Steel Beam And Steel Column Without Concrete Encasement

Steel "zipper column" resists unbalanced post-buckling forces in the braces.

Steel brace

Zipper-col with inverted Y-bracing

Figure 5.33. Composite concentrically braced frames: (a) V-bracing; (b) inverted V-bracing; (c) X-bracing; (d) diagonal bracing; (e) two-story X-bracing; (f) zipper column with inverted V-bracing.

Composite braces of either concrete-filled steel tubes or concrete-encased steel braces may be combined with steel frame elements. Composite columns may also be used in conjunction with composite floors and steel bracing members, as used frequently in the design of tall buildings.

The lateral load capacity of a concentrically braced composite frame is somewhat limited for seismic loading. This is because the energy-dissipation capacity of brace elements deteriorates during repeated inelastic cycles. For small or moderate earthquakes where the braced frame elements remain essentially elastic, the response of these frames can be expected to be satisfactory. Certain techniques such as filling steel tubes with concrete may be used to inhibit the onset of local buckling and thereby improve the cyclic response of the brace elements.

Design of connections should be similar to that of steel-braced frames, where the connections are intended to develop the capacity of the brace elements. Where composite elements are used, the connection design must consider the increased capacity caused by the addition of concrete to the steel bracing elements.

The design of elements in composite concentrically braced frames is similar to the design of corresponding elements in steel and concrete systems. Encased composite columns should have a minimum ratio of structural steel to gross column area of 4%. Transfer of forces between the structural steel and reinforced concrete portions of the section should be made through shear connectors, ignoring the contribution of bond or friction. The capacity design of reinforced concrete columns should meet the requirements for columns in ordinary moment-resisting frames. The detailing of both composite and reinforced concrete columns should provide ductility comparable to that of composite ordinary moment-resisting frames.

Composite brace design in concentrically braced frames must recognize that these elements are expected to provide the inelastic action during large seismic overloads. Braces consisting of concrete-encased steel elements should include reinforcing and confinement steel sufficient to provide the intended stiffening effect even after the brace has buckled during multiple cycles of seismic motion. As a result, it is recommended that these elements should meet detailing requirements similar to those for composite columns. Composite braces in tension should be designed considering only the structural steel.

The general intent of the connection design is to provide strength to develop the capacity of the braces in tension or compression. For composite brace sections, the additional strength of the concrete must be considered, since it would be unconservative to consider only the strength of the structural steel section. Brace buckling and the resulting large rotation demands which could result at the brace ends should be considered in connection detailing. Two schematics of composite concentric bracing connections are shown in Figs. 5.34 and 5.35.

Transfer of loads between structural steel and reinforced concrete elements of a composite-braced frame should be made only through shear friction and direct bearing. Reliance on bond and adhesion should not be considered because of the cyclic nature of the lateral loading. In addition, where shear-friction equations are used in the calculation of connection transfer forces, AISC 341-02, i.e., AISC Seismic, recommends that a 25% reduction in the typical shear-friction capacities be imposed for buildings in areas of high seismicity.

Composite concentrically braced frames are permitted similar to composite eccentrically braced frames in SDC A, B, or C without any restrictions on height. The values of

Concrete Filled Tube Columns

Elevation

Figure 5.34. Concrete-filled composite column-to-steel concentric brace connection.

Elevation

Figure 5.34. Concrete-filled composite column-to-steel concentric brace connection.

Composite Construction Column Junction
Figure 5.35. Concrete-encased composite column-to-steel concentric brace connection.

seismic design factors are: R = 5, = 2, and Cd = 4.5. Buildings in SDC D or E are permitted with a height limitation of 160 feet. The height limit for buildings in SDC F is 100 ft.

5.5.2.2. Eccentrically Braced Frames

The beam elements of composite eccentrically braced frames will generally consist of structural steel elements. Any concrete encasement of the beam elements should not extend into the link regions where large inelastic action is developed (Fig. 5.36). The column and brace elements of these frames can be of either structural steel or composite construction with structural steel and reinforced concrete. The analysis, design, and detailing of the frames is similar to that required for steel eccentrically braced frames. Since the force transfer mechanisms between the elements of a composite frame rely on bearing and shear friction, special attention must be paid to the design of these connections to realize the intended inelastic action in the ductile link members.

Composite action of the concrete slab with the structural steel link beam may become significant in determining the initial capacity of the link section. This should be considered in sizing the brace and column elements.

