Journal of Military Technology Vol. 5, No. 1, Jun.

2022

Effect of Different Bracing Systems

on the Performance of Metallic Tower
Cătălin BACIU and Marin LUPOAE

1Abstract—Having multiple destinations, as observation behavior for the entire structure.

facility, industrial bunkers, equipment bearer (for Concentrically Braced Frames (CBF) are the most
telecommunication, measurement instruments, etc.), leisure commonly utilized bracing systems, having a high level of
climbing walls or even for specific training of special forces, lateral stiffness and a low level of ductility. For CBFs to be
metallic towers are designed in various forms and dimensions,
utilized in high seismic regions, special detailing is required
with different bracing systems. Usually these structures have
limited footprint, but with considerable height. The efficiency to ensure that the frames behave properly (in the prescribed
of bracing systems refers to limiting lateral deformation, but manner) [1]. The dissipative elements are limited to braces
also providing sufficient ductility in order to dissipate a larger under tension (Fig. 1a) or braces under tension and
energy quantity induced by severe wind gusts or seismic compression (Fig. 1 b,c,d).
motions. This paper presents a comparative analysis on a steel
tower in unbraced frames solution or with different bracing
systems, under wind and seismic loadings.

Index Terms—bracing systems, link, steel frame structure,

storey drift, vibration mode.

I. INTRODUCTION
Metallic towers are light structures, basically executed in
modules in special factories or workshops, then easily
assembled on sites. The cross-section could be triangular a) b) c) d)
Figure 1. Bracing systems with CBF
(with 3 support pillars) or square (with 4 support pillars) and
could be constant or variable on the vertical; for heights Eccentrically Braced Frames (EBFs) have become a
exceeding 25 meters reducing the horizontal cross-section of widely accepted form of seismic force resisting system in
the cells or using tie rods fixed on the ground are efficient the last four decades. An EBF is a brace frame system in
solutions. which one end of the brace is connected eccentrically to
Rational conformation of metallic towers, with different another brace end or to a frame corner. EBFs successfully
forms and dimensions, depends on the requirements combine the high level of ductility of MRFs and the high
imposed by the building destination, by specific demands level of stiffness of CBFs. The cross brace of an EBF
and by the type of loads and their way of action. Thus, the provides the elastic stiffness of CBF and the eccentricity of
response of a tall metallic structure to wind or seismic loads the cross brace (elements in red color in Fig. 2) creates a
is strongly influenced by the presence and by the type of the link that is responsible for ductility [1].
bracing systems.
Multi-storey bare frame structures (without braces),
named Moment Resisting Frames (MRF) are characterized
by a relatively large number of rigid beam-column joints,
that offer a good structural redundancy and a high capacity
of energy dissipation. Nevertheless, these are flexible
structures and, especially for tall towers, the lack of braces
determines large deformations and efforts on elements.
Introducing a proper bracing system, which works together
with the frames, stiffens the structure and improves the state a) b) c) d)
of stresses and deformations, determining in the end Figure 2. Dissipative bracing systems with EBFs
reducing the steel consumption.
In terms of braces currently used for metallic structures, The length of the beam between two braces or between
there are classical solutions (Fig. 1) and modern, more brace and the frame node is known as a link, which acts like
efficient ones (Fig. 2), which provide a better dissipative a seismic fuse, brace forces being introduced to the frame
through shear and flexure in the link. Depending on the type
of developed plastic mechanism, the dissipative links are
C. BACIU, Department of Constructions, Military Engineering and
Geomatics, Military Technical Academy “Ferdinand I”, Bucharest, classified as follows: short links (shear deformation
Romania (e-mail: catalin.baciu@mta.ro) dissipates energy), long links (bending deformation
M. LUPOAE, Department of Constructions, Military Engineering and dissipates energy) and intermediate links (both shear and
Geomatics, Military Technical Academy “Ferdinand I”, Bucharest, bending deformation dissipates energy) [2].
Romania (e-mail: marin.lupoae@mta.ro)

