated by cracks [5].

EXPERIMENTAL RESEARCH OF THE RESPONSE OF A MODEL OF A PREFABRICATED WALL STRUCTURE TO THE EFFECTS OF TECHNICAL SEISMICITY

In 2007, the research plan MSM6840770001 “Reliability, optimization and durability of building materials and structures”

Fig. 3 Relationship of the number of cycles of repetitive loading nop on the magnitude of shear force Top

Fig. 4 a) Experimental system, diagram of a plan and elevation arrangement of a model of a prefabricated wall structure on a 1:3 scale; b) Joint of wall and floor unit, linking bar, wall units faces coated with separation paint Legend: The model of a prefabricated structure (Fig. 4) was composed of three transverse walls with an axial distance of 1.4 m (corresponding to a span of 4.2 m) and a longitudinal wall weakened by a door opening located in one transverse module. The arrangement of the prefabricated units, reinforcement of wall and floor units, reinforcement of the floor slab, and shaping of the contact interfaces corresponds to the pre-cast panel system T06B. The structural height of a storey was 0.933 m (corresponding to a structural height of 2.8 m). The wall and floor units with a thickness 50mm (corresponding to the unit thickness of 150mm) were made of C16/20 concrete. The grout was made of concrete C16/20, and the grout reinforcement of the steel was of a E 10216 quality. The composition of the units and the arrangement of the bearing system are evident from Fig. 4. Prior to assembly, the contact interfaces of the floor and wall units were finished with 2 coats of separation paint (simulation of the shrinkage crack at the contact of the units).

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included the implementation of the second phase of research of the residual structural safety of a model of a prefabricated wall structure on a 1:3 scale exposed to the effects of repetitive loading and the effects of technical seismicity. The repetitive loading of the experimental model was carried out by a pair of steel tie rods exerting an inclined force with the horizontal and vertical components acting at the upper free end of the model. The vertical component stabilized the experimental model of the structure against tilting (exerting compressive stresses in the bed joints and substituting the effect of a vertical load1), Fig. 5). The repetitive loading was exerted by a pair of “enerpac RCH – 603“ hydraulic cylinders mounted on inclined steel tie rods. The dynamic load was exerted by the “TIRA vib“ electrodynamic exciter, type TV5550/LS, 750 kg in weight fitted with a mobile weight of 13.2 kg. The exciter’s frequency range was 0-3 kHz; the maximum deflection of the mobile weight was 50.8 mm. The exciter was mounted by means of mandrels onto the floor structure of the topmost storey of the model and adjusted to a horizontal oscillation. The plan for the experimental research on the 7-storey prefabricated wall structure’s model is shown in Table 1.

RESPONSE OF A PREFABRICATED STRUCTURE TO A REPETITIVE DYNAMIC LOAD

In the individual cycles and states of loading, relative shifts in the selected vertical joints of wall units, horizontal deformations at three levels, vertical deformations at the gable wall footing and normal stresses in wall units were measured on the experimental model (Fig. 6). In the course of the dynamic loading, the horizontal response of the structure was measured by three Wilcoxon accelerometers, model CMMS 793 L, with an output sensitivity of 51 mV/ms-2. One of the accelerometers was mounted onto mobile exciter parts scanning the weight motion, while the second accelerometer was mounted onto the 4th storey of the model and the third onto the 7th storey of the model. The test served for the determination of the first and the second natural frequencies (Fig. 7).In the 1st state of loading there was a growth in total deformation of 12.3% and permanent deformation of 50% as compared to the

Fig. 5 Diagram of loading a model of an experimental structure with inclined forces exerted by steel tie rods; picture of the mounted electrodynamic exciter

Tab. 1 Overview of states of loading

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total and permanent deformation in the first loading cycle of the 1st state of loading. In the 2nd state of loading in the 10th loading cycle under loading by 2x 30 kN, there was an increase in the total horizontal deformation by 121.1% as compared to the total deformation in the first loading cycle of the 1st state of loading and in permanent deformation by 200%.After five loading cycles of the 3rd state of loading there was a growth in the total horizontal deformation and the permanent horizontal deformation by 82.5% or by 387.5% respectively as compared to the first loading cycle of the 1st state of loading.In the 4th state of loading the total horizontal deformation under

loading by 2x 30 kN reached a value of 3.12 mm, and the permanent horizontal deformation reached a value of 2.64 mm, i.e. by 173.7% and by 1785.1% higher as compared to the first loading cycle of the 1st state of loading, and by 15.5%, or by 247.3% higher as compared to the first loading cycle of the 4th state of loading.

Fig. 6 Arrangement of the measuring devices

Fig. 7 Shape of the first and second natural frequency (a, b), oscillation record for experimental determination of natural frequencies (c), record of structure’s oscillation due to dynamic load (d)

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The failure of the joints of the load-bearing units after the 1st and 2nd state of loading (a), or after the 3rd and 4th state or loading (b) is schematically displayed in Fig. 8.

