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AISC Design Guide 6_Load+and+Resistance+Factor+Design+of+W-Shapes+Encased+in+Concrete

Steel Design Guide Series

Load and Resistance Factor Design of

W-Shapes

Encased in Concrete

Steel Design Guide Series Load a nd R esistance Factor D esign of

W-Shapes Encased in Concrete

Lawrence G. Griffis

Walter P. Moore and Associates, Inc.

Houston, Texas

A M E R I C A N I N S T I T U T E O F S T E E L C O N S T R U C T I O N

Published by the American Institute of Steel Construction, Inc.

at One East Wacker Drive, Suite 3100, Chicago, IL 60601-2001.

TABLE OF CONTENTS

INTRODUCTION (1)

SCOPE (1)

PART 1: USE AND DESIGN OF COMPOSITE COLUMNS (1)

Composite Frame Construction (1)

Practical Uses of Composite Columns (2)

Advantages, Disadvantages, and Limitations (2)

Practical Design Considerations (3)

Fire Resistance (3)

Longitudinal Reinforcing Bar Arrangement (3)

Ties (4)

Longitudinal Reinforcing Bar Splices (4)

Connection of Steel Beam to Encased

Wide Flange (5)

Shear Connectors (5)

Base Plate (6)

Erection and Temporary Wind Bracing During

Composite Frame Construction (1)

Load and Resistance Factor Design (LRFD) of

Composite Columns (7)

Comparison Between LRFD and Strain

Compatibility Methods (8)

Description of the Composite Beam-Column

Load Tables (10)

REFERENCES (11)

NOMENCLATURE (12)

PART 2: SUGGESTED DETAILS FOR COMPOSITE COLUMNS (13)

PART 3: DESIGN EXAMPLES (18)

PART 4: LRFD COMPOSITE BEAM-COLUMN DESIGN TABLES (29)

Instructions for Using LRFD Composite Beam-

Column Design Tables (29)

PART 5: COMPOSITE COLUMN PROGRAM CMPOL (310)

PREFACE

This booklet was prepared under the direction of the Com-

mittee on Research of the American Institute of Steel Con-

struction, Inc. as part of a series of publications on special

topics related to fabricated structural steel. Its purpose is to

serve as a supplemental reference to the AISC Manual of Steel

Construction to assist practicing engineers engaged in build-

ing design.

The design guidelines suggested by the authors that are

outside the scope of the AISC Specifications or Code do not

represent an official position of the Institute and are not

intended to exclude other design methods and procedures. It

is recognized that the design of structures is within the scope

of expertise of a competent licensed structural engineer,

architect, or other licensed professional for the application of

principles to a particular structure.

The sponsorship of this publication by the American Iron and

Steel Institute is gratefully acknowledged.

The information presented in this publication has been prepared in accordance with recognized engineering principles and is for general information only. While it is believed to be accurate, this information should not be used or relied upon for any specific application without competent professional examination and verification of its accuracy, suitability, and applicability by a licensed professional engineer, designer, or architect. The publication of the material contained herein is not intended as a representation or warranty on the part of the American Institute of Steel Construction, Inc. or the American Iron and Steel Institute, or of any other person named herein, that this information is suitable for any general or particular use or of f reedom infringement of any patent or patents. Anyone making use of this information assumes all liability arising from such use.

LOAD AND RESISTANCE FACTOR DESIGN OF W-SHAPES ENCASED IN CONCRETE

INTRODUCTION

Structural members comprised of steel shapes in combination with plain or reinforced concrete have been utilized by engi-neers for many years. Early structures simply took advantage of the protection that the concrete afforded to the steel shapes for resistance to fire and corrosion. But research on the strength of such members was conducted in the early 1900s,1 and design provisions were formulated by 1924.2 More re-cently, with the advent of modern composite frame construc-tion in high rise buildings, engineers developed new rational methods to take advantage of the stiffening and strengthening effects of concrete and reinforcing bars on the capacity of encased steel shapes.

