In the braced-frame category, ordinary concentrically-braced frames (OCBFs) and special concentrically-braced frames (SCBFs) are intended to exhibit modest and high levels of inelastic response, respectively.
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What is a special concentrically braced frame?
Special Concentrically Braced Frames (SCBFs) are a special class of CBF that are proportioned and detailed to maximize inelastic drift capacity. This type of CBF system is defined for structural steel and composite structures only.
Does ASCE 7 allow the use of ordinary Concentrically braced frames?
ASCE 7 also allows the use of Ordinary Concentrically Braced Frames (OCBFs). However, SCBFs are designed with relatively large Rfactors, and as a consequence are expected to experience relatively large inelastic deformation demands during extreme ground shaking.
What are the different types of braced frames?
There are two main varieties of braced frames – concentric and eccentric. Concentric bracing consists of diagonal braces located in the plane of the frame. Both ends of the brace join at the end points of other framing members to form a truss, creating a stiff frame.
What are the different types of concentric bracing?
Concentric bracing may be arranged in several different configurations – such as X, K or one-directional diagonal bracing – and the bracing members may be designed to act in tension or compression or both. Balanced diagonal bracing is the most common for medium-rise structures because it provides the same strength in both directions.
What is ordinary concentrically braced frames?
Braced frames are a common seismic lateral force resisting system used in steel structures. Ordinary concentric braced frames (OCBFs) and special concentric braced frames (SCBFs) are two major types of frames. Brace layouts vary for both OCBFs and SCBFs.
What is the difference between a moment frame and a braced frame?
In moment resisting frames, the structural members are joined together using rigid joints which transfer moment....Table 1: Difference Between Braced Frame and Moment Resisting frame.Braced frameMoment resisting frameThe connection does not transfer moments.Moments are transferred through the connection.10 more rows•May 18, 2019
What is braced frame system?
A braced frame is a structural system designed to resist wind and earthquake forces. Members in a braced frame are not allowed to sway laterally (which can be done using shear wall or a diagonal steel sections, similar to a truss).
What is a multi tiered braced frame?
Multi-tiered braced frames (MT-BFs) consist of multiple vertically oriented bracing panels that lack intersecting perpendicular framing or diaphragms at the levels between the bracing panels. Due to the ductility demands during a seismic event these frames require special consideration.
What is the difference between ordinary and special moment resisting frames?
Ordinary Moment Resisting Frame (OMRF) is a moment-resisting frame not meeting special detailing requirements for ductile behavior. Special Moment Resisting Frame (SMRF) is a moment- resisting frame specially detailed to provide ductile behavior and comply with the requirements given in IS-4326 or IS-13920 or SP6.
What is a special moment frame?
Structural steel special moment frames (SMF) are typically comprised of wide-flange beams, columns, and beam-column connections. Connections are proportioned and detailed to resist internal forces (flexural, axial, and shear) that result from imposed displacement as a result of wind or earthquake ground shaking.
What are the different types of bracing?
Bracing can be classified into three types:Plan bracing.Torsional bracing.U-frame bracing.
How many types of braces are there?
There are 5 main types of braces available today: Metal braces. Ceramic braces. Self-ligating braces. Lingual braces.
Which types of bracing are commonly used in tall buildings?
There are two major bracing systems:Vertical bracing system.Horizontal bracing system.
What is the purpose of cross bracing?
Cross bracing is used to keep buildings stable when the wind blows and during seismic events, such as an earthquake. It also limits the building's lateral movement, reducing the likelihood of damage to the structure's components and cladding.
What is tension only bracing?
Braces in a tension-only braced frame are designed to resist, in tension, 100 per cent of lateral shear forces in the braced frame. Since the resistance of braces acting in compression is ignored in the design, tension-only braced systems must be direct-acting and concentrically braced frames.
What is horizontal bracing?
Horizontal bracing is employed to resist horizontal / lateral loads on the structure and distribute them to the outer columns and thereby into the vertical stabilizing system. Horizontal bracing will also maintain the planar integrity of the structure and prevent it from deforming out of shape.
What brace configurations are used in opposing pairs?
Other bracing configurations, such as the X-brace, multistory X-brace and chevron brace directly achieve this balance. X-bracing is most commonly used with light bracing on shorter structures. Research shows that the buckling capacity of X-bracing is best estimated by using one half the brace length when the braces intersect and connect at mid section (Palmer 2012). However, the inelastic deformation capacity of the X-braced system is somewhat reduced from that achievable with many other braced frame systems because the inelastic deformation is concentrated in one-half the brace length because the other half of the brace cannot fully develop its capacity as the more damaged half deteriorates. The compressive buckling resistance of most other brace configurations is best estimated by considering true end-to-end length of the brace with an effective length factor, K, of 1.0 (i.e., neglecting rotation stiffness of the brace- to-gusset connection.) Concentration of inelastic deformation in a limited number of stories occurs with braced frames. Experiments suggest that multistory X-bracing offers a slight advantage in that it provides a somewhat more robust path for transferring story shear to adjacent stories even after brace buckling and fracture because the remaining tension brace may directly transfer its force to the next story. Chevron or inverted-chevron bracing (inverted V- or V-bracing) has intersecting brace connections
Why use a braced frame for seismic retrofit?
Seismic Retrofit Braced frames can be an effective system for seismic retrofit due to their high stiffness and because they can be assembled from pieces of relatively small size and weight. SCBFs may be considered for seismic retrofit in cases in which the building deformations corresponding to brace axial ductility are not detrimental to the building performance. In many retrofit projects this is not the case due to the presence of brittle, archaic materials and sensitive finishes not detailed to accommodate significant drift. In such cases, the added drift capacity provided by the careful proportioning and detailing required for the SCBF system is of little benefit, and a conventional braced frame system or other stiff system should be considered instead.
