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Volume 11, Number 3, March 2001

Structural identification via modal analysis in structural mechanics is gaining popularity in
recent years, despite conceptual difficulties connected with its use. This paper is devoted to illustrate both
the capabilities and the indeterminacy characterizing structural identification problems even in quite simple
instances, as well as the cautions that should be accordingly adopted. In particular, we discuss an application
of an identification technique of variational type, based on the measurement of eigenfrequencies and mode
shapes, to a steel frame with friction joints under various assembling conditions. Experience has suggested,
so as to restrict the indeterminacy frequently affecting identification issues, having resort to all the a
priori acknowledged information on the system, to the symmetry and presence of structural elements with
equal stiffness, to mention one example, and mindfully selecting the parameters to be identified. In addition,
considering that the identification techniques have a local character and correspond to the updating of a
preliminary model of the structure, it is important that the analytical model on the first attempt should be
adequately accurate. Secondly, it has proved determinant to cross the results of the dynamic identification
with tests of other typology, for instance, static tests, so as to fully understand the structural behavior and
avoid the indeterminacy due to the nonuniqueness of the inverse problem.

Key Words
structural identification; experimental modal analysis; steel frame.

A. Morassi, Department of Civil Engineering, University of Udine, Via delle Scienze 208, 33100 Udine, Italy

The impact behavior of a multigirder concrete bridge under single and multiple moving
vehicles is studied based on correlated road surface characteristics. The bridge structure is modeled as
grillage beam system. A 3D nonlinear vehicle model with eleven degrees of freedom is utilized according
to the HS20-44 truck design loading in the American Association of State Highway and Transportation
Officials (AASHTO) specifications. A triangle correlation model is introduced to generate four classes of
longitudinal road surface roughness as multi-correlated random processes along deck transverse direction.
On the basis of a correlation length of approximately half the bridge width, the upper limits of impact factors
obtained under confidence level of 95 percent and side-by-side three-truck loading provide probability-based
evidence for the evaluation of AASHTO specifications. The analytical results indicate that a better
transverse correlation among road surface roughness generally leads to slightly higher impact factors.
Suggestions are made for the routine maintenance of this type of highway bridges.

Key Words
highway bridge; road surface roughness; correlation analysis; dynamic analysis; impact study.

Chunhua Liu and Ton-Lo Wang, Department of Civil and Environmental Engineering, Florida Int

Viscoelastic (VE) dampers have shown to be capable of providing structures with considerable
additional damping to reduce the dynamic response of structures. However, the VE material appears to be
sensitive to the variations in ambient temperature and vibration frequency. To minimize these effects, a
new VE material has been developed. This new material shows less sensitivity to variations in vibration
frequency and temperature. However, it is highly dependent on the shear strain. Experimental studies on
the seismic behavior of a 2/5 scale five-story steel frame with these new VE dampers have been carried
out. Test results show that the structural response can be effectively reduced due to the added stiffness
and damping provided by the new type of VE dampers under both mild and strong earthquake ground
motions. In addition, analytical studies have been carried out to describe the strain-dependent behavior of
the VE damper. The dynamic properties and hysteresis behavior of the dampers can be simulated by a
simple bilinear model based on the equivalent dissipated energy principle proposed in this study.

Key Words
VE damper; hysteresis; energy dissipation; shaking table study.

K.C. Chang and M.H. Tsai, Department of Civil Engineering, National Taiwan University, Taipei, Taiwan 10617, R.O.C.
M.L. Lai, 3M Company, St. Paul, Minnesota, U.S.A.

Columns in multi-storey frames are presently categorised as either braced or unbraced,
usually by means of the stability index criterion, for estimating their effective length ratios by design aids
such as

Key Words
effective length; K-factor; column; multi-storey frame; partial bracing; stability index; fuzzy logic; membership function; degree of bracing.

Devdas Menon, Department of Civil Engineering, Indian Institute of Technology, Madras 600036, India

The purpose of this experimental study is to investigate the damage mechanism due to shear-fatigue
behavior of high-strength reinforced concrete beams under repeated loading. The relationship between
the number of cycles and the deflection or strain, the crack growths and modes of failure with the increase
of number of cycles, fatigue strength, and S-N curve were observed through a fatigue test. Based on the
fatigue test results, high-strength reinforced concrete beams failed at 57-66 percent of static ultimate strength
for 2 million cycles. The fatigue strength at 2 million cycles from S-N curves was shown as about 60
percent of static ultimate strength. Compared to normal-strength reinforced concrete beams, fatigue capacity
of high-strength reinforced concrete beams was similar to or lower than fatigue capacity of normal-strength
reinforced concrete beams. Fatigue capacity of normal-strength reinforced concrete beams improved by over
60 percent.

Key Words
high-strength concrete; repeated loading; fatigue strength; S-N curve; number of cycles; deflection; strain; shear reinforcement.

Kae-Hwan Kwak and Jong-Gun Park, Department of Civil and Environmental Engineering, Won Kwang University, 344-2, Shinyong-Dong, Iksan, Challabuk-Do, Korea

This paper presents the experimetal result on the viscoplastic response and collapse of the
titanium alloy tubes subjected to cyclic bending. Based on the capacity of the bending machine, three
different curvature-rates were used to highlight the viscoplastic behavior of the titanium alloy tubes. The
Curvature-controlled experiments were conducted by the curvature-ovalization measurement apparatus which
was designed by Pan et al. (1998). It can be observed from experimental data that the higher the applied
curvature-rate, the greater is the degree of hardening of titanium alloy tube. However, the higher the
applied curvature-rate, the greater is the degree of ovalization of tube cross-section. Furthermore, due to
the greater degree of the ovalization of tube cross-section for higher curvature-rates under cyclic bending,
the number of cycles to produce buckling is correspondingly reduced. Finally, the theoretical formulation,
proposed by Pan and Her (1998), was modified so that it can be used for simulating the relationship
between the controlled curvature and the number of cycles to produce buckling for titanium alloy tubes
under cyclic bending with different curvature-rates. The theoretical simulation was compared with the
experimental test data. Good agreement between the experimental and theoretical results has been achieved.

Key Words
titanium alloy tube; viscoplastic collapse; cyclic bending; curvature; ovalization.

Kuo-Long Lee, Department of Mechanical Engineering, Far East College, Tainan, Taiwan, R.O.C.
Wen-Fung Pan, Department of Engineering Science, National Cheng Kung University, Tainan, Taiwan, R.O.C.

This paper addresses an efficient modification method that eliminates the undesirable effects of
strains due to various non-conforming modes so that the non-conforming element can pass the patch test
unconditionally. The scheme is incorporated in the element formulation to establish new types of non-conforming
hexahedral elements designated as NHx and NVHx for the regular element and variable node
element, respectively. Non-conforming displacement modes are selectively added to the ordinary
(conforming) element displacement assumptions to improve the bending behavior of the distorted solid
element. To verify the validation of proposed direct modification method and the improvement of element
behavior, several numerical tests are carried out. Test results show that the proposed method is effective
and its applications to non-conforming solid elements guarantee for the element to pass the patch test.

Key Words
direct modification method; non-conforming modes; effective modification method; solid elements.

Chang-Koon Choi, Keun-Young Chung and Tae-Yeol Lee, Department of Civil Engineering, Korea Advanced Institute of Science and Technology, Taejon 305-701, Korea

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