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The link, vertical or horizontal, representing the main that the collapse was due to failure of link end connection
dissipative part of the bracing system, is in a state of static and the lateral strength achieved by the tested EBs was
equilibrium under axial force – N, shear force – V and significantly larger than the expected one (because of the
bending moment – M (Fig. 3). shear over-strength exhibited by the tested steel links).
A representative paper for Romanian research in the
domain of using of EBFs [6] presents numerical simulation
and experimental tests on planar steel frame, where the
connections beam-column are dog-bone type. Push-over and
Time-History analyses are carried out for both fixed and
removable links. The final results determine the influence of
Figure 3. Static equilibrium of the link eccentric braces and dog-bone connections to the dissipative
response of the analyzed structure. A further study was
For usual cases, the above classification is detailed materialized in the paper [7], where composite steel-
depending on the value of eccentricity, e, as follows [2]: concrete eccentrically braced frames are used in
a) short dissipative element, when experimental and numerical tests. Results show that the
presence of concrete slab decisively influences the initiation
M pl ,link and the development of plastic hinges in steel section.
e < 1.6 ⋅ (1)
V pl ,link
b) long dissipative element, when
M pl ,link
e > 3⋅ (2)
V pl ,link
c) intermediate dissipative element, when
M pl ,link M pl ,link
1.6 ⋅ ≤ e ≤ 3⋅ (3)
V pl ,link V pl ,link

where M pl ,link is the plastic bending moment and V pl ,link

is the plastic shear force, both values corresponding to hinge
formation.
Interesting valuable scientific papers analyze the use of
EBF systems for steel and reinforced concrete (RC) frame
structures. Z. Khan, B. R. Narayana, S. A. Raza [3] studied
the seismic behavior of a RC building equipped with
different bracing systems by performing linear static and
non-linear static analysis, comparing various parametric
results such as Storey drift and Storey forces. Pushover
curves obtained both in X and Y directions emphasized the
contribution to structural rigidity of centrically X-shaped
and V-shaped bracing systems.
A nonlinear pushover analysis is carried out by M. Mubeen,
K. N. Khan, M. I. Khan [4] for high rise steel frame building
with different patterns of eccentric bracing systems,
resulting that the models with bracings have lesser
vulnerability as compared to the frames without bracings.
Compared with steel bare frame model, the eccentrically
bracing systems (Eccentric Backward Brace, Eccentric Figure 4. Metallic tower to be designed (lateral view)
Model A – Moment Resisting Frames
Forward Brace, Eccentric V-Brace and Inverted V-Brace)
reduced the maximum displacements up to 90%, but If all the above papers generally use I-beam and wide
increased the base shear capacity up to 49%. In conclusion, flange beam for the links of eccentric braces, the papers [8],
the Eccentric Inverted V Brace model has increased the [9] describe the development and results of a finite element
structural performance level as compared to other bracing parametric study of eccentrically braced frame links having
models. hollow rectangular cross sections and the experimental
The significant improvement to the seismic response of verification of the proposed design requirements. A finite
RC structures equipped with dissipative bracing systems, element parametric study consisting of over 200 models of
such as eccentric braces (EBs) and buckling restrained EBF links with tubular cross sections has been conducted.
braces (BRBs) is illustrated in the paper of Mazzollani, Results of the parametric study lead to certain
Della Corte and D’Aniello [5]. The experimental tests were recommendations for compactness ratio limits for tubular
carried out on two similar two-storey one-bay RC structures, cross sections used as links in EBFs [8]. In order to verify
respectively equipped with EBs and BRBs. For eccentric proposed design requirements, experimental tests were carried
braces a Y-inverted bracing configuration, with a vertical out [9]. Results indicated that tubular links satisfying the
steel link, was adopted. The experimental test has shown proposed compactness and stiffeness requirements can achieve

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the target plastic rotations for wide-flange links when subjected dynamic factor generated by the specific actions of the
to the loading protocol. trainings;
- wind, seismic, snow and hoarfrost loads.
II. STUDY CASE The metal columns, placed on every corner of the tower,
Starting from a list of certain requirements, a metallic are made of square hollow section - Tp120×6. Perimeter
tower is to be designed with a special destination (training horizontal beams are fixed between the columns, both at the
troops for climbing, abseiling): 12.50 m total height, 3 × 4 m bottom of each section (rectangular pipe Td120×80×6) and
plan dimensions, 4 intermediate platforms and the fifth on at the top of them (square pipe Tp80×6). Between the lower
top. A metallic staircase is used to access the upper floors secondary beams there are arranged support elements
(Fig. 4). (square pipe Tp60×5 at maximum distances of 600 mm) for
In order to facilitate the effective site execution of the the floor grilles.
tower, European profiles and rolled steel pipes were used, in a All the tower elements are made of S235 structural carbon
truss system solution, with columns, beams, braces and access steel. The welds are continuous (corner joints), with the
stairs; the structure is modular, with sections with a maximum thickness equal to at most 0.7 of the minimum thickness of
height of 2.50 m, so that they can be easily transported and the elements to be joined. High-strength M12 pretightening
assembled. On three lateral sides (one long and two short) screws - group 8.8 are used to connect the modules
three-layer sandwich panels are placed: on the long side there (8 screws on each flange), Fig. 6.
will be window openings, one of the short sides is prepared
for climbing, and the opposite side, on the top platform, a
console platform is arranged with a minimum length of
1.70 m for controlled assisted jumps. Metallic grind is placed
on every platform and on stairs (Fig. 5).

