Design Code Provisions Research Articles

Performance-based multiobjective optimum design of steel structures considering life-cycle cost

A new methodology for the performance-based optimum design of steel structures subjected to seismic loading considering inelastic behavior is proposed. The importance of considering life-cycle cost as an additional objective to the initial structural cost objective function in the context of multiobjective optimization is also investigated. Life-cycle cost is considered to take into account during the design phase the impact of future earthquakes. For the solution of the multiobjective optimization problem, Evolutionary Algorithms and in particular an algorithm based on Evolution Strategies, specifically tailored to meet the characteristics of the problem at hand, are implemented. The constraints of the optimization problem are based on the provisions of European design codes, while additional constraints are imposed by means of pushover analysis to control the load and deformation capacity of the structure.

Load and Resistance Factor Calibration For Wood Bridges

The paper presents the calibration procedure and background data for the development of design code provisions for wood bridges. The structural types considered include sawn lumber stringers, glued-laminated girders, and various wood deck types. Load and resistance parameters are treated as random variables, and therefore, the structural performance is measured in terms of the reliability index. The statistical parameters of dead load and live (traffic) load, are based on the results of previous studies. Material resistance is taken from the available test data, which includes consideration of the post-elastic response. The resistance of components and structural systems is based on the available experimental data and finite element analysis results. Statistical parameters of resistance are computed for deck and girder subsystems as well as individual components. The reliability analysis was performed for wood bridges designed according to the AASHTO Standard Specifications and a significant variation in reliability indices was observed. The recommended load and resistance factors are provided that result in consistent levels of reliability at the target levels.

A Rational Approach for Determining Response Modification Factors for Seismic Design of Buildings Using Current Code Provisions

A short review of response modification factors currently found in code design provisions for seismic design of buildings is given. A proposal for a new, rational procedure for determining R factors is presented. It requires both experimental and analyses for implementation. It is based on a probabilistic foundation developed for the SAC Phase 2 project. The target threshold is to provide a 90% confidence level for satisfying the Collapse Prevention performance objective for an earthquake with a 2,475-year return period. Adoption of this procedure would result in a uniform level of safety against collapse over all materials and building systems. The method is applied to steel moment-resisting frame buildings as an example.

Predicting shear capacity of NSC and HSC slender beams without stirrups using artificial intelligence

The use of high-strength concrete (HSC) has significantly increased over the last decade, especially in offshore structures, long-span bridges, and tall buildings. The behavior of such concrete is noticeably different from that of normal-strength concrete (NSC) due to its different microstructure and mode of failure. In particular, the shear capacity of structural members made of HSC is a concern and must be carefully evaluated. The shear fracture surface in HSC members is usually trans-granular (propagates across coarse aggregates) and is therefore smoother than that in NSC members, which reduces the effect of shear transfer mechanisms through aggregate interlock across cracks, thus reducing the ultimate shear strength. Current code provisions for shear design are mainly based on experimental results obtained on NSC members having compressive strength of up to 50MPa. The validity of such methods to calculate the shear strength of HSC members is still questionable. In this study, a new approach based on artificial neural networks (ANNs) was used to predict the shear capacity of NSC and HSC beams without shear reinforcement. Shear capacities predicted by the ANN model were compared to those of five other methods commonly used in shear investigations: the ACI method, the CSA simplified method, Response 2000, Eurocode-2, and Zsutty's method. A sensitivity analysis was conducted to evaluate the ability of ANNs to capture the effect of main shear design parameters (concrete compressive strength, amount of longitudinal reinforcement, beam size, and shear span to depth ratio) on the shear capacity of reinforced NSC and HSC beams. It was found that the ANN model outperformed all other considered methods, providing more accurate results of shear capacity, and better capturing the effect of basic shear design parameters. Therefore, it offers an efficient alternative to evaluate the shear capacity of NSC and HSC members without stirrups.