The design of composite columns must consider the maximum load that will be generated by yielding and strain hardening of the link beam elements, similar to those required for steel columns. Encased composite columns should have a minimum ratio of structural steel-to-gross column area of 4% unless they are designed as reinforced concrete columns. Transfer of forces between the structural steel and reinforced concrete portions of the section should be made through shear connectors, ignoring the contribution of bond or friction. The capacity design of reinforced concrete and encased composite columns in these frames should meet the requirements for columns in ordinary moment-resisting frames. The detailing of both encased composite and reinforced concrete columns should provide ductility comparable to that of intermediate moment-resisting frames. In addition, for higher-performance categories, these columns should meet the transverse reinforcement requirements for special moment-resisting frames. This requirement is extended to all performance categories when the link element is located adjacent to the column.

Composite brace design in eccentrically braced frames must recognize that these elements are intended to remain essentially elastic during large seismic overloads, since they are designed to be strong enough to yield the link beam elements. The design strength of these braces must consider the yielding and significant strain hardening which can occur

Steel link beam

Steel link beam

Walkway Bracing
Steel link beam
Contreventements Charpente Tallique
Figure 5.36. Examples of composite eccentrically braced frames.

in properly designed and detailed link elements. Both axial and bending forces generated in the braces by the strain-hardened link beams must be considered. Braces should therefore be designed to meet detailing requirements similar to those for columns. Composite braces in tension should be designed considering only the structural steel.

The general intent of the connection design is to provide strength to develop the capacity of the link-beam elements. For composite braces, the additional strength of the concrete must be considered, since it would be unconservative to consider only the strength of the structural steel section. Where the shear link is not adjacent to the column, the connections between the braces and columns are similar to those in composite concentrically braced frames. Where the shear link is adjacent to the columns, the connections should be detailed similar to composite beam-column connections in special moment-resisting frames. The large rotation demands that could result at the ends of the link beams should be considered in detailing the connections of composite eccentrically braced frames. Schematic details for two locations of link are shown in Fig. 5.37.

Composite eccentrically braced frames are permitted for buildings in SDC A, B, or C without any height restrictions. The values for seismic design factors are: R = 8, = 2, and Cd = 4. For buildings in SDC D or E, the height limit is 160 feet, and for

Short Steel Links Behaviour
Figure 5.37. Schematic details of link beams: (a) link at center of beam; (b) link adjacent to column.

those in SDC F, the limit is 100 feet. Note that the height limits for both concentric and eccentric composite braced frames are the same.

5.5.3. Composite Shear Walls

One of the most common types of composite shear walls consists of a structural steel frame in which some bays are encased in a reinforced concrete wall. In essence, this results in a reinforced concrete shear wall with structural steel boundary elements and coupling beams. The steel coupling beam is subjected to high shear and moment at each end, requiring a moment-resisting connection to the column. A strong shear connection is also invariably required to resist high shear forces.

If the coupling beams were pin-connected at each end to the boundary elements, they would be ineffective in improving the lateral resistance of the wall; the two wall piers would resist lateral loads independently. On the other hand, if the coupling beams are infinitely stiff, they can fully couple the two piers and make them work as a single unit. If the coupling beam stiffness is in between the two extremes, as is the case in most practical buildings, a portion of the lateral forces will be resisted by the overall system and a portion by the individual elements, typically resulting in an economical structure.

Adding reinforced concrete or structural steel to an existing structural system to achieve composite action of shear walls is a prevalent method of retrofit for resisting lateral loads, particularly in seismic strengthening of buildings.

AISC 7-02, IBC-03, and NFPA 5000 recognize three types of composite shear walls with their attendant seismic factors as follows:

• Composite steel plate shear walls, R = 6.5, = 2.5, and Cd = 5.5.

• Special composite-reinforced concrete shear walls with steel elements, R = 6, = 2.5, and Cd = 5.

• Ordinary composite-reinforced concrete shear walls with steel elements, R = 5, Qo = 2.5, and Cd = 4.25.

The height limitations for the first two types of composite shear walls are 160 ft for buildings in SDC D or E, and 100 ft for buildings in SDC F. There are no height limits

Concrete stiffening on one

Concrete stiffening on one

Composite Shear Wall Fema
Shear studs
Composite Shear Wall Resisting System
(b)
Composite Braced Frames

Figure 5.38. Composite steel plate shear walls.

