Design Of Braced Frames Research Articles

Stressed Skin Design Versus Braced Frame Design Through Efficient Numerical Modelling

Steel frame structures are traditionally designed with bracings that stabilize the main bearing structure. Another approach is to apply the “stressed skin design” where the cladding structure takes the role of the bracings. In this research, the two approaches were analysed and compared in order to find advantages of any. A typical steel frame structure was chosen, and for the cladding, trapezoidal sheet metal was selected. Optional bracings were also considered. All static analyses were done using the finite element method (FEM) and ANSYS Workbench software, including geometric and material nonlinearity. The structure was loaded perpendicularly to its gable, thus simulating typical wind action. Number of fasteners connecting the cladding and the frame is crucial for the “stressed skin design” concept, so this parameter was varied through the analysis. The stiffness of the fastener devices was simulated in two ways: (1) simplified, by merging of the adjacent nodes of the cladding and the frame, and (2) more accurately, using special joint elements with prescribed stiffness. Obtained stresses and deformations were compared, and they showed obvious advantages of the “stressed skin design” over braced frame design, both in structural, and in economical aspect. In addition, important practical guidelines for the “stressed skin design” using FEM were proposed.

Multi-objective optimal design of braced frames using hybrid genetic and ant colony optimization

In this article, multi-objective optimization of braced frames is investigated using a novel hybrid algorithm. Initially, the applied evolutionary algorithms, ant colony optimization (ACO) and genetic algorithm (GA) are reviewed, followed by developing the hybrid method. A dynamic hybridization of GA and ACO is proposed as a novel hybrid method which does not appear in the literature for optimal design of steel braced frames. Not only the cross section of the beams, columns and braces are considered to be the design variables, but also the topologies of the braces are taken into account as additional design variables. The hybrid algorithm explores the whole design space for optimum solutions. Weight and maximum displacement of the structure are employed as the objective functions for multi-objective optimal design. Subsequently, using the weighted sum method (WSM), the two objective problem are converted to a single objective optimization problem and the proposed hybrid genetic ant colony algorithm (HGAC) is developed for optimal design. Assuming different combination for weight coefficients, a trade-off between the two objectives are obtained in the numerical example section. To make the final decision easier for designers, related constraint is applied to obtain practical topologies. The achieved results show the capability of HGAC to find optimal topologies and sections for the elements.

Member Sizing Optimization for Seismic Design of the Inverted V-braced Steel Frames with Suspended Zipper Strut

Seismic design of braced frames that simultaneously considers economic issues and structural performance represents a rather complicated engineering problem, and therefore, a systematic and well-established methodology is needed. This study proposes a multi-objective seismic design method for an inverted V-braced frame with suspended zipper struts that uses the non-dominated sorting genetic algorithm-II(NSGA-II). The structural weight and the maximum inter-story drift ratio as the objective functions are simultaneously minimized to optimize the cost and seismic performance of the structure. To investigate which of strength- and performance-based design criteria for braced frames is the critical design condition, the constraint conditions on the two design methods are simultaneously considered (i.e. the constraint conditions based on the strength and plastic deformation of members). The linear static analysis method and the nonlinear static analysis method are adopted to check the strength- and plastic deformation-based design constraints, respectively. The proposed optimal method are applied to three- and six-story steel frame examples, and the solutions improved for the considered objective functions were found.

Seismic design and performance of multi-tiered steel braced frames including the contribution from gravity columns under in-plane seismic demand

