Studiu comparativ Cypecad vs. Etabs - Analiza seismica si design-ul unei cladiri cu 10 etaje CYPE
Estudio comparativo entre CYPECAD Y ETABS software
Pórtico Simple
EXAMPLE
SEISMIC ANALYSIS & DESIGN OF 10 STOREY RC BUILDING
MKS system of units
Comparativestudybetween CYPECAD Y ETABS software
Seismic analysis and design of 10 storey RC building
1. Introduction
The following report is based on a comparison between the software program of CYPE
Ingenieros S.A. and ETABS CSI, using ETABS’ example: ‘Seismic Analysis & Design of 10 storey
RC building (equivalent lateral force)’, which can easily be accessed on Internet.
2. Dimensions
Figure 1. Floor plan Dimensions [metres] (Source: CYPECAD)
Figure 2. Floor height dimensions [metres] (Source: CYPECAD)
Comparativestudybetween CYPECAD Y ETABS software
Seismic analysis and design of 10 storey RC building
3. Materials
The MKS system of units is used (metres, kilograms and seconds). Regarding the design
codes to be applied, ACI 318-08 of the American Concrete Institute has been selected for
CYPECAD and ACI 318-02 for ETABS soft
ware.
Concrete:
280 kg/cm2
4200 [kg/cm2] grade 60 (60000 psi)
Reinforcement steel:
Modulus of Elasticity: 250998 kg/cm2
Density:2.5 t/m3
Poisson’s ratio: 0.2
Fixity at the supports:
CYPECAD, by default, considers nodes fixed at the foundations as having external fixity.
ETABS does not take into account this fixity and must be defined by users. In this case, rigid
soil is assumed to be present and so, all the elements are restrained.
Stiffness of the elements
The stiffness of the elements has been adjusted in both programs so they are the same, even
though, by default, the values provided in each program are different.
Beams:
Moment of Inertia y = 0.0054 m4
Moment of Inertia z = 0.00135 m4
Torsion constant = 0.003708
Cover = 4 cm
Section property modifiers: To be able to work with fissured sections, following
that suggested by ACI, the properties of the sections can be affected by applying
coefficients which multiply the values of the gross section. In this example, only
the t
orsional stiffness of the beam is to be cancelled by reducing it to 1%:
Bending = 1
Torsion = 0.01 Axial = 1
Columns:
Moment of Inertia x = 0.002133 m4
Moment of Inertia y = 0.002133 m4
Torsion constant = 0.003708
Cover = 4 cm
Section property modifiers:
Bending = 1
Torsion =0.16 Axial = 2
Panels:
15 cm thick concrete slab
Comparativestudybetween CYPECAD Y ETABS software
Seismic analysis and design of 10 storey RC building
Stiffness = 1
Walls:
20 cm thick concrete walls
Nodes:
The beam-column and column-beam connection is defined as rigid for both software
programs by applying a coefficient of 1.
The size of the node is considered in both CYPECAD and ETABS. A stiffness factor equal
to 1 is provided in both software programs.
CYPECAD, having defined the floor slabs, automatically defines each linear element as a
rigid diaphragm.
In ETABS, a rigid diaphragm must be assigned at floor level to be able to consider the
general behaviour to act as a rigid body in the plane. Furthermo
re, in ETABS, users must bear in
mind that some of the nodes on the floor may not coincide with the corners of the area elements of
the level. Hence, it is recommended a point rigid diaphragm be assigned as well as an area
diaphragm.
Figure 3. Rigid diaphragm assignation (Source: ETABS)
4. Panels
CYPECAD allows users to define joist floor slabs, waffle slabs and flat slabs. In the case of
flat slabs, the panel is considered to be a rigid diaphragm with two-way spanning loads. The
parameters that have to be specified to define the slab are the depth and reinforcement direction.
Comparativestudybetween CYPECAD Y ETABS software
Seismic analysis and design of 10 storey RC building
Figure 4. Panel manager (Source: CYPECAD)
ETABS allows for users to define the slab sections and cover. The floor slab sections can
consist of one-way spanning or two-way spanning reinforcement. Therefore, to define two-way
spanning ribs, it must only be edited at the beginning of the job when the data of t
he grid is defined.
Figure 5. Building plan grid systems (Source: ETABS)
For this example, a two-way spanning flat slab is selected. Users have to define the
material, thickness and type of behaviour of the element (membrane, plate or shell).
ETABS allows for an area with membrane behaviour be used as an instrument to distribute
loads. This is useful when automatically distributing the loads of a floor slab in one or two
directions to its beams using the tributary area criteria. Therefore, area-type elements are defined as
membranes.