Figure 5.38. Composite steel plate shear walls.

for buildings in SDC A, B, or C using these two types of walls. Ordinary composite shear walls are not permitted for buildings in SDC D, E, or F. They are permitted for buildings in SDC A, B, or C without any height limit.

Composite steel plate shear walls are appropriate when extremely high shear forces must be resisted by a limited length of walls. An example of this use may be found in the 76-story Bank of China Tower, Hong Kong, in which the entire base shear is transferred to the building core at the base.

Possible details for concrete-encased shear plates are shown in Fig. 5.38. In these details, structural steel framing surrounds the steel plates with entire steel assembly encased in reinforced concrete. The steel columns not only resist gravity loads but also act as boundary members resisting overturning forces. The shear-wall web is a steel plate welded to the boundary members. A simple practical detail would be to provide a steel tab continuously fillet-welded in the shop to the beams and columns. The shear-wall steel plate can then be attached to the tabs of the beams and columns with erection bolts. Field fillet welds can then be installed between the steel plate and the tabs. If the plates need to be installed in pieces because of size limitations in shipping or erection, field splices can be of simple fillet welds using a common back-up plate. If there are openings in the wall, additional steel boundary members or flanges must be installed as required.

Continue cross ties for a distance of

Continue cross ties for a distance of

(a)

Continue cross ties for a distance 2h to

Continue cross ties for a distance 2h to

Figure 5.39. Composite shear walls with steel boundary elements.

Figure 5.39. Composite shear walls with steel boundary elements.

Figure 5.40. The Renaissance project, San Diego, CA; typical floor framing plan.

To prevent buckling of the steel plate, the completed steel assembly is encased in reinforced concrete. This also fireproofs the steel. The encasement should be thick enough to provide the stiffness needed to prevent buckling and should be reinforced for strength. Common details would include a regular pattern of welded studs on each side of the plate or a regular pattern of holes in the plate to pass reinforcing bars hooked at each end. This provides a composite sandwich of steel and concrete with the entire thickness effective in preventing buckling of the composite plate. Schematic details of composite shear walls with structural steel boundary elements are shown in Fig. 5.39.

Figure 5.41. The Renaissance project, San Diego, CA; (a) plan at outrigger level; (b) transverse wall elevation showing composite outriggers; (c) composite outrigger details; (d) section through composite outrigger.

Figure 5.41. (Continued ).

5.5.4. Example Projects

5.5.4.1. The Renaissance Project, San Diego, CA*

An example of composite construction in a high-seismic zone is the Renaissance project, a residential development in downtown San Diego, CA. It is constructed primarily of cost-in-place conventional and post-tensioned concrete with some structural steel. It has certain unique design features, including the use of steel link beams embedded in reinforced

* Photographs and figures courtesy of Eric Lehmkuhl, S.E., Associate KPFF Consulting Engineers, San Diego, CA.

Figure 5.41. (Continued).

concrete shear walls and story-high composite outrigger beams coupling the shear walls to exterior composite columns. The development consists of two 24-story towers placed within a three-story-high podium, and houses residential and retail facilities. Additionally, a two-level below-grade parking structure is present under the entire podium. Structural engineering for the project is by KPFF Consulting Engineers, San Diego, CA.

#7 (2 PLCS) extend into slab

#7 (2 PLCS) extend into slab

Note:

1. No P/T tendons allowed perp to coupling beams over doors

Slab top bar

Wall reinf.

Slab top bar

Wall reinf.

Short bar beyond

Figure 5.42. Renaissance project: (a) section through link beam at door opening; (b) coupling beam embedded in shear walls.

Slab bottom bar Tendon

Ties through WF

Short bar beyond

Figure 5.42. Renaissance project: (a) section through link beam at door opening; (b) coupling beam embedded in shear walls.

Typical framing for residential floors (see Fig. 5.40) consists of an 8-in. (203-mm)-thick two-way flat plate. Stud-rail reinforcement is used to resist punching shear at column heads. A 12-inch (305-mm)-thick slab is used at the third level to resist a relatively heavy landscape loading. The slab also acts as a diaphragm by distributing the lateral loads to the podium shear walls. A 12-in. (305-mm)-thick slab also occurs at the 22nd floor to support a two-story steel structure. The entire structure is founded on a variable thickness mat up to 7 ft (2.13 m) thick under the towers. Structural steel framing is used for a two-story structure atop the 22nd level, and inside the tower core walls.

Figure 5.43. Renaissance Towers, San Diego, CA; (a,b,c,d) construction photographs.

