Seismic Design of Steel Column-Tree Moment-Resisting Frames

For moment-resistant frames with column beams, short beam struts are welded to the column in the workshop and then the center section of the beam spans are bolted to the column beams in the field. The focus of the report is on the seismic behavior and design of special moment-resistant steel column-tree frames. The information in this report on the seismic design of column tree systems is based on available data on the behavior of components of the system.

The publication of this report was made possible in part by support from the Structural Steel Educational Council (SSEC). One of the most common types of steel construction system is the moment-resisting frame system shown in Figure 1.1. One of the very effective shop welded and field bolted systems is the column timber system.

In a column-tree system, short segments of beams or a short built-up beam, usually two to four feet long, are attached to the columns in the shop. Types of moment-resisting column-tree frames based on connection detailsThe connecting connection of column-trees to beams can be fully glued, welded and.

Figure 1.1. A Typical Steel Moment Frame

Types of Column-Tree Moment-Resisting Frames Based on Splice Details The splice connection of the column-trees to girders can be fully bolted, welded and

Types of Column-Tree Moment-Resisting Frames Based on Ductility

When a framing system cannot be categorized as a special moment-resisting frame and is therefore categorized as an ordinary moment-resisting frame, the reduction factor R used in seismic design is given as 6 of the seismic design codes (UBC-94) . The reduction factor for ordinary moment-resisting frames is half the reduction factor for special moment-resisting frames. As a result, the design seismic forces for the same building using common moment frames will be twice the design seismic forces if special moment frames are used.

Therefore, it makes economic sense and is safer to use special ductile moment frames instead of ordinary moment frames.

Types of Column-Tree Moment-Resisting Frames Based on Stiffness

Therefore, if the beam stem length is less than 15% of the span, the ¥ parameters of the above equations are very close and can be assumed to be approximate. The above categorization is based solely on the elastic rotational stiffness of single-span connections and girders. In seismic design, however, the plastic capacity of connections and girders must also be taken into account when categorizing the span.

It is suggested that at design the values of m and cz are the average value of m and tx for the mid-floor spans of the frame. With these trusses, the welded connection of the girder is designed to be stronger than the connected beams. As a result, the connection after assembly does not play a significant role in the seismic performance of the frame.

To use the connection to control and improve the seismic performance, semi-rigid versions of the column-tree moment-resisting frame system were proposed by A. In the proposed semi-rigid column-tree, the bolted connection of the beam was placed away from the column, is semi-rigid. One of the main advantages of a semi-rigid column-boom system over the standard rigid system is that the semi-rigid bolted connection, located on the beam connection, can act as a.

The strong column-weak beam frames are very frequently used, and many structural engineers believe that these systems have superior seismic behavior compared to the weak column-strong beam frames. Most current codes (UBC, 1994) also promote the use of the strong column-weak beam philosophy. In the strong column weak beam frame, the moment capacity of the beams in a joint is less than the moment capacity of the columns.

One of the advantages of the column beam system is that by selecting an appropriate moment capacity for the connection of the beam, the connection will act as a moment fusion and prevent large moments from developing at the face of the column.

Figure 1.6 shows the above three regions of the moment-rotation behavior based on the relative rotational stiffness of the connection and the girder in the frame

SEISMIC BEHAVIOR OF STEEL COLUMN-

TREE MOMENT-

RESISTING FRAMES

Expected Seismic Behavior of RIGID Column-Tree Moment Frames

If the above conditions are met, the column timber splices are stiffer and stronger than the girders. This means that the resulting column-timber moment-resisting system will behave like a traditional ductile frame. The plastic hinges are expected to form on the face of columns, while girder splices are expected to remain elastic.

Therefore, the splices in this case do not act as fuses, but are simply setup splices that allow the frame to be manufactured as a shop-welded steel frame. Laboratory shaking table tests and analytical studies (Nader and Astaneh-Asl, 1992 and 1996) have shown that these limited joint slips do not result in appreciable increase in drift during the earthquakes. Expected Seismic Behavior of SEMI-RIGID Column-Tree Moment Frames If one of the equations 2.1 and 2.2 above is not satisfied in a column tree, the frame may.

Expected Seismic Behavior of SEMI-RIGID Column-Tree Moment Frames If in a column tree either one of Equations 2.1 and 2.2 above is not satisfied, the frame can

CODE PROVISIONS RELEVANT TO STEEL

MOMENT FRAMES

SEISMIC DESIGN OF STEEL COLUMN-TREE

MOMENT-RESISTING FRAMES

Design Considerations
Criteria for Design of Components of Column-tree Frames The girder splices are suggested to be designed to satisfy the following
Design of Girder Splice
a. Slippage of Flange Bolts
c. Bearing Yielding of Bolt Holes in Girder Flange and Splice Plates
g. Shear Yielding of the Column Panel Zone
i. Block Shear Failure of Flange Splice Plates
k. Fracture of the Welds Connecting the Splice Plates to Girder Flanges

Fracture of Net Area of the Flange Splice Plates

m. Block Shear Failure of Girder Flanges
n. Fracture of Edge Distance or Bolt Spacing in Flanges of the Girder
p. Fracture of Web Bolts
q. Fracture of Net Area of Web Plate

The plastic moment capacity of the beam joint need not be greater than the plastic moment capacity of the beam. The joint of the girder must be designed to regulate the capacity of the gross area of the plates in the joint. The slab zone in the column must have a shear strength 1.2 times the shear strength due to the Mp beams connected to the slab zone.