Lateral-torsional buckling of unrestrained steel beams under fire conditions: improvement of EC3 proposal

The final draft of the EN version of part 1.1 of Eurocode 3 has introduced significant changes in the evaluation of the lateral-torsional buckling resistance of unrestrained beams at room temperature that reduce the over-conservative approach of ENV 1993-1-1 in the case of non-uniform bending. Numerical modelling of the lateral-torsional buckling of steel beams at elevated temperature has shown that the beam design curve from prEN 1993-1-2 is over-conservative for loadings other than uniform bending. In line with the safety format of the lateral-torsional buckling code provisions for cold design, an alternative proposal for rolled sections or equivalent welded sections subjected to fire is presented in this paper, that addresses the issue of the influence of the loading type on the resistance of the beam, achieving better agreement with the real behaviour while maintaining safety.

Numerical modelling and design of unlined cold-formed steel wall frames

Cold-formed steel wall frame systems using lipped or unlipped C-sections and gypsum plasterboard lining are commonly utilised in the construction of both the load bearing and non-load bearing walls in the residential, commercial and industrial buildings. However, the structural behaviour of unlined and lined stud wall frames is not well understood and adequate design rules are not available. A detailed research program was therefore undertaken to investigate the behaviour of stud wall frame systems. As the first step in this research, the problem relating to the degree of end fixity of stud was investigated. The studs are usually connected to the top and bottom tracks and the degree of end fixity provided by these tracks is not adequately addressed by the design codes. A finite element model of unlined frames was therefore developed, and validated using full scale experimental results. It was then used in a detailed parametric study to develop appropriate design rules for unlined wall frames. This study has shown that by using appropriate effective length factors, the ultimate load and failure modes of the unlined studs can be accurately predicted using the provisions of Australian or American cold-formed steel structures design codes. This paper presents the details of the finite element analyses, the results and recommended design rules for unlined wall frames.

Reply to the discussion by R.D. Watts on "Proposed Canadian code provisions for seismic design of elements of structures, nonstructural components, and equipment"

It is true that only a few deaths have been reported from failures of nonstructural components during earthquakes. However, a large percentage of people requiring medical treatment in emergency rooms following an earthquake have injuries resulting from movement and falling of nonstructural components. In public places, failure of nonstructural components such as ceilings leads to panic because of the perception that “the building is falling down”. This has caused serious injuries due to crushing and trampling under foot. The nonstructural components section of the earthquake provisions seeks to address these issues of life safety and public safety. In this, the proposed provisions of the Canadian code are consistent with seismic code provisions throughout the world. The cost data referred to by Mr. Watts are not supported by cost data assembled by the author over the past ten years. In response to Mr. Watts’ specific comments: 1) The code provisions require that the importance factor for nonstructural components shall be as a minimum the value of the importance factor used for the structure. There will be cases when it will be appropriate for the nonstructural components to be designed for a higher importance factor than the structure, and the designer is free to use judgement to do this. 2) The distribution and amplitude of forces applied to nonstructural components in buildings is based on empirical data and studies of structural dynamics. Briefly, it is expected that a single-storey building will have a shorter period than a four-storey building. The onestorey building will be designed for a higher percentage of base shear, and it is expected that the forces at the roof of the one-storey building will be higher than the first floor of the four-storey building. 3) Usually the common elements, such as ceiling and pipe restraints in a multi-storey building are designed for the maximum force levels at the top of the building. The connection details are kept the same throughout the building. There is very little difference in costs for these connections over the practical range of forces. The simplicity of design greatly reduces the chance of errors. For large pieces of equipment or architectural components, the difference in cost of seismic restraint from the top to the bottom of the building can be significant. The code provisions enable the designer to take advantage of this.

