ABSTRACT: Earthquake induced forces are mass proportional forces. As a result lightweight building structures have a fundamental advantage compared to heavy solid structures. Within a European research project an earthquake resistant lightweight house was developed and a two stories mock-up was tested on a 3D shaking table. Two variants for the structural system at three different stages of construction were tested at different excitation levels with a maximum peak ground acceleration of up to 1 g. The main purpose of the tests was to determine the load bearing capacity of a building that is made of a combination of small sized hot-rolled and cold formed steel members and drywalls elements and that is designed according to a standard seismic code. In addition, the dynamic properties (natural frequencies, mode shapes, damping values) of the mock-up were determined from the test results and compared to calculated properties. The large-scale tests successfully demonstrated that a combination of lightweight steel constructions and drywall elements is suitable for residential buildings in seismic prone areas. The low mass of such constructions leads to a reduction of the inertia loads and by this to a reduction of the required cross sections. In addition measured damping ratios of up to D = 15 % showed the ability of the drywall systems to effectively dissipate energy.

KEY WORDS: Shaking table, seismic design, lightweight building, drywall systems, natural frequencies, damping.

1 INTRODUCTION In seismic active areas of southern Europe residential buildings made of lightweight steel constructions and drywall elements are a good alternative to a conventional design with concrete skeleton constructions and infilling masonry. Beside the low weight leading to low inertia loads and the resistance against out of plane motion, the main advantage of this type of structure is the ability to dissipate energy by its high ductility and the flexible attachment of the drywalls.

Within the European research project I-SSB (Integrated Safe & Smart Built) the concept of an aseismic lightweight building was realized by a large-scale test building (mock-up). The mock-up was designed according to the Greek seismic code EAK2000 and tested on a shaking table at the Laboratory of Earthquake Engineering of the National Technical University Athens (NTUA) with artificial seismic time histories with maximum peak ground accelerations (PGA) of up to ag = 1 g.

The main purpose of the tests was to determine the load bearing capacity of a building made of a combination of small sized hot-rolled and cold formed steel members and drywall elements that is designed according to a standard seismic code. In addition, the measured dynamic properties (natural frequencies, mode shapes, damping values) shall be compared to the properties determined with a numerical model.

2 STRUCTURAL DESIGN OF THE MOCK-UP The mock-up is a two-storey steel structure covered by a gable roof. The total height of the mock-up is 7.5 m. The foot print area is quadratic 3.6 m x 3.6 m and limited by the size of the shaking table.

In order to minimize the seismic inertia loads the objective was to have the mass of the load bearing structure of the mock-up as low as possible. Therefore, a steel skeleton structure was chosen with columns made of hot rolled, slender steel members (HEA 120) and girders and truss beams made of cold formed steel (CFS) with a thickness of 1.5 to 2.5 mm. Due to the hinged connections bracings respectively frames with truss girders were necessary for the horizontal load transfer.

Each storey consists of steel frames, which at ground floor are X-braced in both horizontal directions to transfer loads to the foundation. At the second storey the steel frames parallel to the roof are X-braced as well, while for the other direction two variants are investigated. In variant 1 shown in Figure 1 the frames are intended to act as moment resisting frames. In variant 2 shown in Figure 2 X-bracings are used.

Seismic design of lightweight buildings with drywall systems

Carsten Block1, Fritz-Otto Henkel1

1Wölfel Beratende Ingenieure, Max-Planck-Str. 15, 97204 Höchberg, Germany email: block@woelfel.de

Proceedings of the 9th International Conference on Structural Dynamics, EURODYN 2014Porto, Portugal, 30 June - 2 July 2014

A. Cunha, E. Caetano, P. Ribeiro, G. Müller (eds.)ISSN: 2311-9020; ISBN: 978-972-752-165-4

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Figure 1. Mock-up bare steel framed structure, Variant 1, Configuration I.

Figure 2. Mock-up bare steel framed structure, Variant 2 with X-bracing, Configuration I.

Figure 3. Mock-up with clay tiles and “simulated” live loads, Variant 1, Configuration II.

The outer walls of the mock-up consist of a double layer KNAUF wall system W388 where the outer layer is a cement board AQUAPANEL (t = 12.5 m) with a plaster finish (see Figure 4). At the inside a double-layer planking with gypsum boards (2 x 12.5 mm) is used. The interior walls are single metal stud frames with single layer cladding type W111.

Figure 4. Example for system W388 (left) and W111 (right).

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Figure 5 shows the complete mock-up and Table 1 summarizes the weights of the different structural parts of the mock-up. The total mass of the mock-up is 9 tons.

Figure 5. Complete mock up, Configuration III.

Table 1. Weight of mock-up.