Yield of the gross area of the web joint due to combined shear and bending. Slip of flange bolts, followed by yielding of the flange plates, are the most desirable failure modes (first two failure modes in the above list). Fracture of the net area of the beam (the last item in the above list) is the least desirable failure mode.

Based on experience and intuition, it is suggested here that the slip moment is less than 0.8 times the plastic moment capacity of the splice. In order to increase the ductility of the connection, yielding of flange splice plates should be the governing failure mode. By doing this, the yielding of the splice plates acts as a fuse to protect the welded connection between the girder and the column.

The clearance of the bolt hole bearings is useful in reducing the seismic response during extreme events. To avoid early buckling of flanged joint plates, it is recommended that the plate slenderness, ratio KL/r, not exceed 20. The Uniform Building Code allows limited yielding of plate areas in special moment frames (UBC, 1994).

The welds on splice plates are usually fillet welds and must be designed to develop 1.25 times the axial flow capacity of the plates. The splice plates must be designed in such a way that breaking of the plates does not occur before the beam yields. It is suggested that for ductile behavior the strength of the bolts should be greater than the strength of the plates.

Figure 4.1. Failure Modes of Top and Bottom Flange Plate Connections Ductile Failure Modes:

Establishing Stiffness of the Girder splice Connection

However, if more accurate calculation of displacements is desired, especially drift values under factored loads, the flexibility of the connection due to slippage of bolts should be included. It should be added that throughout this report emphasis was placed on seismic design. However, the final design of the connection will be governed by load combinations including the wind load.

Such an approach will ensure that the connections will not slip during the service wind and small to moderate earthquakes. However, the bolt slip during the large earthquakes can be beneficial to distribute energy in the form of friction, extend the period of the structure as well as isolate the connections and cut off the flow of seismic energy in the structure. Seismic design of steel column-beam moment-resisting frames © by Abolhassan Astaneh-Asl, Steel Tips, Apd11997 24.

AISC (1994), Manual of Steel Construction - Load and Resistance Factor Design, 2e editie, 2 delen, American Institute of Steel Construction, Chicago. Astaneh-Asl, A The Innovative Concept of Semi-rigid Composite Beam", Proceedings, Structures Congress, ASCE, Irvine, april. Astaneh-Asl, A Seismic Design of Bolted Steel Moment-Resisting Frames", Steel Tips Report, Constructief staal Educatief Raad, Moraga, CA, juli.

D Seismic Behavior and Design in Semi-Rigid Frames", Proceedings, AISC, 1991 National Conference on Steel Structures, Washington, D. O, by Abolhassan Astaneh-Asl, Steel Tips, April A Comparative Study of the Seismic Performance of Steel Structures with Semi-Rigid Joints", Proceedings, ASCE- Structures Congress, 91, Indianapolis, April 29-1. May, p. Astaneh-Asl Design of Single Plate Shear Connections with Snug-tight Bolts in Short Slotted Holes," Report No.

SAC Joint Venture Interim Guidelines: Evaluation, Repair, Modification and Design of Welded Steel Moment Frame Structures", Rapport FEMA 267, Federal Emergency Management Agency, Washington D.C.

Index of Steel Tips Publications

1991) "A Comparative Study of the Seismic Performance of Steel Structures with Semi-rigid Joints", Proceedings, ASCE- Structures Congress, 91, Indianapolis, 29 April-1 Mei, pp.

APPENDIX

A NUMERICAL EXAMPLE

A Numerical Example

The factored shear and bending moment in the connection is shown in the above figure. The lefi side connection of the joint, which has the greatest forces, is designed in this example. Maximum factored bending moment in the connection: Mu = 636 •kips No significant axial load exists in the beam.

Seismic Design of Steel Column-Tree Moment Resistant Frames © by Abolhassan Astaneh-Asl, Steel Tips, Aptfl 1997 28. Maximum considered bending moment in the beam connection: Ms = 422 ft-kips No significant axial load exists in the beam connection. The above service torque will be used when designing beam connection bolts to ensure that the connection does not slip under the service loads.

Seismic design of steel column-timber moment-resisting frames © by Abolhassan Astaneh-Asl, Steel Tips, Ap/fl 1997 3 0. Seismic design of steel column-timber moment-resisting frames © by Abolhassan Astaneh-Asl, Steel Tips, Apr. # 1997 31. To design the bolts, use the tables in Volume II of the AISC-LRFD Manual (AISC, 1994).

Seismic Design of Moment Resisting Steel Column-Tree Frames © by Abolhassan Astaneh-Asl, Steel Tips, Ap•f11997 32. Full penetration welds connecting the beam flanges to the column face should be made using material and in compliance with procedures resulting in ductile welds. The fillet welds connecting the web of the beam to the column flange are designed according to the criterion.

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