Discussion of "Proposed Canadian code provisions for seismic design of elements of structures, nonstructural components, and equipment"

The author, W.E. McKevitt, is to be congratulated on explaining the mechanics of the new seismic provisions for nonstructural components. The writer, however, still finds many of the provisions confusing. On a general basis, should we even be designing all these elements for seismic loads or should we only be designing all elements on key facilities and specific elements on other buildings for these forces? The writer’s observations and interpretation of various damage reports over the years are as follows: (i) very few people have been killed or seriously injured by failure of these elements, (ii) the costs of implementing the code have been consistently underestimated, and (iii) the benefits have been overrated. There are elements (e.g., parapets, heavy cladding) that represent real threats to life safety, but many elements either do not have the mass to produce serious injuries or have relatively low failure rates. Even windows, which many people feel will shatter, rarely fail, particularly in buildings that are up to current code standards. Some large fixed panes can shatter, but smaller, and, particularly, operable windows almost never shatter. The costs of code implementation have been consistently underestimated and benefits overrated for a variety of reasons: (1) there are few data on the actual failure rates of restrained, both properly and improperly, and unrestrained elements; (2) there are few data on the probability that the restraints will still be effective when an earthquake strikes (some restraints are either modified or removed for a variety of reasons); (3) the cost of multiple or unused installations is ignored, e.g., the equipment needs replacing or the building is renovated or destroyed before the next earthquake; (4) the increased maintenance costs, as the trades either have to work around all these restraints or remove and replace them; and (5) the time value of money. On specific provisions the writer’s concerns are as follows: First, why is the importance factor the same for the main structural system and nonstructural components? Most welldesigned structural systems incorporate redundancy where one component failure does not bring down the whole system. By contrast, a single failure of a water line can, and has, shut down a whole hospital. Second, while it makes intuitive sense that there will be an amplification of response with height in a building, the factor proposed results in some odd designs. For example, similar weight items are designed for higher forces on the ceiling of a single-storey doughnut shop than the first floor ceiling of a four-storey hospital. Third, is it desirable to have the restraint for common elements varying on different floors? The more variations the trades have to build, the greater the chances for errors.

United States–Japan Cooperative Earthquake Engineering Research Program on Composite and Hybrid Structures

The United States‐Japan Cooperative Earthquake Research Program began in 1979 under the auspices of the UJNR panel on wind and seismic effects. The overall objective of the program was to improve seismic safety practices in both countries through cooperative studies to determine the relationship among fullscale, small-scale, and component tests, as well as related analytical and design implication studies. The first four phases of the program were Reinforced concrete ~RC! structures, steel structures, masonry structures, and precast concrete structures. Research on composite steel RC ~SRC! structures was identified as an important area of concern and recommended in the planning group report. Considering the importance of developing seismic design guidelines and code provisions for typical composite and hybrid structures that have been used in recent practice, and the need for developing new and innovative composite structural elements and hybrid systems using advanced new materials and devices, a 5 year research program on composite and hybrid structures was recommended as the fifth phase of the United States-Japan cooperative earthquake research program. The program contents were based on a number of technical meetings of the United States and Japan planning groups and on the outcome of a joint planning group workshop held in Berkeley, Calif. on September 10‐12, 1992 ~Goel and Yamanouchi 1992!. Because of the diverse and broad scope of the subject area, the research program was organized into the following four groups: concrete-filled tube~CFT! column systems; RC/SRC column systems ~RCS!; RC/SRC hybrid-wall systems ~HWS!; and new materials, elements, and systems ~compiled by Research for Innovation!. The first two systems are moment-resisting frames made of steel beams and either CFT or RC/SRC composite columns, whereas, the HWS systems utilize RC or composite shear walls and steel-beam and column frames. The research work on those three systems aimed at developing practical design guidelines as they represent typical composite and hybrid structural systems used in recent practice. The last group aimed at developing new and innovative composite structural elements and hybrid systems using advanced new materials and devices. A theme structure provided a common focus of the overall research program to facilitate cross-comparison between various system types, and to derive structural elements and subassemblages for detailed studies. The work carried out in the four groups listed previously is briefly described in the following sections.