This Guide presents design tables for composite columns, developed under the sponsorship of the American Institute of Steel Construction (AISC) as an aid to the practicing struc-tural engineer in the application of the AISC Load and Resis-tance Factor Design (LRFD) Specification for Structural Steel Buildings.3 The information presented supplements that found in the AISC LRFD Manual.4 Background on the LRFD criteria for composite columns may be found in References 5 and 6. Engineers interested in Allowable Stress Design (ASD) are encouraged to consider the procedure developed pre-viously by the Structural Stability Research Council (SSRC).7 The SSRC procedure is not presently included in the AISC ASD Specification.8

The reader is cautioned that independent professional judg-ment must be exercised when data or recommendations set forth in this Guide are applied. The publication of the material contained herein is not intended as a representation or war-ranty on the part of the American Institute of Steel Construc-tion, Inc.—or any person named herein—that this informa-tion is suitable for general or particular use, or freedom from infringement of any patent or patents. Anyone making use of this information assumes all liability rising from such use. The design of structures should only be performed by or under the direction of a competent licensed structural engineer, architect, or other licensed professional.

SCOPE

This Guide is specifically for composite columns comprised of rolled wide flange shapes encased in reinforced structural concrete with vertical deformed reinforcing bars and lateral ties. Composite columns are defined in Section I1 of the

LRFD Specification as a "steel column fabricated from rolled or built-up steel shapes and encased in reinforced structural concrete or fabricated from steel pipe or tubing and filled with structural concrete." Further, the Specification requires in Section I2.1 that the cross sectional area of the steel shape comprise at least four percent of the total composite cross section. The Commentary to the Specification states that when the steel shape area is less, the column should be designed under the rules for conventional reinforced concrete columns.

Part 1 of this Guide includes a discussion of composite frame construction, practical uses of composite columns, their advantages and limitations, and a review of important practical design considerations. A summary of the pertinent LRFD rules is presented and compared to other methods. A set of suggested design details is given in Part 2, showing placement of reinforcing bars and ties, as well as treatment of joints and base plates. Five design examples are given in Part 3 to illustrate how the tables were derived and how they are applied. Finally, a comprehensive set of t ables is presented in Part 4 to assist the designer in the rapid selection of the most economical section to resist required values of factored load and moment.

PART 1: USE AND DESIGN OF

COMPOSITE COLUMNS

Composite Frame Construction

Although engineers since the 1930s have encased structural steel shapes in concrete for fireproofing and corrosion protec-tion, it was not until the development and popularity of modern composite frame construction in the 1960s that com-posite columns again became a common and viable structural member type. The late Dr. Fazlur Khan, in his early discus-sions of structural systems for tall buildings, first proposed the concept of a composite frame system9, 10 utilizing compos-ite columns as part of the overall wind and earthquake resist-ing frame. Since that time composite frame construction has been adopted for many high rise buildings all over the world.

Its usage, with the composite column as the key element, is well documented in the work of the Council on Tall Buildings and numerous other publications.11-15

The term "composite frame structure" describes a building employing concrete encased steel columns and a composite floor system (structural steel and concrete filled steel deck).

The bare steel columns resist the initial gravity, construction, and lateral loads until such time as the concrete is cast around them to form composite columns capable of resisting the total gravity and lateral loads of the completed structure. In a composite frame building, the structural steel and reinforced concrete combine to produce a structure having the advan-tages of each material. Composite frames have the advantage of speed of construction by allowing a vertical spread of the construction activity so that numerous trades can engage simultaneously in the construction of the building. Inherent stiffness is obtained with the reinforced concrete to more easily control the building drift under lateral loads and reduce perception to motion. The light weight and strength obtained with structural steel equates to savings in foundation costs. Traditionally in steel framed buildings or reinforced con-crete buildings, stability and resistance to lateral loads are automatically provided as the structure is built. Welded or bolted moment connections are made or braces are connected between columns in a steel building immediately behind the erection of the steel frame to provide stability and resistance to lateral loads. Shear walls, or the monolithic casting of beams and columns, provide stability and resistance to lateral loads soon after the concrete has cured for reinforced concrete buildings. However, for composite frame structures, the final stability and resistance to design lateral loads is not achieved typically until concrete around the erection steel frame has cured, which typically occurs anywhere from a minimum of six to as much as 18 floors behind the erection of the bare steel frame. This sequence of construction is shown-schemati-cally in Fig. 1. Thus, as discussed subsequently, temporary Fig. 1. Composite-frame construction sequence.

lateral bracing of the uncured portion of the frame will typically be required.

