|Client:||HRS Real Estate SA, 5612 Giubiasco|
|represented by:||IM, Ingegneria Maggia, 6601 Locarno|
|Reports:||Nr. 100127, January 27, 2010|
|Fig. 1: Outdoor and Indoor Pool Facilities Lido Locarno. Axes 1 to 12/A-B: Indoor Pools. Axes 14 to 20/A-B: Back office/technical maintenance/fitness. The Dancing is indicated in red.|
|Fig. 2: View on the A-20 building corner in a construction stage (Foto courtesy IM). The building was finished to 95% and operational during the tests described here.|
The construction of the new Locarno Lido Out- and Indoor-Swimming Pool facilities at the shores of Lago Maggiore being practically finished the owner decided to add a dancing/fitness facility on top of the building roof. Fortunately, the consulting engineer in charge, Ingegneria Maggia SA, Locarno, (IM), checked the natural fundamental frequency of the roof slab. With a span of roughly 19 m, this steel/concrete composite structure exhibited a fundamental natural frequency of approximately f = 4 Hz (roof slab vertical bending). This value is significantly lower then the f = 7.5 Hz prescribed by the corresponding Swiss Code for a dance floor. To dispose of the data necessary to reliably design rehabilitative measures if necessary, rci dynamics performed an experimental modal analysis on the structure’s roof slab firstly. The dynamic characteristics of the structure with the main load bearing elements being spanned over 19 m can easily be identified using AVT, Ambient Vibration Testing, technology. The test parameters chosen were:
|sensors:||PCB 393B31 (10 V/g), 13 pcs. (3 references vertically, 1 reference 3D, 7 rovers, vertically, see Figs. 3a and 3b)|
|sampling rate:||sR = 250 Hz|
|time window:||T = 10 Minuten|
|software:||Pimento, ArteMis Extractor (Enhanced Frequency Domain Decomposition, EFDD)|
|Fig. 3a: Measurement point grid for the modal analysis. The building axes 16 to 19 are indicated (see Fig. 1).||Fig. 3b: Layout for setup 1. Blue: references, green: rovers. The rovers were moved to the empty grid locations for setup 2.|
|frequency [Hz]||std.dev. freq. [Hz]||damping [%]||std.dev. damp. [%]|
Fig. 4: Experimental modal analysis results.
Fig. 5: Animated mode shapes. Mode 1: upper left, Mode 2: upper right, Mode 3: lower left: Mode 4: lower right.
Jumping tests involving 2 to 18 people were subsequently performed. Synchronisation of the jumping persons to half of the slab’s natural frequency (f = 5 Hz), f = 2.5 Hz, was attempted using a metronome. Unexpectedly, the respective success was limited: The 18 people group did not succeed in performing in a nicely synchronized jumping. This should have been easier with well audible music as a synchronising means.
Rating of the structural vibration’s velocity (Fig. 6) according to SNV 640’312a and DIN 4150-3 ended in the conclusion that the slab vibrations excited through more than 6 jumping persons were not acceptable. Subsequently, four possible remedial measures were investigated into in detail: a) banning of jumping activities, b) applying a monitoring system cutting off electricity after some vibration level having been surpassed, c) mounting of tuned mass dampers, d) adding of several columns reducing the slab span. After comprehensive discussions measure d) was chosen by the owner. Uploading of the following time signals may take some time:
Fig. 6: Acceleration time signal acquired in measurement point 9 during a jumping test as well as the velocity and displacement signals derived from this.