The program carries out a triangular type distribution on each beam, bearing in mind the
areas and loads. The differences in the force distribution ETABS carries out on floor slabs with and
without a reinforcement mesh can be seen in section 4.1.2. Force diagrams of the frame with an
adjacent slab.
Another aspect to be mentioned, which also affects the design, is how the type of behaviour
of the membrane causes ETABS to not take into account these elemen
ts as structural components.
Comparativestudybetween CYPECAD Y ETABS software
Seismic analysis and design of 10 storey RC building
Figure 6. Slab properties panel (Source: ETABS)
4.1. Load distribution comparison in the software programs
The properties of the linear elements defined for the example have been maintained, but the
number of frames has been reduced to analyse the influence of the load distribution on the slab.
4.1.1.
Force diagrams of the frame without an adjacent floor slab
Shown below are the bending moment and shear force diagrams for the self-weight loadcase
of the frame without an adjacent floor.
Figure 7. Simple Frame 3D Model (Source: CYPECAD and ETABS)
Bending moment redistribution: The negative moments are not redistributed.
Single-span frame structure without a floor slab
Comparativestudybetween CYPECAD Y ETABS software
Seismic analysis and design of 10 storey RC building
The bending moments and shear forces obtained in the frame are the same for both
software
programs, as can be seen in Table 11.
Figure 8. Moment and shear forces in the frame without slab due to Self-Weight [metres and tons]
Self-Weight Loadcase
Units
CYPECAD
ETABS
Average
Variance
Deviation
Dev. %
Middle span moment (+)
Tn.m
0,92
0,925
0,92
0
0
0
Extreme face moment(-)
Tn.m
0,85
0,839
0,84
0
0
0
Shear
Tn
1,26
1,26
1,26
0
0
0
Table 1. Moment and shear values obtained in the frame without slab and their statistical comparison
The axial forces in the columns are displayed in Figure 64. If we look at the top of the
column, it can be seen that CYPECAD obtains a smaller axial force than ETABS for the same
shear force diagram.
Comparativestudybetween CYPECAD Y ETABS software
Seismic analysis and design of 10 storey RC building
Figure 9. Axial forces in columns due to Self-Weight [tons]
Self-Weight Loadcase
Units
CYPECAD
ETAB
Average
Variance
Deviation
Dev. %
Axial force (top extreme)
tn
2,76
2,94
2,85
0,0081
0,09
6,32
Axial force (bottom extreme)
tn
3,72
3,90
3,81
0,0081
0,09
5,2
Table 2. Axial values in columns due to Self-Weight and their statistical comparison
CYPECAD, as really occurs in a structure, calculates the axial force in the column due to
the load reaching it from the node and from the corresponding parts of the beams bearing on the
node.
Figure 10. Linear elements considered when computing the load in the column [metres]
(Source:Autocad)
Weight of the node = base x depth x density = 0.40 m x 0.40 m x 0.60 m x 2.5 t/m3 = 0.24 tn.
Weight of the adjacent beams = 2 beams x length x width x depth x density = 2 x 2.8 m x 0.30 m x
0.60 m x 2.5 tn/m3 = 2.52 tn.
Total = weight of the node + weight of the beams = 0.24 tn. + 2.52 tn. = 2.76 tn.
Comparativestudybetween CYPECAD Y ETABS software
Seismic analysis and design of 10 storey RC building
In ETABS, the overlap of non-collinear bar-type elements are taken into account at rigid
ends, as occurs for example, at the connection po
int of the beam with the column. Therefore,
ETABS provides an oversized axial force.
Figure 11. Linear elements considered when computing the load in the column [metres] (ETABS)
Weight of the node + weight of the beams = 2.76 tn.
Weight of the beams inside the node = 2 beams x length x width x depth x density= 2 x 0.2 m x 0.3
m x 0.60 m x 2.5 tn/m3 = 0.18tn.
Total = weight of the node + weight of the beams inside the node = 2.76 tn. + 0.18 tn. = 2.94tn
Three-span frame structure without floor slabs
From the figures and tables below, it can be seen that CYPECAD obtains slightly smaller
moments and shear forces than ETABS for the longitudinal beam (beam 1), whilst the forces in the
transverse beams (beams 2 and 3) are the same with both software programs.