This paper examines the possibility of mobilizing gravity columns in the resistance of in-plane bending moments imposed on the columns of multi-tiered steel braced frames subjected to seismic loading. This would be the case when horizontal struts are used to connect gravity columns to braced frames at every tier level, as is often seen along exterior walls. A seismic design strategy based on the AISC Seismic Provisions is presented for four-tiered prototype steel concentrically braced frames. Three different approaches are proposed for the design of the braced frame and gravity columns. A set of 12 four-tiered X-braced frames, ranging from 15 to 30m in height and located in a high seismic area were designed based on the proposed design approaches. The seismic behavior of the frames is evaluated using nonlinear response history analysis. The results show that the seismic performance of the braced frames is improved as nonlinear seismic demand on the bracing members is reduced when mobilizing the gravity columns for lateral resistance. Furthermore, gravity columns bending moment demands are distributed between braced frame and gravity columns in proportion of their relative flexural stiffness. Adequate seismic performance and cost-effective design can be achieved when columns of both types are designed to resist their respective share of the flexural demand. Alternatively, satisfactory response was achieved when the gravity columns are verified for the seismic induced bending moments acting together with concomitant axial loads.

An Overview on the Seismic Design of Braced Frames

Seismic design of steel braced frames in the modern building codes follow the capacity design approach where some of the members are obliged to dissipate energy whereas others are taken care to be protected. In this paper the seismic design methodologies used by European and American approaches for Concentric Cross Braced Frames (CCBF) and Eccentric Braced Frames (EBF) are highlighted. Synoptic tables for the design of such frames of the most advance seismic codes i.e., Eurocode 8 and the seismic provisions of American Institute of Steel Construction (AISC) are provided. Emphasizes are made on the provisions of Eurocode 8 both for CCBF and EBF. Finally, a general conclusion is drawn when dealing with CCBF and EBF

Theory of plastic mechanism control for the seismic design of braced frames equipped with friction dampers

An innovative approach for the design of a seismic resistant system composed by the combination of a MR-Frame and a bracing system equipped with friction dampers is presented. From a multi-scale point of view, at local scale, supplementary energy dissipation is provided by means of friction dampers, while, at global scale, the development of a global type mechanism is assured involving all the friction dampers equipping the structure. The activation of all the friction dampers requires an advanced design procedure. Toward this end, the theory of plastic mechanism control, which is based on the application of the kinematic theorem of plastic collapse is extended to the concept of mechanism equilibrium curve, is applied. The fulfillment of the design goal has been pointed out by means of both pushover and dynamic non linear analyses whose results are herein presented and discussed.

Design of Braced Frames in Open Buildings for Wind Loading

Open-frame buildings are often used to provide a measure of weather protection over equipment or other sheltered storage on industrial sites. These shelters have a roof covering and minimal wall cladding, if any. ASCE 7 provides no guidance on the wind loads acting perpendicular to the frames that would control the design of the longitudinal braces. This paper summarizes wind tunnel tests that were performed on open-frame, low-rise buildings with a roof covering to determine the base shear and bracing loads parallel to the ridge. Two examples are provided to illustrate how the results of this study may be used to obtain the forces in longitudinal bracing.

Consideration of seismic demand in the design of braced frames

AbstractThis paper addresses several salient issues associated with the design of typical configurations of concentrically braced (CB) frames, with particular emphasis on European seismic provisions. A brief assessment of the underlying design philosophy and strength demand considerations for capacity design checks incorporated within Eurocode 8 is presented first. It is shown that brace overstrength in conventional CB frames is largely dependent on the idealizations related to the treatment of brace buckling in compression, and that the level of such overstrength decreases as brace slenderness increases when tension‐only design approaches are adopted, as is the case with Eurocode 8. Results from extensive non‐linear response‐history analyses on a set of CB frames are presented, and are used to illustrate the main behavioural aspects related to ductility demands on CB buildings for moderately stiff, stiff and soft soil conditions. These results are also used to examine the applicability of the ‘equal displacement’ rule typically adopted in Eurocode 8 for steel structures. The findings indicate that this rule offers reasonable upper‐bound estimates of deformations in CB structures, but only for those with initial periods > 0.5 s built on moderately stiff to stiff soils and for moderate to relatively low slenderness values. The influence of column stiffness continuity and brace capacity‐to‐demand ratio on the distribution of plasticity in CB buildings is also demonstrated, and its implications on the interpretation of code provisions are highlighted. Finally, based on a parametric investigation of simplified frame models, the study shows that properly designed secondary systems such as gravity frames can play a significant role in mitigating the probabilities of dynamic instability in CB buildings.