Update on the AISC Seismic Provisions

Between the advent of incorporating seismic design provisions into the Uniform Building Code (UBC) in the early 1950s, and the late 1980s, the Structural Engineers Association of California (SEAOC) Seismology Committee was almost completely responsible for the content of these provisions. Spurred on by the federally funded National Earthquake Hazard Reduction Program (NEHRP), in the late 1980s and early 1990s seismic design began to be seen as more of a nationwide issue. During these years, the NEHRP program began to fund the Building Seismic Safety Council (BSSC) to develop model building code provisions for seismic design. The BSSC established a nationally represented committee structure, with technical subcommittees addressing each of the main structural materials, including structural steel. To support this effort, the American Institute of Steel Construction (AISC) established a parallel committee (TC 9) and began the development of a set of seismic design provisions for steel buildings. Over the last ten years, a rational, systematic and efficient process has been instituted to incorporate the latest developments in seismic design of steel structures into building code provisions. This system relies on the coordinated efforts of AISC, BSSC, SEAOC and AWS committees. The process provides a single point of responsibility for the development of these provisions, thus eliminating duplicative effort, and more importantly, the development of competing documents that would result in minor differences that would undoubtedly result in major confusion in application by practicing engineers.

Proposed Canadian code provisions for seismic design of elements of structures, nonstructural components, and equipment

Proposed code provisions for the seismic design of elements of structures, nonstructural components, and equipment are presented. In these provisions a new format is introduced which gives a consistent treatment for all elements and components, architectural, mechanical, and electrical. For the first time, soil effects are included in the provisions, which also seek to ensure that designers consider the interrelationship of the nonstructural components and differential displacements within the building structure. The proposed force equation is based on a uniform hazard spectrum approach with force modification factors. Specified force levels are based on data from instrumented buildings recorded during recent earthquakes. Updated requirements for connection design specify forces that are consistent with component design forces. New element and component categories are provided in an expanded table of elements of structures, nonstructural components, and equipment. To assess the impact of the proposed provisions on component design at various locations across the country, calculations are presented for typical multistorey buildings in a number of Canadian cities.Key words: seismic design, elements of structures, nonstructural mechanical electrical architectural components.

Thermal Effects on Skewed Steel Highway Bridges and Bearing Orientation

An investigation is conducted to characterize and quantify external effects in composite steel highway bridges under thermal loading. Based on the results of a literature review, including thermal and thermoelastic analyses as well as current design code provisions, a simple but realistic thermal loading is developed for winter and summer conditions for AASHTO load and resistance factor design (LRFD) Zone 3. Three cases of bearing orientation, representative of current design practice, are examined. Parametric studies are then conducted. Hypothetical bridges are designed for a range of different span lengths, section depths, widths, and skews. Each bridge model is tested under all three constraint cases and both winter and summer thermal loading. Variations in structural response with each parameter are plotted, and the relative influence of each parameter is discussed. Design equations to predict the observed displacements and restraint forces at the bearings are then developed by a systematic regression procedure. The applicability of these proposed design equations is demonstrated by examples.

Seismic demands on steel braced frame buildings with buckling-restrained braces

Some results are highlighted in this paper from a research effort being undertaken to identify ground motion and structural characteristics that control the earthquake response of concentrically braced steel frames and to identify improved design procedures and code provisions. The focus of this paper is on the seismic response of three and six story concentrically braced frames utilizing buckling-restrained braces. A brief discussion is provided regarding the mechanical properties of such braces and the benefits of their use. Results of detailed nonlinear dynamic analyses are then examined for specific cases as well as statistically for several suites of ground motions in order to characterize the effect on key response parameters of various structural configurations and proportions.

Applicability of Code Design Methods to RC Slabs on Secondary Beams. Part II: Moment Variations

Reinforced concrete slabs on secondary beams differ from two-way slabs on beams in the fact that the latter has vertical supports at the intersection of the beams whereas the former does not. In the absence of specific code provisions for the former, it is usually designed in accordance with the provisions developed for the latter. The numerical distributions of moments over five beam-slab systems with different beam to slab depth ratios are examined to assess the applicability of relevant code provisions for design. Moments from the numerical analysis are employed to evaluate two assumptions. First, the secondary beams are assumed rigid, and hence making the beam-slab systems similar to two-way slab on beams. Second, the secondary beams are assumed flexible, hence the systems are considered as single panels. The former representation shows that the error exceeds 200 % in positive bending moments. This means that slab on secondary beams should not be treated as the traditional two-way slab on beams. The latter representation shows that the beam-slab systems essentially behave as single panels. Accordingly, treating the entire floor as a single panel seems to be an essential element in devising a design methodology for this type of construction.