Weight per unit area [kN/m²]

Area[m²]

Weight[kN]

clay tiles 0.55 15.2OSB-cladding, 22 mm 0.13 15.2suspended ceiling, D112, 12.5 mm 0.11 10.9 11.5GIFA-floor board, 28 mm 0.42 10.9suspended ceiling, D112, 12.5 mm 0.11 10.9 5.8Exterior walls, W388 0.63 79.2Interior walls, W111 0.23 16.0 53.6

structural steel 19.190.0total weight [kN]:

Structural Part

roof

1st Floor

walls

For both wall types monotonic and cyclic shear tests at large

scale specimens were performed at EMPA Dübendorf ([2], [3]). The tests show that the wall type W388 has a shear resistance of about 15 kN/m length of wall (Figure 6). This value corresponds well to similar tests performed for metal stud systems with single layer cladding with a shear resistance of up to 10 kN/m [4].

Forc

e F

[kN

]

Displacement s [mm] Figure 6. Shear tests EMPA [2], W388 wall L/H = 2.5/2.72 m.

The mock-up was assessed for the load cases dead weight,

live loads and earthquake. For the seismic design the elastic response spectrum according to [1] for the city of Amfilochia, Greece was chosen with

· soil class B · damping ratio D = 0.04 · seismic zone II · maximum ground acceleration of 0.24 g

and is shown in Figure 7. A typical time history generated from this spectrum is shown in Figure 8.

Figure 7. Elastic design spectrum.

Figure 8. Typical time-history ground acceleration in

x-direction from design spectrum. According to [1] a reduction of the elastic spectrum using a

behavior factor q > 1 is not possible as the CFS members correspond to class 4 cross sections.

Corresponding to German codes the load carrying capacity and the stiffness of the drywalls shall not be taken into account. Only the masses have to be considered. For the dynamic analysis and the assessment of lightweight structures with high mass ratios of mwalls / msteelwork this may lead to

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wrong and potentially non-conservative results. Neglecting the stiffness of the walls the calculated natural frequencies may be too low and the applied spectral values wrong.

3 DYNAMIC BEHAVIOUR OF THE MOCK-UP The shaking table tests of the mock-up were performed at NTUA. The shaking table with dimensions of 4.0 m x 4.0 m allows an independent operation in all six degrees of freedom. The table can shake specimens with a maximum load of 100 kN and apply horizontal and vertical accelerations of up to 2.0 g in a frequency range of 0.1 to 100 Hz. In total three configurations of the mock-up were tested:

I. Bare steel-framed structure (Figures 1 and 2) II. Bare steel-framed structure with floor board and roof

sheathing including clay tiles (Figure 3) III. Completely built-up mock-up including drywall

systems (Figure 5) The acceleration responses of the mock-up were recorded

by several sensors. Using this data the dynamic properties natural frequencies, mode shapes and damping ratios were determined for the different configurations by an experimental modal analysis.

Figure 9 shows exemplarily the determined transfer functions between two sensor locations (MP1 and MP5) for 2-axial seismic tests in x-y direction for all three configurations with a the maximum design ground acceleration of 0.24 g.

Based on the transfer functions the fundamental natural frequency in x-direction for the complete mock-up (Configuration III) at a seismic load level according to the design is 7.0 Hz with a corresponding modal damping ratio of D = 14.5 %.

Configuration I

Configuration II

Configuration III

measuredmodal model, 6.34 Hz, 1.9 %

measuredmodal model, 4.58 Hz, 2.7 %

measuredmodal model, 7.01 Hz, 14.5 %

Frequency [Hz] Figure 9. Transfer functions between sensor locations MP1 and MP5 in x-direction for Configurations I to III.

Figure 10 shows the corresponding first main mode shape for Configuration III calculated with a FE-Model. The calculated natural frequency of 6.7 Hz fits well to the measured value of 7.0 Hz.

Figure 10. Calculated main mode shape in x-direction for Configuration III.

A summary of the fundamental natural frequencies and

damping values for different seismic load levels and configurations is shown in Table 2. For comparison the calculated natural frequencies are included as well.

Expect for the last row of the table the results correspond to variant 1 where the frames in the second storey are intended to act as moment resisting frames without X-bracings. The last row shows the result of variant 2 with X-bracings in x-direction.

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Table 2. Summary of natural frequencies and modal damping ratios.