Figure 12. Moments and shear forces in three-span frame structure without slabs due to Self-Weight
[metres and tons] (Source: CYPECAD)
Comparativestudybetween CYPECAD Y ETABS software
Seismic analysis and design of 10 storey RC bu
ilding
Figure 13. Moments in three-span frame structure without slabs due to Self-Weight [metres and tons]
(Source: ETABS)
Figure 14. Shear forces in three-span frame structure without slabs due to Self-Weight [metres and
tons] (Source: ETABS)
x=0,2
x=3
x=6
x=9
X=12
x=15
x=17,8
CYPECAD
-0.68
0.75
-1.42
0.54
-1.42
0.75
-0.68
ETABS
-0.702
0.794
-1.499
0.647
-1.378
0.794
-0.702
CYPECAD
1.14
-0.12
(-1.38) (1.26)
0.00
(-1.26)(1.38)
0.07
-1.14
ETABS
1.17
-0.09
(-1.35)(1.26)
0.00
(-1.26)(1.35)
0.09
-1.17
Beam 1
Flexion
Shear
Table 3. Moment and shear values in three-span frame structure without slabs due to Self-Weight.
Beam 1.
Beam 3
x=0,2
x=3
x=5,8
CYPECAD
-0.85
0.92
-0.85
Beam 2
Flexion
Shear
ETABS
-0.839
0.925
-0.839
CYPECAD
1.26
0.00
-1.26
ETABS
1.26
0.00
Beam2
-1.26
Beam 1
Table 4. Moment and shear values in three-span frame structure
without slabs due to Self-Weight. Beam 2.
x=0,2
x=3
x=5,8
CYPECAD
ETABS
-0.85
-0.839
0.92
0.925
-0.85
-0.839
CYPECAD
1.26
0.00
-1.26
ETABS
1.26
0.00
-1.26
Beam 3
Flexion
Shear
Table 5. Moment and shear values in three-span frame structure without slabs due to Self-Weight.
Beam 3.
Comparativestudybetween CYPECAD Y ETABS software
Seismic analysis and design of 10 storey RC building
4.1.2.
Force diagrams of the frame with an adjacent floor slab
Below are the force diagrams obtained when a 15 cm thick concrete slab is introduced.
Figure 15. Slab properties (Source: CYPECAD)
Figure 16. Slab properties (Source : ETABS)
CYPECAD automatically discretises the float slab into a bar-type element mesh with a
maximum bar size of 25 cm. Hence, for a length of 600cm, it divides the slab into a 24 x 24 grid.
In ETABS the discretisation is carried out using the mesh areas command.For this example,
even though ETABS has a very similar mesh to CYPECAD, ETABS requires a longer period of
time to process it than CYPECAD.
Figure 17. Slab discretisation (Source: ETABS)
Sin
gle-span frame structure with a floor slab
The distribution sensitivity of the slab loads on the beams and columns is very high
especially at corners where the shear forces are large. For this reason, it is important to take into
account the position of the meshing distribution to ensure a fair comparison between the two
software programs.
Comparativestudybetween CYPECAD Y ETABS software
Seismic analysis and design of 10 storey RC building
Figure 18. Slab meshing distribution (Source: CYPECAD and ETABS)
Figure 74 shows that CYPECAD obtains greater negative moments at the supports
than ETABS, whilst ETABS has greater positive moments and shear forces.
Figure 19. Moment and shear forces in the frame with slab due to Self-Weight [metres and tons]
Self-Weight
Units
CYPECAD
ETABS
Average
Variance
Deviation
Dev. %
Comparativestudybetween CYPECAD Y ETABS software
Seismic analysis and design of 10 storey RC building
Middle span moment (+)
Tn.m
2,54
2,88
2,71
0,0289
0,17
6
,27
Extreme face moment (-)
Tn.m
2,40
2,03
2,215
0,034225
0,185
8,35
Shear
Tn.
2,78
2,95
2,865
0,007225
0,085
2,97
Table 6. Moment and shear values obtained in the frame with slab and their statistical comparison
The axial forces in the columns are displayed in Figure 75.
Figure 20. Axial forces in columns due to Self-Weight [tons]
Self-Weight Loadcase
Units
CYPECAD
ETAB
Average
Variance
Deviation
Dev. %
Axial force (top extreme)
tn
5,81
6,32
6,065
0,130
0,360
5,946
Table 7. Axial values in columns due to Self-Weight and their statistical comparison
As explained using Figures 65 and 66, the difference in the axial load due to the selfweight is because ETABS considers the linear elements inside the nodes.
Weight of the beams inside the node = 2beams x length x width x depth x density = 2 x 0.2 m x 0.3
m x 0.60 m x 2.5 tn/m3 = 0.18 tn.
Difference in axial force between the software programs = 6.32 – 5.81 =0.51 tn.
There is still, however, a differenc
e in weight: 0.51 tn – 0.18tn = 0.33 tn.
