KEY PROJECTS
10. Quantifying the Risk Posed to Tall Steel Frame Buildings in Southern California from Earthquakes on the San Andreas Fault
9. On the Elastic and Inelastic, Critical and Post-Buckling Behavior of Slender Columns and Braces
8. On the Mechanism of Collapse of Tall Steel Moment Frame Buildings
7. Tall Steel Building Response to the SHAKEOUT Scenario Earthquake
5. 1857-like San Andreas Earthquake End-to-End Simulation
3. Behavior of Tall Steel Moment Frame Buildings Under Near-Source Ground Motion (PhD Thesis)
2. System Identification of Dynamic Struct. Systems Using Continuous-Time Domain Methods (MS Thesis)
1. Long-Span Cable-Supported Bridges (B.Tech Thesis)
10. Quantifying the Risk Posed to Tall Steel Frame Buildings in Southern California from Earthquakes on the San Andreas Fault
Funding Source: National Science Foundation (NSF)
The recently released Uniform California Earthquake Rupture Forecast by the Working Group on California Earthquake Probabilities concludes that there is a high probability that an earthquake of magnitude 6 or larger will occur on the southern section of the San Andreas fault in the next 30 years. The research objective of this award is to do a region-wide quantitative risk analysis of six steel buildings in the 20-story class for the hazard posed by the San Andreas fault. For this, end-to-end computer simulations of ground motion from 36 earthquakes and damage to the six buildings will be carried out. The earthquakes vary in magnitudes from 6.0 to 8.0 and originate at three hypocenter locations on the fault. Two rupture propagation directions will be considered. The Los Angeles metropolitan region will be divided into 636 analysis sites on a uniform grid of about 3.5km spacing. Three steel moment frame and three steel braced frame building models will be analyzed at each site for damage caused by the 3-component motion of the 36 scenario earthquakes. The results will be probabilistically analyzed to quantify the expected annualized economic loss for each building at each of the sites. Remedial measures that prevent collapse of the buildings will be investigated. The study will resolve long-standing questions related to failure modes, degradation characteristics, and collapse mechanisms of steel moment frame and braced frame buildings. This will also provide a quantitative measure of the economic risk due to the seismic activity on the San Andreas fault to the hundreds of high-rise buildings that exist in Southern California.
GROUP MEMBERS
Ramses Mourhatch, Graduate Student; Hemanth Siriki, Graduate Student
COLLABORATORS
Dr. James L. Beck, Caltech
Dr. Chen Ji, University of California, Santa Barbara
Dr. Hiroo Kanamori, Caltech
Dr. Dimitri Komatitsch, University of Pau, France
Dr. Marco Stupazzini, Politecnico di Milano
Dr. Jeroen Tromp, Princeton University
9. On the Elastic and Inelastic, Critical and Post-Buckling Behavior of Slender Columns and Braces
Funding Source: Southern California Earthquake Center (SCEC)
An efficient beam element, the modified elastofiber (MEF) element, has been developed to capture the overall features of the elastic and inelastic response of slender columns and braces under axial cyclic loading without unduly heavy discretization. In the first half of this two-part paper, MEF element theory and calibration are presented. The ability of the element to capture elastic buckling of columns is illustrated. The MEF element consists of three fiber segments, two at the member ends and one at mid-span, with two elastic segments sandwiched in between. The segments are demarcated by two exterior nodes and four interior nodes. The fiber segments are divided into 20 fibers in the cross-section that run the length of the segment. The fibers exhibit nonlinear axial stress-strain behavior akin to that observed in a standard tension test of a rod in the laboratory, with a linear elastic portion, a yield plateau, and a strain hardening portion consisting of a segment of an ellipse. All the control points on the stress-strain law are user-defined. The elastic buckling of a member is tracked by updating both exterior and interior nodal coordinates at each iteration of a time step, and checking force equilibrium in the updated configuration. Inelastic post-buckling response is captured by fiber yielding, fracturing, and/or rupturing in the nonlinear segments. The element is calibrated to accurately predict the Euler critical buckling load of box and I-sections with a wide range of slenderness ratios (L/r = 40, 80, 120, 160, and 200) and support conditions (pinned-pinned, pinned-fixed, and fixed-fixed). Elastic post-buckling of the Koiter-Roorda L-frame (tubes and I-sections) with various member slenderness ratios (L/r = 40, 80, 120, 160, and 200) is simulated and shown to compare well against second-order analytical approximations to the solution, even when using a single MEF element to model each leg of the frame. the inelastic behavior of struts under cyclic loading observed in the Black et al., Fell et