Canadian Seismic Design of Steel Structures An Organized Overview

1. Canadian Seismic Design of Steel StructuresAn Organized Overview • By: Alfredo Bohl • University of British Columbia • Department of Civil Engineering • March, 2005 Canadian Seismic Design of Steel Structures - by Alfredo Bohl

2. Introduction • The overview given in this report is based on the provisions contained in: • The upcoming 2005 Edition of the National Building Code of Canada. • Clause 27 of the 8th. Edition of the Handbook of Steel Construction. • Parameters: • V: Design shear force. • S(Ta): Design spectral response acceleration. • Mv: Factor for the higher mode effects on the shear base. • IE: Earthquake importance factor of the structure. • W: Expected weight of the structure. • Rd: Ductility-related force modification factor. • Ro: Overstrength-related force modification factor. Lateral seismic force at the base according to the 2005 NBCC Canadian Seismic Design of Steel Structures - by Alfredo Bohl

3. New force reduction factors in the 2005 NBCC • Ductility-related force modification factor (Rd): • This factor corresponds to the R factor used in the previous 1995 edition. • For steel structures, these have been increased for ductile and moderately ductile systems to 3.5 and 5.0, compared to 3.0 and 4.0 in the previous code. • The design forces for these systems are now lower; however, the details requirements to ensure adequate ductility according to these factors are more demanding. • Overstrength-related force modification factor (Ro): • This factor is related to the calibration factor U used in the previous code. • It takes into account in a more explicit way the overstrength in structures, by identifying the sources of it and assigning factors that consider each of these sources, like the actual strength of the material, rounding up of dimensions of the elements, and redistribution of internal forces. Canadian Seismic Design of Steel Structures - by Alfredo Bohl

4. Steel seismic force resisting systems (SFRS) • Characteristics of SFRS: • The 2005 NBCC recognizes different types of SFRS, their corresponding Rd and Ro factors, and the design and detail requirements for each of them according to the CSA standard CSA-S16-01. • In each of these SFRS, there are certain structural elements which are designed to dissipate energy by inelastic deformation; these must be able to sustain various cycles of inelastic loading with a minimum reduction of strength and stiffness. The other elements and connections must respond elastically to loads induced by yielding elements. • Classification of SFRS according to there ductile behavior: • Ductile or Type D: They can sustain severe inelastic deformations. They have a force reduction factor between 4.0 and 5.0. • Moderately ductile or Type MD: Inelastic deformations are more limited, members are designed to resist greater loads. They have a force reduction factor between 3.0 and 3.5. • Limited ductile or Type LD: These are newly introduced types of frames. Inelastic deformations are even more limited and design loads are greater than in type MD elements. They have a force reduction factor of 2.0. Canadian Seismic Design of Steel Structures - by Alfredo Bohl

5. SFRS with Rd = 5.0 • Ductile moment-resisting frames: • The energy dissipating elements are the beams. • Beams must be capable of plastic hinging without connection failures. • Plastic hinges in columns is only permitted at their base, except for single-storey buildings. • Maximum axial load in columns limited to 0.3AFy for all load combinations, since their flexural resistance deteriorates fast when high axial loads are applied. • Ductile members must be class 1 and capable to undergo inelastic response without stability failures. Canadian Seismic Design of Steel Structures - by Alfredo Bohl

6. SFRS with Rd = 5.0 • Ductile moment-resisting frames: • Limited inelastic deformations are permitted in column joint panel zones if they are properly detailed. • The beam-to-column connections must be capable to develop an inter-storey drift angle of 0.04 rad under cyclic loading. • Advantages: They absorb less shear forces due to their flexibility and have high energy dissipation capacity. • Disadvantages: Large inter-storey drifts may cause severe P-delta effects and non-structural damage. Canadian Seismic Design of Steel Structures - by Alfredo Bohl

7. SFRS with Rd = 5.0 • Ductile plate walls: • Newly introduced system in the CAN/CSA S16-01. • The main energy dissipating element is the web plate, framing elements also dissipate energy once the plate has yielded. • Same requirements for beams, columns, panel zones and connections; except that columns must always be class 1. • Elements are proportioned so that yielding occurs first in the web plate (principle of capacity design). • The top and bottom web plates must also be anchored to stiff elements. Canadian Seismic Design of Steel Structures - by Alfredo Bohl