This is due to how each program considers the weight of the floor slab.CYPECAD carries
out the analysis by taking into account the slab within the edges of the beams and columns, as can
be observed in Figure 76.
Comparativestudybetween CYPECAD Y ETABS software
Seismic analysis and design of 10 storey RC building
Figure 21. Area considered within the perimeter (Source: CYPECAD)
ETABS partly lays the floor slab over the beams and columns, depending on how users draw
the floor slab in the software program.
Figure 22. Area considered area around the perimeter (Source: Autocad)
The weight due to the overlap of the slab that acts on a column is:
[1 beam (5.6 m long x 0.15 m wide x 0.15 m deep) + 1 node (0.2 m wide x 0.2 m long x
0.15mdeep)] x 2.5 tn/m3 = 0.33 tn.
This explains the difference in weight of the floor slab. The total weight of the floor slab overlap
along its four sides is:
[4 beams (5.6 m long x 0.15 m wide x 0.15 m deep) + 4 nod
es (0.2 m wide x 0.2 m long x
0.15mdeep)] x 2.5 tn/m3 = 1.32 tn.
Comparativestudybetween CYPECAD Y ETABS software
Seismic analysis and design of 10 storey RC building
Three-span frame structure with floor slabs
Two more frames are added to the previous structure so the load distribution and force
diagrams along several spans can be observed. In this case, there is agreat difference in the bending
moment and shear force diagrams in ETABS depending on whether the floor slab is meshed or not.
The values for beam 1 for CYPECAD are a bit smaller than those obtained for ETABS, whilst the
moment and shear distribution in beams 2 and 3 are similar. This might be caused by the difference
in how these two softwares construct their model. Etabs is overlapping the beams inside the node
and the slab inside beams untill axis. This results in a higher mass and a slightly different stress
distribution. Cype considers the slab inide the faces of the perimetral beams. Beams are not
considered inside
the node. This result in a better approximation of the real structure and the mass
is closer to the real mass.
Figure 23. Moments and shear forces in three-span frame structure with slabs due to Self-Weight
[metres and tons] (Source: CYPECAD)
Figure 24. Moments in three-span frame structure, with slabs, not meshed, due to Self-Weight [metres
and tons] (Source: ETABS)
Comparativestudybetween CYPECAD Y ETABS software
Seismic analysis and design of 10 storey RC building
Figure 25. Shear forces in three-span frame structure, with slabs, not meshed, due to Self-Weight
[metres and tons] (Source: ETABS)
Figure 26. Moments in three-span frame structure, with slabs, meshed, due to Self-Weight [metres and
tons] (Source: ETABS)
Figure 27. Shear forces in three-span frame structure, with slabs, meshed, due to Self-Weight [metres
and tons] (Source: ETABS)
Beam 1
x=0,2
x=3
x=6
x=9
x=12
x=15
x=17,8
-2.85
1.87
-1.78
-1,78
1,87
-2.85
1.08
Moments ETABS (not meshed)
-1,638
2,035
-3,202
1,287
-3,02
2,22
-1,638
ETABS (meshed)
CYPECAD
Shear ETABS (not meshed)
ETABS (meshed)
-1,646
2,31
-2,62
-2,80
1,989
0,29
-0,02
0,26
-3,118
(-2.22) (1.77)
(2,95)(-2,50)
(2,60)(-2,10)
1,236
0.00
0.00
0.00
-3,118
(-1.77)(2.22)
(2,50)(-2,95)
(2,10)(-2,60)
1,983
0.15
-0,73
-0,43
-1,646
-2.31
2.62
2,80
CYPECAD
Table 8. Moment and shear values in three-span frame structure with slabs due to Self-Weight. Beam
1.
x=0,2
x=3
x=5,8
-2.26
2.40
-2.26
Moments ETABS (not meshed)
-2,14
2,57
-2,14
ETABS (meshed)
CYPECAD
ETABS (not meshed)
-2,148
2.71
3,01
2,524
0.00
0.00
-2,148
-2.71
-3,01
Beam 2
CYPECAD
Shear
Beam 2
Beam 3
Comparativestudybetween CYPECAD Y ETABS software
Seismic analysis and design of 10 storey RC building
ETABS (meshed)
3,23
0,00
-3,23
Beam 1
Table 9. Moment and shear values in three-span frame structure
with slabs due to Self-Weight. Beam 2.
Beam 3
X=0,2
x=3
x=5,8
CYPECAD
Moments ETABS (not meshed)
ETABS (meshed)
-4.28
-4,02
-4,105
4.61
4,72
4,64
-4.28
-4,02
-4,105
CYPECAD
ETABS (not meshed)
6.50
5,60
0.00
0.00
-6.50
-5.60
ETABS (meshed)
7,05
0.00
-7,05
Shear
Table 10. Moment and shear values in three-span frame structure with slabs due to Self-Weight. Beam
3.