al., and Tremblay et al. experiments is shown to be accurately captured by single MEF element models. A FRAME3D model (using MEF elements for braces) of a full-scale 6-story braced frame structure that was pseudodynamically tested by the Building Research Institute of Japan subjected to the 1978 Miyagi-Ken-Oki earthquake record, is analyzed and shown to closely mimic the experimentally observed behavior. To aid in the evaluation of the collapse-prediction capability of competing methodologies, a benchmark problem of a water-tank subjected to the Takatori nearsource record from the 1995 Kobe earthquake, scaled down by a factor of 0.32, is proposed. The water-tank is so configured as to have a unique collapse mechanism (under all forms of ground motion), of overturning due to P−D instability resulting from column and brace buckling at the base.
REFERENCES
Krishnan, S., Submitted. "The Modified Elastofiber Element for Slender Column and Brace Modeling: I. Theory and Calibration", vol. XX(Y), ZZZZ, Journal of Structural Engineering. [PDF draft available on request]
Krishnan, S., Submitted. "The Modified Elastofiber Element for Slender Column and Brace Modeling: II. Validation and Benchmarking", vol. XX(Y), ZZZZ, Journal of Structural Engineering. [PDF draft available on request]
Krishnan, S., 2009. "On the Modeling of Elastic and Inelastic, Critical- And Post-Buckling Behavior of Slender Columns and Bracing Members", Technical Report - CaltechEERL:EERL-2009-03, California Institute of Technology, Pasadena, California, 2009. [PDF Reprint]
8. On the Mechanism of Collapse of Tall Steel Moment Frame Buildings
Funding Source: National Earthquake Hazard Reduction Program (NEHRP), US Geological Survey (USGS)
How would tall steel moment frame buildings collapse under seismic loading? Steel moment frame exhibit shear beam type behavior. It is well known that when a two-sided pulse travels through a uniform shear beam, strain doubling occurs due to constructive interference of the reverse phase of the incident wave with the forward phase that is reflected off the free end. Such strain doubling can lead to damage localization. Moment frame buildings differ from uniform shear beams in three important ways- first, the buildings are not uniform, there is typically stiffness gradation and mass variation over the height of the structure; second, gravity is not present in the shear beam wave propagation problem, whereas, it plays a dominant role in the response of moment frame buildings. Not only do building columns carry axial loads, but gravity also causes second-order overturning moments associated with the self-weight of the structure acting through its deformed configuration under lateral loading, the so-called P-Δ effect; third, steel-frame buildings do exhibit low levels of damping. Damping has the effect of attenuating the response (which impacts the response to multi-pulse excitation more than single-pulse excitation) and lengthening the apparent period. But the low level of damping inherent in steel structures means that it plays a relatively minor role in the damage localization phenomenon. Damage localization in moment frame buildings can result in the formation of a shear-compliant block collapse mechanism, consisting of column yielding at floors corresponding to the top and bottom of the shear-compliant block, with significant yielding of the beams or columns or panel zones at each joint in each of the intermediate floors. We demonstrate that such a shear-compliant block, driven by P- Δ effects from the over-burden dead weight of the floors above the block, is the fundamental mode of collapse of tall steel moment frame buildings. Through parametric studies of 3-D finite element models of tall (approximately twenty story) steel moment-frame buildings subjected to idealized single- and multi-pulses, we find that for collapse to occur, the pulse period must generally equal or exceed the fundamental period of the building. For moderate loading (motions that are not strong enough to cause structural collapse), the Confines of the Most Pronounced Localization of Yielding (CoMPLY) is controlled by the period of the input ground motion relative to the fundamental period of the structures, with behavior that is very similar to that of an idealized shear beam. However, the response to "collapsogenic“ input motions shows a "convergence" of the CoMPLY to the same few stories, rendering it invariant with respect to various measures of the ground motion. This implies that there is a characteristic mechanism of collapse for a given building regardless of the frequency-duration characteristics of the incident ground motion. This characteristic collapse mechanism happens to be a function of the structural system alone and can be predicted using its basic properties. Analyses performed using recorded near-source ground motion waveforms from recent events, as well as simulated motions from a larger (magnitude7.9) far-source event support this idea of a characteristic collapse mechanism.