Figure 5.43. (Continued).

Figure 5.43. (Continued).

The lateral system is a composite shear wall core with story-high composite outrigger beams interconnected to exterior composite columns at the mid-height of the building in the transverse, slender direction of the core. Steel beams, W14 x 311 (W 360 x 196), are used as flanges, and plates up to 1V2-inch (38-mm)-thick are used as webs for the storyhigh outriggers. See Figs. 5.41a through d.

A diagonally reinforced concrete coupling beam was judged by the design engineers to be impractical if not unbuildable because the beams were required to link the shear walls in two directions at the corners. Therefore, steel wide flange beams up to W18 x 258 (W460 x 383) are used as link beams by embedding them in the shear walls as shown in Fig. 5.42a and b. The core walls are 24-inch (0.60-m)-thick from the base to the 16th floor, and then step down to 16 in. thick. The embedment length of the link beams is sufficiently long to develop the full plastic capacity of the steel beam. Typically this is the plastic shear capacity, Vp = 0.6Fy(d - 2tf) tw (see AISC Seismic, Sect. 15), as the beams are designed to function similar in manner to a ductile link of an eccentric braced system.

Ascertaining satisfactory performance of the gravity system—particularly the flat slab system—subjected to deformations due to seismic lateral loads was a concern. The slab system, together with the columns, behaves as a flat slab-frame and is thus subjected to additional punching shears by virtue of the fact that the slab-frame experiences the same lateral deformations as the lateral-load-resisting elements. To determine the additional shears, a two-dimensional model of an equivalent frame with slabs and columns was analyzed by applying building's drifts to it.

Recommendations given in FEMA 356 were used to determine the equivalent width of the slab and the degree of slab craking. The resulting slab moments were limited to the moment resulting from the yielding of top or bottom slab reinforcement. The punching shears derived from the moments were used to design the slab shear reinforcement.

(a)
Concrete Encased Steel Column Square

Figure 5.44. Kalia Towers, Waikiki, HI; (a) Typical floor plan; (b) box column plan section; (c) brace-to-box column connection; (d) construction photo. (The tower utilized a composite system composed of steel beams with a 6"-thick precast, prestressed concrete plank in the guest rooms, and metal deck with steel beams in the corridors. The lateral system was a braced frame with composite box columns filled with concrete.) Photograph and figures courtesy of Gary Y. K. Chock, S.E., Martin & Chock Inc., Honolulu, HI.

Figure 5.44. Kalia Towers, Waikiki, HI; (a) Typical floor plan; (b) box column plan section; (c) brace-to-box column connection; (d) construction photo. (The tower utilized a composite system composed of steel beams with a 6"-thick precast, prestressed concrete plank in the guest rooms, and metal deck with steel beams in the corridors. The lateral system was a braced frame with composite box columns filled with concrete.) Photograph and figures courtesy of Gary Y. K. Chock, S.E., Martin & Chock Inc., Honolulu, HI.

TT Ca

TT Ca

o.o. WIN.; NUMBER Of STUDS VARIES PER BRACE
Figure 5.44. (Continued).

5.5.4.2. Kalia Towers, Waikiki, HI

Kalia Towers, constructed in UBC seismic zone 3, Waikiki, HI, is a 24-story, 300 ft-high, 450-room hotel featuring structural steel with precast, prestressed concrete floor system. An 81/2-ft (2.60-m) ceiling height is achieved while still keeping the overall floor-to-floor height at 9 ft (2.74 m). The floor system consists of steel beams with 6-in.-thick precast, prestressed concrete planks in the guest rooms, and a metal deck with concrete topping in the corridors. The structural steel framing allowed engineers to lay out the steel system so that most of the beams linedup within the demising walls. As a result, the economics that come with keeping overall building heights at a minimum were realized.

Composite box columns filled with concrete are used in a steel bracing system to resist lateral loads. A typical floor plan, details of composite column, and construction photographs are shown in Fig. 5.44. The structural engineering for the project is by Martin and Chock, Inc., Honolulu, HI.

Greener Homes for You

Greener Homes for You

Get All The Support And Guidance You Need To Be A Success At Living Green. This Book Is One Of The Most Valuable Resources In The World When It Comes To Great Tips on Buying, Designing and Building an Eco-friendly Home.

Get My Free Ebook


Responses

  • lena
    What is concrete encasement to column?
    2 years ago

Post a comment