Figure 6. Plan views of the current platform (upper view) and of the top
platform (lower view)

The proper solution to connect the modules using flanges

is according to the actual regulations and is based on the
valuable experimental and numerical results presented by Y.
Chen et al. in the paper [10]. Providing the sufficiently
strong ribs, welds, and flange plates, the capacity of the
flange was found to be mainly predominated by the bolt
strength. Both the experimental and the numerical results
show a linear load interaction curve, in terms of the ultimate
capacity.

III. ANALYZED MODELS

The tower was modeled using the automatic calculation
program ETABS, software dedicated to structural analysis,
Figure 5. Plan views of the current platform (upper view) and of the top
platform (lower view) thus determining the efforts in the components, dynamic
characteristics and displacements of the structure.
For the pre-sizing of the tower elements and for the Four different models were analyzed:
subsequent detailed analysis, the various possible scenarios Model A – Moment Resisting Frames, without braces (Fig. 4, 7a);
during the construction life were taken into account. The Model B – frames with cross braces on every side (Fig. 7b);
main types of loads considered in the analyses, according to Model C – frames with chevron (Lambda type) braces on
the regulations in force, are as follows: every side (Fig. 8a);
- self-weight of structural and non-structural elements; Model D – frames with eccentric braces on every side, (Fig. 8b).
- payload (weight of personnel in the phases of execution, All models are designed with linear elements such as
operation and maintenance of the tower); for the operation columns, beams and braces, the last ones being considered
phase, the establishment of the calculation value for the articulated at both ends. For Model D, the length of the link is
personnel weight loads will also take into account the 1.00 m, falling into the category of long dissipative element.

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Gravitational loads are represented by the own weight of Base shear is the estimation of maximum expected lateral
structural and non-structural elements (P), by the payload force which will occur at the base of a structure due to ground
( L = 1 kN m 2 - value established depending on the motion during the earthquake. When seismic base shear is
estimated, specific behavior factor (q), which accounts for the
destination of the tower). Lateral loads are generated by
ability of a structure to dissipate energy, is applied for every
wind (W) and seismic input (E); the action of the wind took
type of structure, according to actual regulation [2]: for bare
into account the provisions of the design code [11], the
frame (Model A) q = 6 ; for frames with cross braces
reference value of the dynamic wind pressure being 0.5 kPa.
The seismic loading was taken into account in accordance (Model B) q = 4 ; for frames with chevron braces (Model C)
with the specifications of the design code [2], with the peak q = 2.5 and for frames with eccentric braces (Model D)
value of the terrain acceleration a g = 0.25 g , and with the q = 6.
control period Tc = 1 s . The response spectrum method was In order to determine the design spectrum for each model
applied to determine the response of the structure for all four starting from elastic spectrum, the critical damping for this
models. For final loading scenarios, snow or hoarfrost loads type of steel structure is ξ = 3% and the correction factor,
are not taken into account because the tower should be according to the design code [2], becomes:
cleared before being used in troop training. 10
= η = 1.118 (4)
5+ξ
The elastic and design spectra used for each type of
model are displayed in Fig. 9. The same value of behavior
factor for Model A and D leads to the same design
spectrum. As expected, a higher energy dissipation capacity
determines a lower base shear value.

a) b)
Figure 7. 3D views of Model A – Moment Resisting Frames (a) and of
Model B – Frames with cross braces (b)
The load combinations adopted in the analysis are:
a) Fundamental Combination (FC): 1.35P+1.5L;
b) Fundamental Combination with Wind pressure on long
Figure 9. Elastic and design spectra for considered models
façade, along Oy axis (FCW1): 1.35P+1.5L+1.5W_long;
c) Fundamental Combination with Wind pressure on short All models are analyzed for each combination, above
facade, along Ox axis (FCW2): 1.35P+1.5L+1.5W_transv; mentioned, in order to determine their response in terms of
d) Special Combination with seismic load along Ox axis displacements / drifts and efforts. Also modal characteristics
(GSX): P+0.3L+E_x; of structures will offer further knowledge of typical
e) Special Combination with seismic load along Oy axis behavior of models.
(GSY): P+0.3L+E_y.
IV. RESULTS AND DISCUSSIONS
Fundamental vibration periods of the models, shown in
Table I – second column, are strongly influenced by
structural stiffness; introducing braces into a bare frame
structure increases the rigidity. The most flexible model is
obviously A, and models B and C have almost the same
fundamental periods. As a consequence, the displacements
and rotations are larger for flexible structures.
TABLE I. FUNDAMENTAL VIBRATION PERIODS
AND BASE SHEAR
Periods Base Shear
Model
[s] [kN]
A 1.26 9 (7%)
B 0.20 21 (12%)
C 0.19 31 (18%)
a) b)
Figure 8. 3D views of Model C – Frames with chevron braces (a) and of D 0.47 17 (10%)
Model D – Frames with eccentric braces (b)