REFERENCES
Muto, M. and Krishnan, S., 2009. "Localization of Damage in Steel Moment Frame Buildings and Implications for Collapse", Technical Report - CaltechEERL:EERL-2009-05, California Institute of Technology, Pasadena, California, 2009. UNDER PREPARATION.
GROUP MEMBERS
Dr. Matthew Muto, Post-Doctoral Scholar
7. Tall Steel Building Response to the SHAKEOUT Scenario Earthquake
Funding Source: Multi-Hazard Demonstration Project, US Geological Survey
In order to prepare for the next big earthquake on the San Andreas fault, the US Geological Survey (USGS) conducted a year-long "DARE TO PREPARE" campaign that culminated in the Great Southern California Shakeout Scenario in 2008 (Porter et al. 2009). The scenario earthquake, chosen based on a wide variety of observations and constraints, was a magnitude 7.8 earthquake on the San Andreas fault with rupture initiating at Bombay Beach and propagating northwest through the San Gorgonio Pass a distance of roughly 304 km, terminating at Lake Hughes near Palmdale, sections of the San Andreas fault that last broke in 1680, 1812, and 1857. Through community participation in two Southern San Andreas Fault Evaluation (SoSAFE) workshops organized by the Southern California Earthquake Center (SCEC), a source model specific to the southern San Andreas fault was constructed with constraints from geologic, geodetic, paleoseismic, and seismological observations. Using this source model, Rob Graves of URS Corporation simulated 3-component seismic waveforms on a uniform grid covering southern California (Graves et al. 2008). Peak velocities of the synthetic ground motion were in the range of 0-100 cm/s in the San Fernando valley, and 60-180 cm/s in the Los Angeles basin. Corresponding peak displacement ranges were 0-100 cm and 50-150 cm. For the shakeout drill, USGS commissioned us to provide a realistic picture of the impact of such an earthquake on the tall steel buildings in southern California. We selected 784 sites across southern California to place 3-D computer models of three steel moment frame buildings in the 20-story class (an existing building designed according to the 1982 UBC, the same building redesigned using the 1997 UBC, and a hypothetical L-shaped building also designed according to the 1997 UBC), and analyzed these models subject to the simulated 3-component ground motion, orienting them in two different directions, considering perfect and imperfect realizations of beam-to-column connection behavior. We averaged the response from these 12 cases (3 buildings x 2 orientations x 2 connection susceptibility realizations), and combined this with the observed response of tall steel buildings in past earthquakes to provide a qualitative picture of one plausible outcome in the event of the big one striking southern California.