DISCUSSION OF EXPERIMENTAL RESEARCH RESULTS

The results of the experimental research carried out on an experimental model exposed to the effects of a repetitive monotonously growing load ranging between 0-2x30 kN and 0-2x80 kN and a dynamic load with a frequency of 15 Hz (8 x 105 cycles in all), the aim of which was an investigation of the response and impact of these effects on the residual rigidity and structural safety of a prefabricated system may be summed up into the following conclusions: • The analysis and comparison of the experimentally determined

increments of the total and permanent deformations on the top of the experimental model in individual loading cycles of the 1st, 2nd and 3rd states of loading for the case of a selected frequency value of 15 Hz (experimentally measured 1st and 2nd natural frequencies at the 7th storey level are f1 = 5.62 Hz and f2 = 13.92 Hz, while at the 4th storey level f1 = 5.37 Hz and f2 = 13.92 Hz) suggests the relatively low impact of the dynamic

effects on a gradual decrease in the rigidity of the load-bearing system resulting from joint degradation (appearance of structural cracks and their propagation in the joints of load-bearing units). The relatively high frequency of the dynamic load exerted by the electrodynamic exciter with a very low oscillation amplitude to which the experimental model was exposed in the 2nd state of loading did not cause any prominent, visually observable, failure of the joints of the load-bearing prefabricated units.

• In the course of the 3rd loading cycle, no progressive growth in deformations and no prominent decrease in the rigidity and resistance of the load-bearing wall system was recorded. The growth in the total and permanent deformations caused by the impact of the repetitive low-cyclic load during the 1st – 3rd state of loading, i.e., after 24 cycles, amounted to 82.5% and 387.5% as compared to the first loading cycle of the 1st state of loading. The growth of the total and permanent deformations caused by the impact of the dynamic load in the 2nd state of loading, i.e. after 80x104 cycles with a high frequency and a very low amplitude, which amounted to 90.9% and 166%, proves that

Fig. 8 Growth in the horizontal deformation (deflection fh) in the individual states of loading at the 7th storey level (fh x NT), time pattern of the relationship of the total (horizontal) deformations in individual cycles (fh, total x NT, fh, perm. x NT) on the upper free end

Fig. 9 Diagram of in decrease rigidity in the vertical joints of load-bearing wall units of an experimental system in individual states of loading

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the investigated effect of the dynamic load did not cause stresses in the vertical joints of the wall units exceeding the limit of their linearly elastic action.

• Unlike a dynamic load with a high frequency, a low-cyclic repetitive shear load, where at least in some cycles load exceeding the proportional elastic limit of the T x δ relationship is reached in vertical joints of wall units (Fig. 2), causes a progressive decrease in the joint rigidity having consequently substantially more serious effect on a gradual decrease of the structural safety of the load-bearing system as compared to the dynamic effects caused by technical seismicity (e.g., effects of traffic, Fig. 8).

• Based on the analysis of the experimental results of the response of a prefabricated experimental system to a dynamic load, a relatively high level of the reliability and resistance of similar load-bearing prefabricated wall systems of multi-storey buildings to the effects of standard technical seismicity with the frequency spectrum of seismic response and the magnitude of the seismic load within the verified scope may be reported.

These conclusions cannot be applied in their full scope to all prefabricated wall systems, in particular to the cases of load-bearing prefabricated wall structures (precast-panel buildings) with an insufficient horizontal reinforcement of the floor slab, or with explicitly degraded joints of load-bearing units. • The experimental loading of the experimental model within

the 4th state of loading manifested an exceptionally serious

(stabilization) effect of the longitudinal walls even in the phase where they were separated on the top storeys of the experimental model from adjoining transverse walls and the floor structure by a continuous crack in the joints (Fig. 10). At this phase, the separated longitudinal walls acted to stiffen loosely - inserted diaphragms with a characteristic effect of a compressive diagonal prominently stabilizing the structure in the longitudinal direction against the effects of the horizontal load (Fig. 11 and Fig. 12).

The paper was written with support from the Research plan MSM 6840770001 “Reliability, optimization and durability of building materials and structures”.

Fig. 10 Diagram of the gradual failure of the joints and units of an experimental model in the course of 1st and 2nd (a) and in the course of 3rd and 4th (b) state of loading

Fig. 11 Time pattern of the principal stresses in the longitudinal wall loaded by an inclined force 2x30 kN acting on the upper free end (disintegrated joints with lowered rigidity 10-3)

Fig. 12 Isolines of principal stresses in the longitudinal wall loaded by inclined force 2x30 kN on the upper free end (disintegrated joints with lowered rigidity 10-3)

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REFERENCES

[1] Geophysical Institute of the Academy of Sciences, Czech Republic, Seismic Department.

[2] Kaláb Z.: Assessment of Seismic Load of the Mediaeval Mine Jeroným in the Czech Republic, Acta Montanistica Slovaca No. 1, Vol. 8, 2003.

[3] Eurocode 8: Design of Structures Resistant to Earthquakes, ČNI 2005.

[4] Witzany, J., Behaviour of Joints of Concrete Units Loaded by Shear under Repetitive Loading. In: Pozemní stavby journal 8-1987, pp. 343 – 348.

[5] Witzany, J., Regeneration of the Load-Bearing Structure of Precast-Panel Buildings. In: Pozemní stavby journal 9-1989, pp. 373 – 378.

[6] Witzany, J., Zigler, R., Pašek, J.: Experimental Research of 3D Behaviour of a 1:3 Model of a Prefabricated Wall Structure of a Multi-Storey Building. In: Stavební obzor. 2001, Vol. 10, No. 12, pp. 21-23. ISSN 1210-4027.

[7] Witzany, J.: Behaviour of Joints of Concrete Units Loaded by Shear under Repetitive Loading. In: Pozemní stavby journal 8 – 1987.

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