Practical Uses of Composite Columns

Practical applications for the use of composite columns can be found in both low rise and high rise structures. In low rise structures such as a covered playground area, a warehouse, a transit terminal building, a canopy, or porte cochere, it may be necessary or desirable to encase a steel column with concrete for aesthetic or practical reasons. For example, ar-chitectural appearance, resistance to corrosion, or protection against vehicular impact may be important. In such structures, it may be structurally advantageous to take advantage of the concrete encasement of the rolled steel shape that supports the steel roof structure by designing the member as a compos-ite column resisting both gravity and lateral loads.

In high rise structures, composite columns are frequently used in the perimeter of "tube" buildings where the closely spaced columns work in conjunction with the spandrel beams (either steel or concrete) to resist the lateral loads. In some recent high rise buildings, giant composite columns placed at or near the corners of the building have been utilized as part of the lateral frame to maximize the resisting moment pro-vided by the building's dead load. Composite shear walls with encased steel columns to carry the floor loads have also been utilized in the central core of high rise buildings. Frequently, in high rise structures where floor space is a valuable and income producing commodity, the large area taken up by a concrete column can be reduced by the use of a heavy encased rolled shape to help resist the extreme loads encountered in tall building design. Sometimes, particularly at the bottom floors of a high rise structure where large open lobbies or atriums are planned, a heavy encased rolled shape as part of

a composite column is a necessity because of the large load

and unbraced length. A heavy rolled shape in a composite column is often utilized where the column size is restricted architecturally and where reinforcing steel percentages would otherwise exceed the maximum code allowed values.

Advantages, Disadvantages, and Limitations

Some of the advantages of composite columns are as follows:

1. Smaller cross section than required for a conventional

reinforced concrete column.

2. Larger load carrying capacity.

3. Ductility and toughness available for use in earthquake

zones.

4. Speed of construction when used as part of a composite

frame.

5. Fire resistance when compared to plain steel columns.

6. Higher rigidity when part of a lateral load carrying

system.

7. Higher damping characteristics for motion perception in

tall buildings when part of a lateral load carrying system. 2

8. Stiffening effect for resistance against buckling of the

rolled shape.

There are also, of course, some disadvantages and limita-tions. In high rise composite frame construction, design en-gineers sometimes have difficulty in controlling the rate and magnitude of column shortening of the composite column with respect to adjacent steel columns or shear walls. These problems are exacerbated by the wide variation in construc-tion staging often experienced in the zone between the point where the steel erection columns are first erected and the point where concrete is placed around the steel to form the com-posite column. This variation in the number of floors between construction activities has made it difficult to calculate with accuracy the effect of column shortening. Creep effects on the composite columns with respect to the all-steel core columns, or between shears walls, can also be troublesome to predict for the designer. The net effect of these problems can be floors that are not level from one point to another. One solution to these problems has been the measurement of column splice elevations during the course of construction, with subsequent corrections in elevation using steel shims to compensate for differences between the calculated and measured elevation. As with any column of concrete and reinforcing steel, the designer must be keenly aware of the potential problems in reinforcing steel placement and congestion as it affects the constructability of the column. This is particularly true at beam-column joints where potential interference between a steel spandrel beam, a perpendicular floor beam, vertical bars, joint ties, and shear connectors can all cause difficulty in reinforcing bar placement and lead to honeycombing of the concrete. Careful attention must be given to the detailing of composite columns by the designer. Analytical and experi-mental research is needed in several aspects of composite column design. One area requiring study is the need, or lack thereof, of a mechanical bond between the steel shape and the surrounding concrete. Several papers16, 17 have discussed this question, but additional work is required to quantify the need for shear connectors with a practical design model for routine design office use. There presently is a question about transfer of shear and moment through a beam-column joint. This concern is of particular importance for seismic regions where large cyclical strain reversals can cause a serious degradation of the joint. Initial research has been completed at the Uni-versity of Texas at Austin24 and is ongoing at Cornell Univer-sity on physical test models to study various joint details in composite columns.