REFERENCES
Muto, M. and Krishnan, S., Submitted. "Hope for the Best, Prepare for the Worst: Response of Tall Steel Buildings to the ShakeOut Scenario Earthquake", vol. XX(Y), ZZZZ, Earthquake Spectra. [PDF draft available on request]
Muto, M., and Krishnan, S., 2008. "Response of tall steel buildings in southern California to the magnitude 7.8 shakeout scenario earthquake", Inaugural International Conference of the Engineering Mechanics Institute, University of Minnesota, May 18-21 2008. [PDF Reprint]
GROUP MEMBERS
Dr. Matthew Muto, Post-Doctoral Scholar
COLLABORATORS
Dr. Rob Graves, URS Corporation
6. The Caltech Virtual Shaker
The Caltech Virtual Shaker (CVS) is a publicly accessible online science gateway created to facilitate the analysis and visualization of structural models under various kinds of earthquake ground motion. It consists of a structural model database and a ground motion waveform database. Registered users can upload new structural models and ground motion time-histories, can select existing models from the database and analyze them under selected ground motion waveforms. The analyses are conducted on a high-performance computing cluster on the Caltech campus. Upon completion, results are emailed back to the user. Newly submitted models become available to all registered users. This is a community-driven venture that is expected to be self-sustaining in the long run, with models contributed by users and the software and CPU cycles being provided by CVS.
RELATED LINKS:
http://virtualshaker.caltech.edu
5. 1857-like San Andreas Earthquake End-to-End Simulation
What would happen to engineered highrise buildings in the Los Angeles and San Fernando basins if an 1857-like magnitude 7.9 earthquake were to occur on the San Andreas fault?
Two such earthquakes have been simulated and the nonlinear response of two 18-story steel moment-frame buildings has been modeled with the aid of high-performance computing.
Key information:
1. Two scenarios - one with rupture initiating at Parkfield in central California and propagating north-to-south a distance of 290 km and the other with the slip distribution and rupture direction flipped such that rupture starts north of Los Angeles and proceeds south-to-north terminating at Parkfield.
2. Both are magnitude 7.9 earthquakes. Kinematic source model of the Denali (Alaska) fault earthquake of 2002 has been mapped on to the San Andreas fault.
3. Domain of simulation includes a uniform grid of 636 analysis sites spaced at 3.5 km in each direction.
4. 3-D models of two 18-story steel moment-frame buildings (one existing building designed according to the 1982 Uniform Building Code, and the other being the same building redesigned according to the stricter 1997 UBC) have been analyzed subjected to 3-component ground motion from the two San Andreas fault earthquakes at each of the 636 sites.
5. Building response summed up in the form of maps of peak interstory drift ratio.
Watch movies of the seismic wave propagation and building response.
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NORTH-TO-SOUTH RUPTURE SCENARIO
Fault rupture and seismic wave propagation:
Play Movie (18 MB)
Movies of the shaking of existing and new 18-story steel moment-frame
building models at various sites in Southern California. Of these, the
existing building models in Thousand Oaks and Northridge are seen collapsing,
while there is significant permanent tilt in most other cases.
Anaheim: Play Movie
(8 MB)
Baldwin Park: Play
Movie (8 MB)
Downtown Los Angeles:
Play Movie (8 MB)
Long Beach: Play Movie
(8 MB)
Northridge: Play Movie
(8 MB)
Santa Ana: Play Movie
(8 MB)
Thousand Oaks: Play Movie
(8 MB)
West Los Angeles: Play Movie
(8 MB)
SOUTH-TO-NORTH RUPTURE SCENARIO
Fault rupture and seismic wave propagation:
Play Movie (18 MB)
SOFTWARE USED
SPECFEM3D for the seismic wave propagation.
FRAME3D for the building analyses.