Seismic response of stainless steel braced frames

The present paper investigates the feasibility of the application of stainless steel (SS) in the seismic design of braced frames, either concentrically (CBFs) or eccentrically (EBFs) braced. A sample of regular multi-storey CBFs and EBFs was designed in compliance with modern seismic standards based on capacity-design rules. The results of pushover and inelastic response history analyses demonstrate that systems employing SSs exhibit enhanced plastic deformations and excellent energy absorbing capacity with respect to mild steel braced frames. The augmented strain hardening of SS, which is nearly twice that of carbon steels, is beneficial to prevent local buckling in steel members, especially those subjected to high axial compression. The performed analyses also demonstrate that in CBFs with SS braces and columns the increase in overstrength is about 40% with respect to the configuration in mild steel. For EBFs, the use of SS in the diagonals or in braces and links increases the global overstrength of the lateral resisting system by 20%. When the EBFs employ braces and columns in SS the increase can be as high as 50%.

Impact of Recent Research Findings on Eccentrically Braced Frame Design

Design provisions for eccentrically braced frames (EBFs) are based largely on results from ASTM A36 links tested at the University of California, Berkeley (UCB) in the 1980's. To address issues related to the use of ASTM A992 steel in modern EBFs, new experimental testing and finite element modeling of EBF links were performed. A total of twenty-four link specimens were tested at the University of Texas at Austin (UTA). UTA experimental data was used to validate finite element modeling techniques used in a parametric study conducted at the University of California, San Diego (UCSD). This paper highlights findings from these experimental and analytical investigations. The impact of flange width-thickness ratio, intermediate stiffener spacing, and loading protocol on link rotation capacity and failure mode are discussed. Recommendations are given for intermediate stiffener detailing that will improve link rotation capacity. Results justify the flange width-thickness requirement for EBF links in the 2005 AISC Seismic Provisions for Structural Steel Buildings, which is relaxed from the previous limit and will facilitate more economic design of EBFs. The studies also indicate that the link overstrength value adopted in the Provisions, which was originally based on A36 links, is equally valid for A992 links.

An assessment of seismic design practice of steel structures in the United States since the Northridge earthquake

AbstractThe 1994 Northridge earthquake served as a watershed event in the practice of seismic design of structural steel buildings. Damage to steel moment frame and concentrically braced buildings in this earthquake as well as similar types of damage observed in the 1995 Hyogoken‐Nambu earthquake illustrated the need to address design, construction, and quality shortcomings in design standards, methods of construction, and inspection practices. Research demonstrated that moment frame performance could be improved by shifting the location of the plastic hinge from the face of the column, use of higher notch‐tough welding electrodes, rigorous attention to proper fabrication and erection practices, and improved inspection practices emphasizing visual inspection. Changes to braced frame design provisions have emphasized development of full member strength in the connections, which has led to development of buckling restrained systems to better match tensile and compression brace capacities. This paper assesses the impact of post‐earthquake observations and some of the research initiatives that have changed the way in which structural steel seismic systems are designed. It attempts to identify the trends relative to selection of various steel seismic systems in response to perceived performance issues, limitations imposed by design standards and regulatory agencies, and economic factors. Copyright © 2004 John Wiley & Sons, Ltd.