Retrofitting Building Frames for Snow Loads

Many older buildings and farm buildings have been designed with snow load provisions that are lower than the present design code provisions. As a result, some of these buildings have experienced complete or partial roof failure. This paper examines the snow load requirements of two governing building codes and presents a case study where a low-cost, fast-track solution strategy was devised to repair and to retrofit a partially damaged roof in Fargo, N. Dak.

On the damping adjustment factors for earthquake response spectra

AbstractDamping adjustment factors for converting 5%‐damped earthquake response spectra to other damping levels are evaluated in this paper. The evaluation is based on statistical analysis of spectra of 1047 horizontal components of earthquake ground shaking recorded between 1933 and 1994. The dependency of damping adjustment factors to response period is studied. Results of the evaluation are compared to the damping adjustment factors embodied in the current seismic design code provisions and guidelines which are based on the spectrum amplification factors of Newmark and Hall. Copyright © 2001 John Wiley & Sons, Ltd.

Closure to “Canadian Bridge Design Code Provisions for Fiber‐Reinforced Structures” by Baidar Bakht, George Al‐Bazi, Nemy Banthia, Moe Cheung, Marie‐Anne Erki, Martin Faoro, Atsuhiko Machida, Aftab A. Mufti, Kenneth W. Neale, and Gamil Tadros

Closure to “Canadian Bridge Design Code Provisions for Fiber‐Reinforced Structures” by Baidar Bakht, George Al‐Bazi, Nemy Banthia, Moe Cheung, Marie‐Anne Erki, Martin Faoro, Atsuhiko Machida, Aftab A. Mufti, Kenneth W. Neale, and Gamil Tadros

Design Strategies for Controlling Structural Instabilities

An overview of buckling behaviour is presented, with emphasis on its analysis and design implications and ways of incorporating these considerations in the design of spatial structures. In the first part of the paper the fundamental stability concepts are briefly reviewed. The major types of instability are presented and typical structures that are prone to fail in such modes are identified. The influence of initial imperfections is discussed, as well as the interaction between different failure modes. In the second part, practical issues pertaining to structural analysis and design are addressed. These include hints to identify potential instability modes and obtain approximate buckling loads by analytical considerations, guidelines for selection of appropriate analysis methods, as well as discussion of pertinent design approaches, code provisions and issues that are not covered by present codes.

Review of Earthquake Performance, Seismic Codes, and Dynamic Analysis of Elevators

Elevators are among the most important mechanical systems in building structures that are quite susceptible to earthquake-induced damages. This survey paper is written primarily for the earthquake engineers who have not been directly involved in the design and maintenance of elevator systems but who are professionally interested in their functioning and performance. It describes the important components of an elevator system and highlights those that are most susceptible to earthquake-induced ground motions. A comprehensive review of the observed performance of elevators in past earthquakes reported by many investigators in the open literature is also presented. The evolution of code provisions for seismic design of these systems, measures adopted by the industry to mitigate the seismic effects and enhance seismic performance, and rationale of simple formula used for the current designs of counterweight guide rails and their supports are reviewed. A comprehensive review of the technical studies conducted to examine the dynamic behavior of these studies is also presented.

Canadian Bridge Design Code Provisions for Fiber-Reinforced Structures

This paper presents a synthesis of the design provisions of the Canadian Highway Bridge Design Code for fiber-reinforced structures. These include structures reinforced with fiber-reinforced polymer and fiber-reinforced concrete (FRC). The provisions apply to fully, or partially, prestressed concrete beams and slabs, non-prestressed concrete beams, slabs, and deck slabs, FRC deck slabs of slab-on-girder bridges, stressed wood decks, and barrier walls. Test methods to confirm the tensile strength of fiber-reinforced polymer and the postcracking strength of FRC are given.