REFERENCES
Krishnan, S., Ji, C., Komatitsch, D., Tromp, J., Muto, M., Mitrani-Reiser, J., and Beck, J. L., 2008. "Simulation of an 1857-like Mw 7.9 San Andreas fault earthquake and the response of tall steel moment frame buildings in southern California - A prototype study", Proceedings of the 14th World Conference in Earthquake Engineering, Beijing, China, October 17-21, 2008. [PDF Reprint]
Muto, M., Krishnan, S., Beck, J. L., Mitrani-Reiser, J., 2008. "Seismic loss estimation based on end-to-end simulation", IALCCE08: First International Symposium on Life-Cycle Civil Engineering, Varenna, Lake Como, Italy, June 11-14, 2008. [PDF Reprint]
Krishnan, S., Ji, C., Komatitsch, D., and Tromp, J., 2006. "Performance of Two 18-Story Steel Moment-Frame Buildings in Southern California During Two Large Simulated San Andreas Earthquakes", vol. 22(4), November 2006, Earthquake Spectra. [PDF Reprint]
Krishnan, S., Ji, C., Komatitsch, D., and Tromp, J., 2006. "Case Studies of Damage to Tall Steel Moment-Frame Buildings in Southern California During Large San Andreas Earthquakes", vol. 96(4), August 2006, Bulletin of the Seismological Society of America. [PDF Reprint]
Krishnan, S., Ji, C., Komatitsch, D., and Tromp, J., 2006. "Impact of a Large San Andreas Fault Earthquake on Tall Buildings in Southern California", EERI's 8th U.S. National Conference on Earthquake Engineering, April 2006. [PDF Reprint]
Krishnan, S., Ji, C., Komatitsch, D., and Tromp, J., 2005. "Performance of 18-Story Steel Moment-Frame Buildings During a Large San Andreas Earthquake - A Southern California-Wide End-to-End Simulation", Technical Report - CaltechEERL:EERL-2005-01, California Institute of Technology, Pasadena, California, 2005. [PDF Reprint]
COLLABORATORS
Dr. Jeroen Tromp, Princeton University
Dr. Chen Ji, University of California, Santa Barbara
Dr. Dimitri Komatitsch, University of Pau, France
4. P-FRAME3D & FRAME3D
FRAME3D is a program for the three-dimensional nonlinear analysis of steel buildings. It aims to overcome the computational challenges posed by full 3D analysis of buildings subject to earthquake ground motion. The element library consists of a plastic hinge beam element, an elastofiber beam element, a modified elastofiber element, a panel zone element, a 4-noded diaphragm element to model floor slabs, and an elastic translational/rotational spring element to model foundations and supports. The program utilizes a Netwon-Raphson iteration strategy applied to an implicit Newmark time-integration scheme to solve the nonlinear equations of motion at each time-step. Geometric nonlinearity and shear deformation are included in the formulation.
P-FRAME3D is the parallel version of FRAME3D currently under development. Upon completion, P-FRAME3D can be executed on a parallel computer cluster to solve complex structural engineering problems such as the strong shaking of a super-highrise building or a long-span cable-supported bridge under a large earthquake occurring on a nearby fault.
REFERENCES
Krishnan, S., 2009. "FRAME3D V2.0 - A Program for the Three-Dimensional Nonlinear Time-History Analysis of Steel Buildings: User Guide", Technical Report - CaltechEERL:EERL-2009-04, California Institute of Technology, Pasadena, California, 2009. [PDF Reprint]
RELATED LINKS:
FRAME3D Program - http://www.frame3d.caltech.edu.
3. Behavior of Tall Steel Moment Frame Buildings Under Near-Source Ground Motion
Strong ground motion from a nearby fault has frequency content in the same range as the natural frequencies of tall buildings. This may have serious repercussions and is the topic of my Ph.D. dissertation. Buildings are designed per building code standards. But, are the code provisions adequate? Strong motion from large earthquakes has been recorded only in recent times in the near-source region. Have the current codes used this information to update tall structure design guidelines? Considerable damage has been observed in tall buildings from the Northridge, Kobe, Turkey, and Taiwan earthquakes. How will tall buildings designed per the latest code regulations perform if they were to be shaken by any of these earthquakes? My thesis attempts to answer these questions.