5. Forces in the 10 storey building
5.1. Basic loadcase: Self-weight, without floor slabs
10thfloor
1stfloor
The values obtained for the bending moments and shear forces in the first floor and last floor
in CYPECAD are slightly less than those obtained in ETABS.
x=0,2
x=3
x=6
x=9
X=12
x=15
x=17,8
CYPECAD
-1.03
0.73
-1.22
0.61
-1.21
0.73
-1.04
ETABS
-1.14
0.76
-1.09
0.71
-1.30
0.76
-1.14
CYPECAD
1.26
0.02
(-1.26) (1.26)
0.00
(-1.26)(1.26)
-0.03
-1.26
ETABS
1.31
0.09
(-1.21)(1.26)
0.00
(-1.26)(1.21)
-0.08
-1.31
10th floor beam
Moments
Shear
Table 11. Moment and shear values in the 10storey building without slabs due to Self-Weight.10th
floor.
x=0,25
x=3
x=6
x=9
X=12
x=15
x=17,750
CYPECAD
-0.86
0.71
-1. 35
0.57
-1.35
0.71
-0.86
ETABS
-0.92
0.75
-1.35
0.67
-1.35
0.75
-0.908
CYPECAD
1.19
-0.03
(-1.33) (1.26)
0.00
(-1.26)(1.33)
0.04
-1.19
ETABS
1.23
-0.02
(-1.29)(1.26)
0.00
(-1.26)(1.29)
-0.01
-1.23
1st floor beam
Moments
Shear
Table 12. Moment and shear values in the 10 storey building without slabs due to Self-Weight.1st floor.
Given the hyperstaticity of the structure and that the shear forces differ slightly in each
software program for the same structure, the way in which the loads are distributed to the columns
Comparativestudybetween CYPECAD Y ETABS software
Seismic analysis and design of 10 storey RC building
is not the same in CYPECAD and ETABS. This can be seen in Figure 83, where the axial force at
the top of the selected column is greater than the value that should be obtained in accordance with
what has been discussed and displayed in Table 12 of this report.
Figure 28. Axial forces in columns due to Self-Weight [tons]
Self-Weight
Loadcase
Axial force (top)
Axial force (top) (From
Table 2)
Units
tn
CYPECAD
2,76
ETAB
3,05
Average
Variance
Deviation
Dev. %
tn
2,76
2,94
2,85
0,0081
0,09
6,32
Table 13. Axial values in columns due to Self-Weight and their statistical comparison
Therefore the axial force due to the self-weight and the total sum of all the columns is
analysed (the weight of the walls for the lift shaft is omitted and the weight of the floor slab is
considered in this area) and the difference is found to be due to how ETABS considers the linear
elements lying within the nodes.
Comparativestudybetween CYPECAD Y ETABS software
Seismic analysis and design of 10 storey RC building
.
Figure 29. Column forces. Self-Weight loadcase. Building without slabs (Source: CYPECAD)
Figure 30. Column forces.Self-Weight loadcase. Building without slabs(Source: ETABS)
Self-Weight Loadcase
Axial force (top extreme)
Units
tn
CYPECAD
784,48
ETAB Difference
826,6
42,12
Table 14. Total axial force due
to Self-Weight in columns and the difference observed between both
software programs
In this case, the weight increases considerably due to how ETABS considers the linear
elements inside nodes and because of the numerous beam-column connections that are present.
Type of beamcolumn
connection
Nº of beams per
connection
Number of connections
7 storeys x 4 connec. + 3 storeys x 5
43
connec
7 storeys x 8 connec. + 3 storeys x 6
3 beams-column
3
74
connec.
7 storeys x 4 connec. + 3 storeys x 4
4 beams-column
4
40
connec.
Total of beams connected with columns in whole building
Table 15. Procedure to work out the number of beams connected to columns
2 beams-column
2
Nº of beams
per type of
connection
86
222
160
468
Weight of 1 beam inside a node = 1 beam x length x width x depth x density = 1 x 0.2 m x 0.3 m x
0.60 m x 2.5 tn/m3 = 0.09 tn
Therefore, 468 beams connected to nodes x 0.09 tn = 42.12 tn.
Comparativestudybetween CYPECAD Y ETABS software
Seismic analysis and design of 10
storey RC building
5.2. Basic loadcase: Self-weight, with floor slabs
10th floor beam
1st floor beam
As occurs in the structure without floor slabs in section5.1. Basic loadcase: Self-weight,
without floor slabs, the bending moment and shear force values in CYPECAD are slightly smaller
than in ETABS.