Practical Design Considerations

Fire Resistance

Composite columns, like reinforced concrete columns, have an inherent resistance to the elevated temperatures produced in a fire by virtue of the normal concrete cover to the reinforc-

ing steel and structural steel. It is standard practice to provide

a minimum of one and one-half inch of concrete cover to the

reinforcing steel of a composite column (concrete cover is specified in ACI 318-89 Section 7.7.1).18 Chapter 43 of the Uniform Building Code states that reinforced concrete col-umns utilizing Grade A concrete (concrete made with aggre-gates such as limestone, calcareous gravel, expanded clay, shale, or others containing 40 percent or less quartz, chert, or flint) possess a four-hour rating with one and one-half inch cover. A four-hour rating is the maximum required for build-ing structures.

Tables of fire resistance rating for various insulating mate-rials and constructions applied to structural elements are published in various AISI booklets19, 20, 21 and in publications of the Underwriters Laboratory, Inc.

Longitudinal Reinforcing Bar Arrangement

Composite columns can take on just about any shape for which a form can be made and stripped. They can be square, rectangular, round, triangular, or any other configuration, with just about any corresponding reinforcing bar arrange-ment common to concrete columns. For use in composite frame construction, however, square or rectangular columns

Fig. 2. Longitudinal bar arrangement in composite columns. 3

are the most practical shape, with bar arrangements tending to place the vertical reinforcing bars at or near the four corners of the column. Figure 2 shows preferred arrangements which allow spandrel beams and a perpendicular floor beam to frame into the encased steel shape without interrupting the continuous vertical bars. Such arrangements also generate the maximum design capacity for the column.

Although there are no explicit requirements for longitudi-nal bar spacing in the LRFD Specification, it is advisable to establish minimum limits so that concrete can flow readily in spaces between each bar and between bars and the encased steel shape.

Minimum spacing criteria will also prevent honeycombing and cracks caused by high bond stresses between bars. Past experience with reinforced concrete columns has shown that the requirements established by the ACI 318 Code have provided satisfactory performance. These spacing and cover requirements have been used in the formulation of this design aid and as diagramed in Fig. 3 and listed below:

1. Minimum concrete cover over vertical bars and ties shall

be 1?-in. (LRFD Specification, Section I2.1.b).

2. Clear distance between longitudinal bars shall not be less

than 1? bar diameters or 1?-in. minimum (ACI 318-89 Section 7.6.3).

Fig. 3. Composite column cover and bar spacing requirements.

3. The clear distance limitations apply also to contact lap

splices and adjacent bars (ACI 318-89 Section 7.6.4).

4. Clear distance between longitudinal bars and steel shape

shall be 1? bar diameters or 1?-in. minimum.

Ties

Reinforcing steel cages (longitudinal bars and ties) must usually be set after and around the steel column. Because the steel column is erected in an earlier erection sequence, only open U-shaped ties are suitable for composite columns. Ties are used to provide lateral stability of the longitudinal bars and confinement of the concrete. The requirements of the LRFD specification and certain requirements of the ACI 318-89 code not specifically addressed by the LRFD specifi-cation should be satisfied as follows:

1. The cross sectional area of the tie shall be at least 0.007

square inches per inch of tie spacing (LRFD Specifica-

tion I2.1.b).

2. The spacing of the ties shall not be greater than two-

thirds of the least dimension of the cross section (LRFD

Specification I2.1.b).

3. The spacing of ties shall not be greater than 16 longitu-

dinal bar diameters or 48 tie bar diameters (ACI 318-89

Section 7.10.5.1).

4. Ties shall be at least #4 in size for #11, #14, #18, and

bundled longitudinal bars, and #3 in size for all other

bars (ACI 318-89 Section 7.10.5.1).