8. SFRS with Rd = 5.0 • Ductile plate walls: • Columns must be stiffened at the base, so that the plastic hinges form at a certain distance above the base plate. • Advantages: They have very large stiffness, reducing the amount of non-structural damage during an earthquake. • Disadvantages: They may be more expensive; and calculating the tension fields in the plate web and determining the yielding sequence of the plate and the framing system is still a problem, due to the limitations of the strip model. Canadian Seismic Design of Steel Structures - by Alfredo Bohl

9. SFRS with Rd = 4.0 • Ductile eccentrically braced frames: • The energy dissipating elements are the links, which are the beam segments between the brace connections and the beam. • Links must be class 1. • Link rotation limits depend on if it yields in shear or flexure. • Full-depth stiffeners at both ends of the link and intermediate stiffeners are required to make sure that it will have a ductile behavior. • Beams outside the link, braces and columns must be stronger than the link. Canadian Seismic Design of Steel Structures - by Alfredo Bohl

10. SFRS with Rd = 4.0 • Ductile eccentrically braced frames: • If the link is directly connected to the column, the link beam-to-column connection must be able to develop anticipated plastic deformation. • The columns must be designed for secondary moment effects due to the frame drift. • Advantages: Combines the ductile behavior of the moment-resisting frame and the stiffness of the concentrically braced frame. • Disadvantages: Since all the energy dissipation is restricted to the link, the collapse mechanism forms once this element has yielded; while other SFRS are more redundant. Canadian Seismic Design of Steel Structures - by Alfredo Bohl

11. SFRS with Rd = 3.5 • Moderately ductile moment-resisting frames: • Requirements are the same as for ductile moment-resisting frames, except for the following: • Beams must be class 1 or 2. • Maximum axial load in columns limited to 0.5AFy for all load combinations, since their flexural resistance deteriorates fast when high axial loads are applied. • The beam-to-column connections must be capable to develop an inter-storey drift angle of 0.03 rad under cyclic loading. Canadian Seismic Design of Steel Structures - by Alfredo Bohl

12. SFRS with Rd = 3.0 • Moderately ductile concentrically braced frames: • The energy dissipating elements are the diagonal braces. • Only configurations that allow inelastic response without losing stability are permitted, like tension-compression, chevron or tension-only bracing systems. • Because ground motions may occur in any direction, the dimensions of the diagonal braces must be such that the shear resistance in each storey provided by the tension forces developed in these elements is similar for storey shears acting in opposite directions. Canadian Seismic Design of Steel Structures - by Alfredo Bohl

13. SFRS with Rd = 3.0 • Moderately ductile concentrically braced frames: • In tension-compression systems, braces must be class 2 to delay local buckling, but they must yield before the other elements. • In chevron systems, the beams must be strong enough to resist yielding and buckling forces from the braces together with gravity loads, without considering the support from the braces. • In tension-only systems, the energy dissipation capacity is very limited, the braces must be able to carry all the seismic loads. Canadian Seismic Design of Steel Structures - by Alfredo Bohl

14. SFRS with Rd = 3.0 • Moderately ductile concentrically braced frames: • Brace connections must have a ductile rotational performance if high inelastic response is expected. • Beams, columns and connections must resist forces induced by yielding of the braces. • The columns must be designed for secondary moment effects due to the frame drift. • Advantages: They have very large stiffness. • Disadvantages: They tend to have a soft-storey response, so height restrictions apply depending on the bracing system used. Canadian Seismic Design of Steel Structures - by Alfredo Bohl

15. SFRS with Rd = 2.0 • Moment-resisting frames with limited ductility: • Requirements are the same as for moderately ductile moment-resisting frames, except for the following: • Beams must be class 1 or 2, while columns must be class 1 and I-shaped. • In high seismic areas, buildings with this system cannot exceed 12 storeys. • The beam-to-column connections must be capable to develop an inter-storey drift angle of 0.02 rad under cyclic loading. Canadian Seismic Design of Steel Structures - by Alfredo Bohl

16. SFRS with Rd = 2.0 • Limited ductility plate walls: • Requirements are the same as for ductile plate walls, except for the following: • The energy dissipating element is the web plate, not the framing elements. • Beams, columns and their connections do not have any special requirements, since they are not expected to yield. • Buildings with this type of system cannot exceed 12 storeys. • Limited ductility concentrically braced frames: • Requirements are the same as for moderately ductile concentrically braced frames, except for the following: • Height restrictions are relaxed, since elements are designed for higher forces. • Diagonal braces can be class 2 or lower in low seismic areas. • Brace connections do not need to have a ductile rotational performance in low seismic areas if braces are slender. Canadian Seismic Design of Steel Structures - by Alfredo Bohl