Tall buildings by their nature are computationally intensive to analyze. They consist of thousands of degrees of freedom and when subjected to strong ground motion from a nearby source, exhibit inelastic response. Modeling this inelastic response requires an iterative approach that is computationally expensive. Furthermore, a large class of buildings, classified as irregular, exhibits complex behavior that can be studied only when the structures are modeled in their entirety. To this end, a three-dimensional analysis program, FRAME3D, has been developed incorporating two special beam-column elements -- the plastic hinge element and the elastofiber element that can model beams and columns in buildings accurately and efficiently, a beam-column joint element that can model inelastic joint deformation, and 4-noded elastic plane-stress elements to model floor slabs acting as diaphragms forcing the lateral force resisting frames in a building to act as one unit. The program is capable of performing time-history analyses of buildings in their entirety.
Six 19-story irregular steel moment frame buildings (with buildings 2A and 3A being variants of buildings 2 and 3, respectively) have been designed per the latest code (Uniform Building Code, 1997). Two of these buildings have reentrant corners and the other two have torsional irregularity. Their strength and ductility are assessed by performing pushover analyses on them. To assess their performance under strong shaking, FRAME3D models of these buildings are subjected to near-source strong motion records from the Iran earthquake (Mw = 7.3, Tabas Station) of 1978, the Northridge earthquake (Mw = 6.7, Sylmar Station) of 1994 and the Kobe earthquake (Mw = 6.9, Takatori Station) of 1995. None of the buildings collapsed under these strong events in the computer analyses. However, when compared against the acceptable limits for various performance levels in FEMA 356 document, the damage in terms of plastic deformation at the ends of beams and columns and at joints would render the buildings inadequate in terms of life safety in quite a few cases and would even indicate possible collapse in a couple of cases. Thus, in these terms, the code falls short of achieving its life safety objective, and the near-source factors introduced in the code in 1997 in recognition of the special features of near-source ground motion seem to be inadequate. The ductility demand, in terms of plastic rotation at the ends of beams and columns and in joints, on these buildings during this class of earthquakes is up to 6% of a radian, which is far greater than a typical limiting plastic rotation of 3% associated with fracture and consequent failure of large wide-flanged steel sections during experiments. Thus, if strength degradation due to fractures, local buckling, etc., were to be included in the analysis, then the results would likely to be worse, as far as the ability of these buildings to withstand these earthquakes without collapse is concerned.
Due to damage localization, the peak drifts observed in the structure far exceeded the inelastic drift limit in the code of 0.02 (in some cases up to 3 times). This points to serious non-structural damage to facades, interior dry wall, etc. Furthermore, large roof permanent offsets after the events indicate significant post-earthquake repair requiring considerable disruption and building closure. Column yielding was minimal thus validating the strong-column weak-beam criterion in the code. Redundancy factors used to assess the redundancy in the system need to take into account the case of torsionally sensitive structures where frames in both principal directions are simultaneously activated. Stress concentration was not observed at the reentrant corners in L-shaped buildings.
Finally, the data catalogued in my thesis could be useful for future code development and tall structure design guidelines.
REFERENCES
Krishnan, S., 2003. "Three-Dimensional Nonlinear Analysis of Tall Irregular Steel Buildings Subject to Strong Ground Motion", Technical Report - CaltechEERL:EERL-2003-01, California Institute of Technology, Pasadena, California, 2003. [PDF Reprint]
Krishnan, S., 2007. "Case Studies of Damage to 19-Story Steel Moment-Frame Buildings Under Near-Source Ground Motion", vol. 36(7), July 2007, Earthquake Engineering and Structural Dynamics. [PDF Reprint]
Krishnan, S. and Hall, J. F., 2006. "Modeling Steel Frame Buildings in Three Dimensions - Part I: Panel Zone and Plastic Hinge Beam Elements", vol. 132(4), April 2006, Journal of Engineering Mechanics. [PDF Reprint]
Krishnan, S. and Hall, J. F., 2006. "Modeling Steel Frame Buildings in Three Dimensions - Part II: Elastofiber Beam Element and Examples", vol. 132(4), April 2006, Journal of Engineering Mechanics. [PDF Reprint]
RELATED LINKS:
Building Animations - http://www.frame3d.caltech.edu/anim.html.