Direction f0 [Hz] D [%] f0 [Hz] D [%] f0 [Hz] D [%]

x 6.5 - 4.7 - 6.7 -

z 10.4 - 7.3 - 6.6 -

x 6.4 1.4 4.6 2.4 7.4 13.0

z 10.3 1.7 7.3 7.8 6.7 13.8

x 6.4 1.6 4.6 2.5

z 10.3 1.9 7.3 8.7

x 6.3 1.9 4.6 2.7 7.0 14.5

z 10.4 2.2 7.2 8.9 6.7 16.3

x 4.6 2.8 7.0 15.9

z 7.3 8.4 6.4 8.9

x 11.1 2.2 8.3 3.3 7.5 11.5

z 10.0 3.2 7.1 9.0 6.9 14.7

Measurement2D-Excitation

ag = 0.24 gvariant 2 with X-

bracings

-

-

-

Measurement2D-Excitation

ag = 0.12 g

Measurement2D-Excitation

ag = 0.16 g

Measurement2D-Excitation

ag = 0.24 g

Measurement3D-Excitation

ag = 0.24 g

Configuration I Configuration II Configuration III

FE-Analysis

-

As expected the results show a linear behavior up to a seismic level of 0.24 g. Also the damping ratios of D = 1.4 - 2.2 % of the bare steel-framed structure (Configuration I) for load levels smaller than the design values correspond to common values.

A comparison of natural frequencies of Configuration I and II (with additional masses on roof and life load on floor) indicates that the vibrating mass is doubled. The damping ratios for Configuration II in x-y-direction are in an expected range of D = 2 - 3 %. However, the values in z-y-direction are unexpectedly high with D = 8 – 9 %. One possible explanation for the higher energy dissipation could be a shifting of the roof tiles for excitations in parallel to the ridge line.

For Configuration III all outer walls and one interior wall per floor are installed. A realistic amount of doors and windows are considered. Overall, the mass is twice as high as for Configuration II. If we compare the measured natural frequencies of Configuration III and II from this perspective it becomes clear that the drywall systems not only add mass but considerably increase the stiffness of the structure. In this particular case the stiffness is increased by a factor of about 2. The relatively flexible and screwed design of the wall panels and the screwed connection to the supporting structure lead to a significant energy dissipation during the seismic excitation. The measured damping ratios are about D = 15 % while the measured natural frequencies do not indicate any plastic deformation of structural parts.

Comparing the results for variant 1 with frames in x-direction for a 2D-excitation with ag = 0.24 and the results for variant 2 with X-bracings (last row of Table 2) the stiffening effect of the X-bracing can be observed. For both Configurations I and II the natural frequencies in x-direction significantly increase. For Configuration III the increase in stiffness is minor as the bracings to not introduce high

additional stiffness into the system compared to the drywall systems.

Nonlinear effects could only be observed during the stepwise increase of the seismic load level beyond the design values (ag = 0.24 g, 0.36 g, … 1.08 g). Starting at 0.6 g the fundamental natural frequencies dropped [5]. At a level of 0.96 g the main natural frequency in x-direction for Configuration III was 5.0 Hz compared to 7.0 Hz at a level of 0.24 g. Nevertheless, during the visual inspections of the mock-up after each test no visual damages could be observed up to the maximum ground acceleration of 1.08 g. At this level the technical limitations of the hydraulic system were reached and therefore the actual ultimate load carrying capacity of the mock-up could not be tested.

4 SUMMARY The large-scale tests successfully demonstrated that a combination of lightweight steel constructions and drywall elements is suitable for residential buildings in seismic prone areas. The relatively low mass of such constructions leads to a reduction of the inertia loads and by this to a reduction of the required cross sections. In addition the measured damping ratios of up to D = 15 % showed the ability of the drywall systems to dissipate energy.

The tests showed that the conventional approach for the dynamic analysis and assessment of lightweight structures by considering only the masses of the drywall systems is not necessarily a conservative approach. For structures with high mass ratios of mwalls / msteelwork this may lead to wrong and potentially non-conservative results. Neglecting the shear stiffness of the walls, the natural frequencies are underestimated and the applied spectral values may be wrong.

In a following research project the hysteretic non-linear behavior of drywall systems will be investigated in more detail. Based on component tests and tests on drywall systems analysis methods will be developed to consider the non-linear load displacement behavior of drywall systems in numerical analyses.

ACKNOWLEDGMENTS The project I-SSB (Integrated Safe & Smart Built) was co-funded by the European Commission within the Sixth Framework Programme, project number IP 026661.

REFERENCES [1] EAK 2000: Greek Code for Seismic Resistance Structures [2] C. Papanicolaou, A. Schuhmacher, Report covering first series of cyclic-

static test on building components, Research Report D5.2.4.1, I-SSB Project, Proj.No. 026661-2, 2008

[3] A. Schuhmacher, Second series of cyclic-static test on building components: Parts 1 and 2 (EMPA), Research Report D5.2.4.2, I-SSB Project, Proj.No. 026661-2, 2009

[4] R. Serrette, K. Ogunfunmi, Shear Resistance of Gypsum-Sheathed Light-Hauge Steel Stud Walls, J. Struct. Eng., Vol. 122, No. 45, 1996

[5] C. Ebert, F.-O. Henkel, Modale Identifikation mit Zeitbereichsverfahren am Beispiel von Rütteltischversuchen einer Stahl-Leichtbaukonstruktion, 12. D-A-CH Conference, Hannover, 2011

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