17. Physical tests for connections in moment-resisting frames • Characteristics: • Connections in type D and MD frames must be tested to ensure that they satisfy certain deformation criteria under cyclic loading. • Testing procedures are described in the FEMA 350 document. • The test assemblies must represent the prototype characteristics, and the test loading the deformation magnitude and cyclic nature. • For each given combination of beam and column size, tests of at least two specimens must be performed. The results obtained must be able to predict the mean value of the drift angle capacity of the connection. Canadian Seismic Design of Steel Structures - by Alfredo Bohl

18. Physical tests for connections in moment-resisting frames • Characteristics: • The size of the beam used in the specimen must be at least the largest depth and heaviest weight used in the structure. • The column must provide a flexural strength consistent with the requirements of strong-column-weak-beam connections, and must have a height similar to the real column, so that the drift angles obtained are representative of the real structure. • The mean drift angle capacity must not be less than a certain limit. • Tests results must be supported with analytical design procedures. Canadian Seismic Design of Steel Structures - by Alfredo Bohl

19. Moment-resisting connections for seismic applications • Bolted unstiffened end plate connection: • The beam is welded to an end plate, extended above and below the flanges. The beam flange-to-plate joints have complete-penetration-groove welds, and the beam web is connected to the plate with fillet or complete-joint-penetration-groove welds. Then, the end plate is bolted to the column using eight bolts. • Design principle: Member sizes are selected to preclude brittle failure modes, and for yielding to occur as a combination of beam flexure and panel zone yielding. This applies also for the other connections. Canadian Seismic Design of Steel Structures - by Alfredo Bohl

20. Moment-resisting connections for seismic applications • Bolted stiffened end plate connection: • The beam is welded to an end plate. The beam flange-to-plate joints have complete-penetration-groove welds, and the beam web is connected to the plate with fillet or complete-joint-penetration-groove welds. The end plate extensions at the top and bottom of the beam are stiffened with vertical stiffeners that extend outward from beam flanges. Then, the end plate is bolted to the column using 16 bolts. • Elements must have similar sizes and details to those that were tested, to predict their behavior. This applies also for the other connections. Canadian Seismic Design of Steel Structures - by Alfredo Bohl

21. Moment-resisting connections for seismic applications • Reduced beam section connection: • The flexural resistance of the beam is reduced at a certain distance from the connection, so that yielding and plastic hinging occurs in the beam. The top and bottom beam flanges have circular radius cuts for this purpose. The flanges of the beam are connected to the columns only with complete joint penetration groove welds. A shear tab, that can be bolted or welded, is used for the web connection. • This type of connection cannot be used for type LD frames, but the previous two connections may be used for these cases. Canadian Seismic Design of Steel Structures - by Alfredo Bohl

22. Special seismic steel framing systems • Special truss moment frames: • This system is designed in such a way that the inelastic deformation is moved to some segments of the truss that are specially designed. This truss has several diagonal members in a segment at the midspan designed for this purpose, they absorb most of the energy and dissipate it by yielding. After the earthquake, the diagonal members that were damaged can easily be repaired or replaced. • This system has the advantage that it weighs less than common framing systems. Also, it provides substantial cost and time savings compared to these systems. Canadian Seismic Design of Steel Structures - by Alfredo Bohl

23. Special seismic steel framing systems • Friction-damped steel frames: • Friction dampers are designed in such a way they have moving parts that will slide over each other during a strong earthquake. Friction is created between these sliding elements, which dissipates energy built up in the structure. Examples of these are the basic sliding joint, the rotation sliding joint, the dual level joint, the Pall friction device and the Sumitomo friction device. • They have the advantage that their behavior is not seriously affected by repeated cycles of displacement, the friction force between surfaces can be controlled, and they are not affected by fatigue. Basic sliding joint Rotating sliding joint Pall friction device Canadian Seismic Design of Steel Structures - by Alfredo Bohl

24. Conclusions • There may be cases in which it might not be possible to use the prequalified connections, and physical tests are usually very expensive and cannot be afforded by small engineering companies. More research is needed to develop design procedures for various types of connections with different element sections. • Systems like the special truss moment frame and the friction-damped steel frame provide safe and economic solutions compared to conventional framing systems, and may be implemented in future codes. • An overall overview of the seismic design of steel structures in Canada has been carried out. The design procedures for SFRS, moment-resisting connections, and some special framing systems, which are spread in various documents and publications, have all been organized in this report. • Each of the SFRS presented have their own advantages and disadvantages. Canadian Seismic Design of Steel Structures - by Alfredo Bohl

25. End of the presentationThanks Canadian Seismic Design of Steel Structures - by Alfredo Bohl