Figure 31. Moment and shear forces in the 10 storey building with meshed slabs due to Self-Weight. 1st
floor. [metres and tons] (Source: CYPECAD)
Comparativestudybetween CYPECAD Y ETABS software
Seismic analysis and design of 10 storey RC building
Figure 32. Moments in the 10 storey building with meshed slabs due to Self-Weight. 1st floor. [metres
and tons] (Source: ETABS)
x=0,2
x=3
x=6
x=9
x=12
x=15
x=17,8
CYPECAD
-3.03
1.73
-2.07
1.40
-2.07
1.76
-2.82
ETABS
-3.04
2.33
-2.65
2.13
-3.42
2.32
-3.17
CYPECAD
2.74
0.12
(-1.66) (1.76)
0.00
(-1.77)(1.66)
-0.13
-2.71
ETABS
3.14
0.4
(-2.79)(2.95)
0.00
(-2.98)(2.75)
0.01
-3.18
10th floor Beam
Moments
Shear
Table 16. Moment and shear values in the 10 storey building with meshed slabs due to Self-Weight.
10th floor.
1st floor Beam
Moments
Shear
x=0,2
x=3
x=6
x=9
x=12
x=15
x=17,8
CYPECAD
-2.44
1.71
-2.58
1.19
-2.58
1.70
-2.20
ETABS
-2.29
2.29
-3.55
1.95
-3.53
2.29
-2.26
CYPECAD
2.43
-0.13
(-2.00) (1.78)
0.00
(-1.77)(2.01)
0.14
-2.42
ETABS
2.86
-0.11
(-2.93)(2.96)
0.00
(-2.93)(2.96)
0.35
-2.85
Table 17.Moment and shear values in the 10 storey building with meshed slabs due to Self-Weight. 1st
floor.
The axial forces in the columns are displayed in Figure 88 (the weight of the lift shaft has
been omitted in both software programs in order to simplify the example).
Comparativestudybetween CYPECAD Y ETABS software
Seismic analysis and design of 10 storey RC building
Figure 33. Column forces. Self-Weight loadcase. Building with slabs (Source: CYPECAD)
Figure 34. Column forces. Self-Weight loadcase. Building with slabs (Source: ETABS)
Self-Wei
ght Loadcase
Axial force (bottom)
Units
CYPECAD
tn
1844,1
ETAB Difference
2001
156,9
Table 18. Axial values in columns due to Self-Weight
In this case, the load considered by ETABS is even more than before due to the weight of
the linear elements inside the nodes and also because of the floor slab overlaps with beams
and columns.
Weight due to linear elements inside the nodes (from section 5.1 Basic loadcase: SelfWeight, without floor slabs) is equal to 42.12 tn.
The load due to the overlap of the floor slab on the beams is equal to 1.32 tn (see Figure 77). The
number of slabs in this structure is: 7 floors x 9 slabs + 3 floors x 8 slabs = 87 slabs.
Therefore, the weight due to the overlap of the floor slabs for the whole structure:
87 slabs x 1.32 tn= 114.84 tn.
A check is carried out to verify whether the sum of extra weight is equal to the difference obtained
in Table 28.
Weight of the linear elements inside nodes + weight due to floor slab overlaps = 42.12 + 114.84
=156.96
tn.
Comparativestudybetween CYPECAD Y ETABS software
Seismic analysis and design of 10 storey RC building
Time required to analyse the structure
CYPECAD analyses this structure with a 24 x 24 mesh for each floor slab in 12 minutes and
32 seconds.
ETABS, for a 24 x 24 mesh for each floor slab, requires 28 minutes to analyse the structure.
5.3. Concrete walls
CYPECAD performs a thick shell, three-dimensional finite element discretisation of
concrete walls, taking into account deformation due to shear. The finite elements consist of six
nodes; one at each vertex and one at the mid-point of the sides. These create a triangular mesh
which automatically adjusts (becomes finer in order to include supplementary nodes) in area where
the wall interacts with other elements (plates, beams, a.o.)
Figure 35. Wall discretisation made up of triangular finite elements (Source: CYPECAD)
ETABS discretises the wall portions (two-dimensional) as if they were bar-type elements
(one-dimensional) and i
dentifies the portions as pier wall or spandrel wall elements.
