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IBRACON Structures and Materials Journal • 2013 • vol. 6 • nº 1
Platform for dynamic tests: preliminary studies, design and construction
From modal analyzes (see Table 1) it was possible to realize the
influence of various factors on the natural frequencies of the struc-
ture. Among them it is worth mentioning the compressive strength
of the concrete and the slab thickness. It was noted that slabs with
equal sides presented fundamental frequency lower than others of
the same area but, with a rectangular shape.
These results enabled the development of a more complete model,
with the goal of checking how the rigidity of other structural compo-
nents of the structure (beams and columns) could influence the dy-
namic behavior of the platform. For this model were used SOLID65
elements for the slab and pillars and, SOLID45 for metal beams.
So if the choice was for a model of dimensions 6.1 mx 4.9 m, which
was characterized by a fundamental frequency of 4.48 Hz while
the second reached 7.11 Hz, far enough from the first frequency,
decreasing the possibility of coupling modes.
3.2 Complete model
The slab structural scheme chosen for construction with dimen-
sions and details of the support system are shown in Figure 3.
It was considered an overload for sizing of 1.0 kN / m² , which
results in a smaller dimension than recommended by ABNT NBR
6120:1980 [14] for residential slabs. Yet, that overload seems safe,
due to the use to which the platform will be intended for, therefore,
considering a person with a mass of 80 kg, the slab will be able to
support 0535 people per square meter, or a total of 16 people in
the slab. The requested times were obtained through performed
static analysis. For a safe scaling, the positive requested time was
obtained from the model which considers the simple support in
3. Numerical Analysis
3.1 Initial model
The goal of the analysis is to provide subsidies to build a platform
for dynamic tests that will enable the study of loads induced by
human activities. For this reason, it is important that the platform
presents a natural frequency below 5 Hz while the maximum de-
flection should remain at levels close to those recommended by
the Brazilian standards (ABNT NBR 6118:2007, [13]).
Initially, a numerical study is conducted to determine the dimen-
sions of the platform that meet the specifications of size and dy-
namic characteristics mentioned. The model consists of a rectan-
gular solid concrete slab, clamped on two of their edges while the
other two edges of the support slab are free, as shown in Figure 2.
Not considering the pillars and beams, initially.
Figure 2 – Dynamic tests platform - simplified model
Table 1 – Natural frequencies of preliminary models
f (Resistance
ck
typical of the
concrete
compression)
25 MPa
21 MPa
Slab thickness
7 cm
7 cm
Slab width
10 m
10 m
Slab length
5 m
5 m
Model number
1
(fixed in
length)
2
(fixed in
width)
3
(fixed in all
edges)
4
(simple
supports in
all edges)
5
(simple
supports in
all edges)
Frequency 1
2,29 Hz
9,26 Hz
10,22 Hz
5,13 Hz
4,91 Hz
Frequency 2
3,74 Hz
9,72 Hz
13,23 Hz
8,21 Hz
7,86 Hz
Frequency 3
6,33 Hz
11,44 Hz
18,61 Hz
13,34 Hz
12,77 Hz
Frequency 4
8,57 Hz
14,80 Hz
26,33 Hz
17,45 Hz
16,70 Hz
Frequency 5
11,38 Hz
20,19 Hz
26,59 Hz
20,52 Hz
19,64 Hz