Discussion: Seismic response of concentrically braced steel frames

The article discusses the nonlinear response of a number of braced frame structures designed in accordance with CAN/CSA-S16.1-M89 (CSA 1989) and the National Building Code of Canada (NRC 1990), using a time history analysis. The article considers designs based on the rules for ductile braced frames, nominally ductile braced frames, and frames with no provisions for ductility. Firstly, the writer found some aspects of the paper somewhat confusing, particularly in the authors' use of the terms and when referring to the braces acting in compression. From the information supplied it appears that the failure mechanism in is buckling, not yielding. The use of in reference to a buckling-type failure is unusual. Further, the writer does not see how the discussion of compression ductility is relevant to the performance of an X-braced system; the key element is the tension brace, not the brace. Secondly, from the article, it appears that most of the braces either did not yield or only yielded once in tension. While not conclusive, this may indicate that the approach taken in CSA (1989) and other codes for the design of braced frames is too conservative. This hypothesis is supported by the generally good behaviour of braced frame structures in prior earthquakes, despite the bracing being both substantially smaller in size and more slender than required by current standards. One possible explanation for this apparent discrepancy is the recent focus on the energy dissipation capabilities of ductile elements. Due to their pinched hysteresis loop, steel braces are not able to dissipate energy as well as plastic hinges in either steel or confined concrete. What appears to have been neglected, however, is the isolating effect once the braces have yielded in tension, which reduces the energy braces. The reduced energy input into the structure partially offsets the poorer dissipation capacity of braced frames when compared to structures with plastic hinges. If this explanation is correct, then there are only two ways to rationally design a braced frame structure: either elastically or as a ductile braced frame. For elastic design, it would be prudent to use an R value of less than 1 to account for potential peak responses, as essentially no provisions would be made for ductility. For a ductile braced frame, it is imperative that general yielding can occur in the braces, while the connections and other members must be capable of resisting the forces resulting from the yielding of the braces and the impact that occurs as the braces take up load. As many braces are field-bolted, there is a tendency for the braces to fail at the net section through the bolts before general yielding can occur (Ellingwood 1980). To preclude this type of failure, the writer recommends the area around bolt holes be reinforced, so that the net section at the bolts is greater than the gross area of the brace. This should permit yielding across the gross section of the brace, greatly increasing ductility. This line of reasoning, while different from the authors', results in many of the same conclusions expressed by them.

Guidelines for the design of braced frames with semi-rigid connections

This paper describes the mode of application of the two usual design philosophies-elastic and plastic-to braced frames with semi-rigid connections. The attention of the designer is particularly drawn to the influence of the second-order effects on the frame stability. Practical recommendations that allow one to define the collapse load of the frame in the most convenient way are proposed. Examples illustrate the design principles stated.

Design of Braced Multi-Story Frames by the Plastic Method

Since 1956 an extensive research program has been carried out at Lehigh University to study the inelastic behavior and strength of multi-story frames subjected to various types of applied loads and to develop design methods utilizing the reserved strength in the inelastic range. In this program, because of the difference in load-carrying behavior, multistory frames were divided into two types: braced and unbraced frames, and a separate investigation was made for each type. The results obtained from this research together with the recommended design procedures were presented in a comprehensive set of lecture notes and a design aids booklet, which were prepared for a special Summer Conference on "Plastic Design of Multi-Story Frames" held in August, 1965, at Lehigh University. This paper summarizes the procedure described in the lecture notes for the design of braced frames. A braced frame is defined as a frame in which all resistance to lateral force, sway and frame instability is provided by a specially designed bracing system. The required member sizes for the beams and columns in such a frame are often governed by gravity load. It is therefore possible in the initial stage of design to treat the frame and the bracing as two separate load-carrying systems and to perform independent designs for them. The interaction between the frame and the bracing system is then checked after the approximate member sizes are selected.

A Plastic Method For Unbraced Frame Design

Frequently, because of architectural or functional requirements, no structural bracing can be permitted in the plane of a multi-story frame. Furthermore, with the trend in modern building design towards light curtain wall construction, larger window areas, and removable interior partitions, the bracing effect provided by cladding is often small or unreliable. Because of this situation the designer is faced with the design of an unbraced multi-story frame in which the bare structural skeleton of columns and girders must resist all applied gravity and lateral loads. Certain initial steps in the design of unbraced frames are identical to the same steps in braced frame design. For example, the dimensional layouts in both designs, being based on architectural and functional requirements, will likely be similar. The assignment of load systems and their distribution to the plane frame will be the same. The working loads will also be increased by the same load factors in each design.