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In Fig. 10, lateral drifts generated by seismic loads for all Comparing the axial forces in columns at the base of the
four models are compared and the first conclusion is that for tower, Table III, also demonstrates that wind loads generate
all four, the maximum drifts are below the admissible value higher structural efforts than seismic loads for all the
(Ultimate State Limit). The reduced mass of the building models. Moreover, adding braces into the frames drastically
generates low values of inertial lateral loads from reduces the bending moments in columns and beams,
earthquake action, meaning low values of shear base and offering the possibility to use lighter structural elements in a
further, limited deformations even for MRF. Examining more efficient manner. Rising prices in recent times requires
Table I – third column, it results that the highest value of the adoption of proper structural configurations that lead to
shear base is obtained by the system with chevron braces, the lowest possible quantities of materials and labor.
which have the lowest value of behavior factor, while the
TABLE III. MAXIMUM AXIAL FORCES, N [KN]
lowest value of shear base corresponds to the bare-frame AND BENDING MOMENTS, M [KN‧M] IN COLUMNS
system. On the third column in the Table I, the percentage in Load Combination
the brackets represents the ratio between the shear base and GSX GXY FCW1 FCW2
the total weight of the building, or in other words the global Model
N M N M N M N M
seismic coefficient; the highest flexibility of MRF also A 47 5 50 5 225 91 149 70
brings the lowest value of this coefficient. B 60 0.7 66 0.6 224 3.7 145 3
C 55 0.3 63 0.4 192 4.7 128 3.4
D 49 0.8 51 1.1 147 13 111 8.5

In Table IV, axial forces in the braces are compared,

resulting once again that higher wind loads determine bigger
efforts in the structural elements (in this case in braces) than
in the case of seismic scenarios. The cross braces are less
loaded, almost half compared to the eccentric braces.
TABLE IV. MAXIMUM AXIAL EFFORTS IN BRACES [KN]
Load Combination
Model GSX GXY FCW1 FCW2
B 12 15 75 48
C 16 20 102 65
Figure 10. Drifts on Ox direction for all models D 14 19 150 80

On the other hand, the shape of the graph is firmly V. CONCLUSION

changing when braces are added to MRF; if for the bare-
frame structure the maximum drift is formed at one-third The results obtained in this study proved that the presence
above the ground and then the values gradually decrease, for of braces improves the response of structures under lateral
the structures with braces, drift distribution is changing, loading, but the proper solution has to be in accordance with
becoming more uniform on the height of the building. the type and level of loads and with the specific
In terms of lateral displacements, the maximum values requirements.
determined at the upper floor of the tower, Table II, Braces increase structural rigidity, reduce displacements
indicates that for bare-frame structure (Model A) wind loads and efforts in elements (columns, beams), but increase the
generate larger deformations than seismic loads, while for level of shear base. The shape of the graph for drifts on the
each model with braces the displacements are quiet similar. height of the tower is firmly changing from MRF to the
Setting the limit value of maximum lateral displacement at systems with braces (it becomes more uniform, with lower
2.5% of building height, representing 310 mm for analyzed values). For this kind of structure, the cross braces are the
tower, the cases of MRF under wind loads are the only ones most efficient.
with exceeded values (highlight cells of Table II). There can The lower mass of the building reduces the influence of
also be seen an important difference between Model A and seismic loads compared to wind loads. Using in this case a
all the other models, the presence of braces offering higher dissipative system, like EBF, tends to be less efficient than
rigidity. We expected Model D to be less efficient in terms for the case of office or residential buildings, for example.
of displacement than Models B and C, knowing that the A further research direction, which completes the image
main advantage of EBF lays in the higher energy dissipation of tower-type structures is to determine the capacity of the
capacity. structure using a Push-over method for a system with an
important mass.
TABLE II. MAXIMUM LATERAL DISPLACEMENTS [MM] Design of the braces, usually long and slender, has to
Load Combination consider their behavior under compression, when the
Model GSX GXY FCW1 FCW2 possible loss of stability reduces the strength capacity. A
A 198 156 530 431 solution for this matter is the usage of Buckling Restrained
B 14 20 14 7 Braces (BRB). Another direction to develop the study
C 8 13 13 7 presented in the paper is to analyze the structural response
D 42 70 73 35 when BRB system is used.

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