Key Analysis Results - http://www.frame3d.caltech.edu/bldgdb.html.
2. System Identification of Dynamic Structural Systems Using Continuous-Time Domain Methods
Structural system identification is the process of deducing the properties of a structural system from its measured response to ambient vibration by fitting a mathematical model. The objectives of my Masters research were to: (1) investigate the use of existing system identification methods, and (2) develop new system identification methods, in order to evaluate both loading and structural modal parameters of ambient excited structures for which the load process is difficult to measure. An example is the case of offshore platforms subjected to sea waves. The study considers the inverse problem from the viewpoint of continuous-time linear dynamic systems. An existing structural identification technique defined for the deterministic case (when the loading is known) and based on sequences of modal minimizations (called modal sweeps) is formulated for the general case of multiple-input multiple-output (MIMO) systems. A global minimization technique based upon the Levenberg-Marquardt algorithm for nonlinear least-squares problems is developed for the same case. Both identification techniques are applied to a multi-degree-of-freedom (MDOF) shear building model and the results are shown to be consistent. Using the framework of random vibration theory, these techniques are then converted to the stochastic case, namely when the loading process is known only statistically. These identification methods, both individually and combined, were tested based on simulated cases of increasing complexity. The results obtained are promising and indicate that under certain conditions, both load and structural parameters can be estimated from the measured response and the statistical properties of the loading process. However, the load parameters are not as well estimated as the structural parameters, since the load process is further removed from the response process than the structural filter. Although a generic shear building model has been used throughout this study to simulate the dynamic response of real structures, the results obtained are believed to apply to linear multi-degree-of-freedom systems in general.
REFERENCES
Krishnan, S., 1994. "System Identification of Dynamic Structural Systems Using Continuous-Time Domain Methods", Master of Science Thesis, Rice University, Houston, Texas, 1994. [PDF Reprint]
Conte, J. P., and Krishnan, S., 1995. "Modal Identification Method for Structures Subjected to Unmeasured Random Excitation", Proceedings of the 10th ASCE Engineering Mechanics Specialty Conference, Boulder, Colorado, 1995. [PDF Reprint]
1. Long-Span Cable-Supported Bridges
Cable-supported bridge systems are distinguished from conventional bridge systems by their ability to span wide crossings efficiently and aesthetically. The need for long-span bridges may arise due to high foundation cost resulting from wide flood zones, channel instability, deep gorges, etc. Two basic types of cable-supported bridge systems have been commonly used: cable-stayed bridge systems and suspension bridge systems. It is well-established that cable-stayed bridges are suitable for main spans in the range of 250m-500m, and cable-suspended bridges for main spans beyond 1500m. In the intermediate span range of 500m-1500m, both systems are deficient and there is need for a third option. Using the fundamental concept of "cable beams" applied in suspended roof structures, a new cable-net system is proposed. Detailed designs of this system and a cable-stayed bridge system are carried out for a main-span of 800m. By means of a comprehensive comparative analysis of bending moments, axial forces, deflections, and amount of steel used, the paper demonstrates that the cable-net bridge system provides an economically, structurally, and asethetically viable alternative to the cable-stayed and cable-suspended bridge systems for intermediate spans.
REFERENCES
Krishnan, S., 1994. "Cable-Net Bridge System", Submitted to the Student Paper Competition at the 11th Annual International Bridge Conference, 1994. [PDF Reprint]
Krishnan, S., 1992. "Long-Span Cable-Supported Bridges", Bachelor of Technology Thesis, Department of Civil Engineering, Indian Institute of Technology, Madras, India, May 1992.