Comparativestudybetween CYPECAD Y ETABS software
Seismic analysis and design of 10 storey RC building
Figure 36. Wall discretisation made up of pier elements (Source: ETABS)
When a Piers tag is assigned to a shell, it implies the forces (axial, shear and moment) in it
are to act as in a wall, i.e. in its top and bottom sections. It is a way of telling ETABS how the
forces are to be integrated. When a Spandrel tag is assigned to a shell, the forces (axial shear and
moment) in it are to act as in a beam, i.e. in its left and right sections. For this example, the wall is
discretised in 2 x 4 Pier elements per floor.
Figure 37. Pier and spandrel forces (Source: ETABS)
5.3.1.
Axial forces
Comparativestudybetween CYPECAD Y ETABS software
Seismic analysis and design of 10 storey RC building
Figure 38. Axial forces in the shear wall due to Self-Weight [tons] (Source: CYPECAD)
Figure 39. Axial forces in the shear wall due
to Self-Weight [tons] (Source: ETABS)
Seismic loads
Units
CYPECAD
ETABS
Average
Variance
Deviation
Dev. %
tn.
-14,87
-16,80
15,835
1,862
1,365
8,618
Table 19. Axial force due to seismic loads in CYPECAD and ETABS and their statistical comparison
Comparativestudybetween CYPECAD Y ETABS software
Seismic analysis and design of 10 storey RC building
6. Dynamic analysis (seismic load)
6.1. Modal spectral analysis using generic method
6.1.1.
Design acceleration spectrum
The following acceleration spectrum is defined
Period
Acceleration
0.005
0.074
0.143
0.212
0.281
0.350
0.419
0.488
0.557
0.626
0.695
0.764
0.833
0.902
0.971
1.040
1.109
1.178
1.247
1.316
0.000
2.350
3.501
3.289
2.882
2.505
2.190
1.933
1.722
1.547
1.401
1.277
1.171
1.080
1.001
0.932
0.870
0.816
0.767
0.724
1.730
1.799
1.868
1.937
2.006
2.075
2.144
2.213
2.282
2.351
2.420
2.489
2.558
2.627
2.696
2.765
2.834
2.903
2.972
3.041
0.535
0.512
0.490
0.471
0.452
0.435
0.419
0.404
0.390
0.377
0.365
0.353
0.342
0
.332
0.322
0.312
0.304
0.295
0.287
0.280
3.455
3.524
3.593
3.662
3.731
3.800
3.869
3.938
4.007
4.076
4.145
4.214
4.283
4.352
4.421
4.490
4.559
4.628
4.697
4.766
0.241
0.235
0.230
0.225
0.220
0.215
0.211
0.206
0.202
0.198
0.194
0.190
0.187
0.183
0.180
0.176
0.173
0.170
0.167
0.164
5.249
5.318
5.387
5.456
5.525
5.594
5.663
5.732
5.801
5.870
5.939
6.008
6.077
6.146
6.215
6.284
6.353
6.422
6.491
6.560
0.146
0.144
0.142
0.140
0.138
0.136
0.134
0.132
0.130
0.128
0.126
0.125
0.123
0.121
0.120
0.118
0.117
0.115
0.114
0.112
Table 20. Acceleration Spectrum Data. R.S.A. Type I Soil I Damping 10%
6.1.2.
Ductility, acceleration and participation modes
Due to the different way the ductility is interpreted by the software programs, as explained in
the Simple Frame Example, a value of 1 is assigned to the ductility to avoid it affecting the
analyses.
The basic seismic acceleration, interpreted as a fraction of g in CYPECAD, is equal to 0.10
(g) m/s2. In ETABS, the value of the acceleration is
standardised and so is expressed as if it had no
units: 0.10 x 9.81 = 0.981.
Regarding the number of modes with significant contribution, CYPECAD defines them in
two ways: automatically by considering superposed modes whose mass totals are greater than 90%
of the mobilised mass in the seismic movement, or manually by users. In ETABS, users must
consider the adequate number of modes. In this case, the number of modes has been calculated
automatically in CYPECAD (the result has been 4 modes), and has then been extrapolated to
ETABS.
Comparativestudybetween CYPECAD Y ETABS software
Seismic analysis and design of 10 storey RC building
Figure 40. Automatic option for calculating the modes (Source: CYPECAD)
6.1.3.
Seismic mass
So to obtain the vibration modes, both software programs create the mass matrix and
stiffness matrix for each element of the structure. The stiffness matrix has already been checked in
section 3. Materials and, upon observing the inertias, is the same for both so
ftware programs.
However, the mass matrix that is created based on the self-weight loadcase and the corresponding
live loads (in this case non-existent), are different due to the different way in which the self-weight
is considered, as can be seen in Figures 65 and 66 and Figures 76 and 77.