What is the inelastic deformation of a brace?
Inelastic deformation of the brace dominates the inelastic performance of SCBFs during moderate and large earthquakes, and fracture of the brace at mid-length is clearly the anticipated initial failure mode of the braced frame system. A number of brace design issues affect the inelastic deformation and ultimate fracture of the brace as illustrated in the sketch of
What is brace buckling?
System performance is strongly influenced by aspects of brace behavior (Lehman et al. 2008). Brace buckling places large inelastic demands on the brace at the middle of the brace, typically resulting in a plastic hinge at midspan (Figure 3-1a). Brace buckling also places significant demands on gusset plate connections (Figure 3-1b) and adjacent framing members (Figure 3-1c). Limited cracking of the welds joining the gusset plate to the beams and columns generally is expected because of gusset plate deformation. These cracks normally initiate at story drifts in the range of 1.5 % to 2.0 %, but the cracks remain stable if the welds meet size and demand-critical weld requirements in AISC 341. Current design criteria encourage conservative gusset plate design, but overly conservative gusset plate design can increase the inelastic deformation in the beams and columns adjacent to the gusset plate and does not significantly reduce the deformation of the gusset plate or the demands on the weld. Gusset plate damage and the weld cracking are largely driven by the brace end rotations and the opening and closing of the right angle of the connection. Configuration Issues The configuration of braces also affects system performance. Multiple configurations of bracing are used, and these configurations are identified in Figure 3-2. Braces buckle in compression and yield in tension. The initial compressive buckling capacity is smaller than the tensile yield force, and for subsequent buckling cycles, the buckling capacity is further reduced by the prior inelastic excursion. Therefore, bracing systems must be balanced so that the lateral resistance in tension and compression is similar in both directions. This means that diagonal bracing (Figure 3-2) must be used in matched tensile and compressive pairs. As a result, diagonal Figure 3-1– Various aspects of braced frame behavior.
What is SCBF in construction?
2 The SCBF system is generally an economical system to use for low-rise buildings in areas of high seismicity. It is sometimes preferred over Special Moment Frames because of the material efficiency of CBFs and the smaller required beam and column depths. SCBFs are only possible for buildings that can accommodate the braces in their architecture. Buildings for which this a problem may be well suited for Special Moment Frames. Up to the present, SCBFs have been used more extensively than Buckling-Restrained Braced Frames (BRBFs). BRBFs generally offer cost and performance advantages for buildings three stories and higher, but SCBFs continue to be popular because of the level of experience designers and fabricators have with the system. The desired performance of the SCBF system is based on providing high levels of brace ductility to achieve large inelastic drifts. It is not particularly well suited for applications in which the seismic demands are low. The capacity design rules for connections can be uneconomical in cases where brace sizes are governed by wind loads or by slenderness limits. SCBFs are designed using capacity design procedures, with the braces serving as the fuses of the system. Optimal design of SCBFs entails careful selection and proportioning of braces so as to provide limited overstrength and avoid a concentration of inelastic demands. Designers should strive for a small range of brace demand-to-capacity ratios so that the resulting system is proportioned to spread yielding over multiple stories rather than concentrating it at a single location. Overstrength can be beneficial, but care should be taken to maintain a well-proportioned design in order to avoid concentration of ductility demands. Braced frames are most effective at the building perimeter, where they can control the building’s torsional response. ASCE 7 allows buildings to be considered sufficiently redundant (and thus avoid a penalty factor) with two braced bays on each of the presumed four outer lines (assuming a rectangular layout). Such a layout is good for torsion control as well. In mid-rise or high-rise buildings, SCBFs are often used in the core of the structure, with a perimeter moment frame used to provide additional torsional resistance. Stacked braced frames (frames in which the braces occupy the same plan location at each level) can have high overturning forces. In many cases it is advantageous to spread the overturning forces out over several bays to reduce foundation and anchorage forces. The design of elements interconnecting these frames is critical to ensure that brace ductility remains the primary source of inelastic drift.
What is a CBF?
CBFs are a common structural steel or composite system in areas of any seismicity. Special Concentrically Braced Frames (SCBFs) are a special class of CBF that are proportioned and detailed to maximize inelastic drift capacity. This type of CBF system is defined for structural steel and composite structures only.
How many gusset plates are there in a SCBF?
8 gusset plate connections (Roeder et al. 2011). The specific distribution clearly depends on the specifics of the design, but this comparison illustrates the importance of the gusset plate and beam-to-column connection in developing the resistance and deformation capacity of the SCBF and the subsequent moment frame behavior developed after brace buckling and fracture.
Which bracing is most common for medium-rise structures?
Balanced diagonal bracing is the most common for medium-rise structures because it provides the same strength in both directions. Efficient energy dissipation is difficult to achieve in concentrically braced frames. Common types of concentric bracing.
Where is the eccentric brace located?
Eccentric bracing consists of diagonal braces located in the plane of the frame where one or both ends of the brace do not join at the end points of other framing members. The system essentially combines the features of a moment frame and a concentrically braced frame, while minimising the disadvantages of each system.
What is bracing in engineering?
Another fundamental concept in engineering – bracing – involves added additional elements to a frame in order to increase its ability to withstand lateral loads . There are two main varieties of braced frames – concentric and eccentric.
What is eccentric connection?
The eccentric connection to the frame means an eccentric brace transfers lateral forces via shear either to another brace or to a vertical column. When properly proportioned, eccentric braced frames may exhibit a more ductile characteristic and greater energy dissipation capabilities than a concentric braced frame in the same material.