Consequently, the period and accelerations that are obtained from the spectrum
curve, as well as the seismic forces which are directly proportional to the weight of the
corresponding mass, will be affected.
6.2. Seismic results for the structure with floor slabs and without a lift shaft
Based on the vibration modes, the program obtains the periods, participation coefficients for
each direction, accelerations and percentage of displaced mass.
As expected, due to the greater amount of seismic mass considered by ETABS, the periods
and accelerations are affected.
Comparativestudybetween CYPECAD Y ETABS software
Seismic analysis and design of 10 storey RC building
Figure 41. Period, mass, accelerati
on and displacement for each mode (Source: CYPECAD)
T:
Lx, Ly:
Lgz:
Mx, My:
R:
A:
D:
Vibration period in seconds.
Normalised participation coefficients in each direction of the analysis.
Normalised participation coefficient corresponding to the degree of rotational freedom.
Displaced mass percentage for each mode in each direction of the analysis.
Ratio between the design acceleration using the assigned ductility and the design
acceleration obtained without ductility.
Design acceleration, including ductility.
Mode coefficient. Equivalent to the maximum displacement of the degree of dynamic
freedom.
Figure 42. Period and mass participation for each mode (Source: ETABS)
Figure 43. Acceleration for each mode (Source: ETABS)
Comparativestudybetween CYPECAD Y ETABS software
Seismic analysis and design of 10 storey RC building
Seismic
Units CYPECAD
loads
Period
Sx
s
1,173
Participating
Sx
%
43,89
Mass ratio
Spectrum
Sx m/s2
0,804
Acceleration
ETABS
Average
Variance
Deviation
Dev.
%
1,175
1,174
0,000002
0,001414
0,12
42,89
43,39
0,5
0,707107
1,63
0,802
0,803
0,000002
0,001414
0,12
Table 21.Period, participating mass ratio and spectrum acceleration due to seismic loads.
6.2.1.
Internal forces in the columns due to seismic loads
As the seismic loads are greater in ETABS due to how it considers more seismic mass, the
internal forces in the columns are also greater.
Detailed column
Figure 44. Axial force due to seismic loads. Vibration mode 1
Seismic loads
Units
CYPECAD
ETABS
Average
Variance
Deviation
Dev. %
Axil Force
tn.
34,12
34,10
34,11
0,0002
0,014142
0,04
Table 22. Axial force due to seismic loads in CYPECAD and ETABS and their statistical comparison
Comparativestudybetween CYPECAD Y ETABS software
Seismic analysis and design of 10 storey RC building
6.2.2.
Reactions due to seismic load
The reactions, just as the internal forces in the columns, are also greater in ETABS.
Figure 45. Reactions due to seismic loads in
column [tons and metres] (Source: CYPECAD)
Figure 46. Reactions due to seismic loads in column [tons and metres] (Source: ETABS)
Comparativestudybetween CYPECAD Y ETABS software
Seismic analysis and design of 10 storey RC building
Figure 47. Comparison of the reactions due to seismic loads in CYPECAD in red and ETABS in blue
[tons and metres]
6.3. Seismic results for the structure with floor slabs and lift shaft
Based on the vibration modes, the program obtains the periods, participation coefficients for
each direction, accelerations and percentage of displaced mass.
Figure 48. Period, mass, acceleration and displacement for each mode (Source: CYPECAD)
T:
Lx, Ly:
Lgz:
Mx, My:
R:
A:
Vibration period in seconds.
Normalised participation coefficients in each direction of the analysis.
Normalised participation coefficient corresponding to the degree of rotational freedom.
Displaced mass percentage for each mode in each direction of the analysis.
Ratio between the design acceleration
column [tons and metres] (Source: CYPECAD)
Figure 46. Reactions due to seismic loads in column [tons and metres] (Source: ETABS)
Comparativestudybetween CYPECAD Y ETABS software
Seismic analysis and design of 10 storey RC building
Figure 47. Comparison of the reactions due to seismic loads in CYPECAD in red and ETABS in blue
[tons and metres]
6.3. Seismic results for the structure with floor slabs and lift shaft
Based on the vibration modes, the program obtains the periods, participation coefficients for
each direction, accelerations and percentage of displaced mass.
Figure 48. Period, mass, acceleration and displacement for each mode (Source: CYPECAD)
T:
Lx, Ly:
Lgz:
Mx, My:
R:
A:
Vibration period in seconds.
Normalised participation coefficients in each direction of the analysis.
Normalised participation coefficient corresponding to the degree of rotational freedom.
Displaced mass percentage for each mode in each direction of the analysis.
Ratio between the design acceleration
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