5. Ties shall be arranged such that every corner and alter-

nate bar shall have lateral support provided by a corner

of a tie, with an inclusive angle of not more than 135°

and no bar shall be further than 6 inches clear on each

side along the tie from such a laterally supported bar

(ACI 318-89 Section 7.10.5.3).

6. A lap splice of two pieces of an open tie shall be at least

equal to 1.3 times the tensile development length for the

specified yield strength (ACI 318-89 Section 12.13.5).

Suggested details for composite column ties are shown in Typical Details 1, 2, and 3 of Part 2.

Longitudinal Reinforcing Bar Splices

The requirements for splicing vertical longitudinal reinforc-ing bars for composite columns shall follow the same rules as apply for conventional reinforced concrete columns as speci-fied in Chapter 12 of the ACI 318-89 Code. Several additional comments should be made for composite columns. First, additional vertical longitudinal restraining bars (LRFD Specification I2.1.b) should be used between the corners where the continuous load carrying bars are located in com-posite frame construction. These bars usually cannot be con-tinuous because of interruption with intersecting framing members at the floor line. They are often required to satisfy the spacing requirements for vertical longitudinal bars shown as follows:

The cross section area of longitudinal reinforcement shall be at least equal to 0.007 square inches per inch of bar spacing (LRFD Specification I2.1.b).

Second, it is suggested that, in high rise composite frame construction, the vertical bar splices be located at the middle clear height of the composite column. This point is usually near the inflection point (zero moment) of the column where the more economical compression lap splices or compression butt splices may be used. The more expensive tension lap or tension butt splices may be required if splices are made at the floor line.

A suggested composite column splice detail is shown in Typical Detail 1 of Part 2.

Connection of Steel Beam to Encased Wide Flange

In composite frame construction, steel spandrel beams and/or perpendicular floor beams often frame into the composite column at the floor level. Sometimes these beams will be simply supported floor beams where conventional double-angle framed beam connections (LRFD Manual, Part 5) or single-plate shear connections may be utilized. More often,however, the steel spandrel beams will be part of the lateral load resisting system of the building and require a moment connection to the composite column. Practicality will often dictate that the larger spandrel beam (frequently a W36 in tall buildings) be continuous through the joint with the smaller erection column (often a small W14) interrupted and penetration welded to the flanges of the spandrel beam. To increase the speed of erection and minimize field welding,the spandrel beam and erection column are often prefabri-cated in the shop to form "tree columns" or "tree beams"with field connections at the mid-height of column and midspan of spandrel beam using high strength bolts. See Typical Detail 5, Part 2.

The engineer must concern himself with the transfer of forces from the floor beams to the composite column. For simply supported beams not part of the lateral frame, the simplest method to transfer the beam reaction to the compos-ite column is through a standard double-angle or single-plate shear connection to the erection column. It is then necessary to provide a positive shear connection from the erection column to the concrete along the column length to ensure transfer of the beam reaction to the composite column cross section. The simplest method to accomplish this is by the use of standard headed shear connectors, preferably shop welded to the wide flange column. For moment connected spandrel beams, the beam shear and unbalanced moment must be transferred to the composite column cross section. Different transfer mechanisms have been tested at the University of

Texas at Austin.

Several suggested details are shown in Details 1 and 2 of Part 2.

Shear Connectors

As discussed in the previous section, it is necessary to provide a positive shear connection transfer from the floor beam to the encased steel column when the beam connection is made directly to the encased steel column. It is likely that a signifi-cant portion of this reaction can be transferred in bond be-tween the encased section and the concrete as reported in Reference 14. An estimate of this value can be made from Equation 5 of Reference 16 which is based on the results of a limited number of push tests in which a steel column is encased in a concrete column.

where

allowable load for the encased shape, lb steel flange width of encased shape, in.concrete compressive strength, psi

encased length of steel shape, in.constant 5

Converting to an average ultimate bond stress "u ," using only the flange surfaces as being effective and applying a safety factor of five as reported in the tests.

Consider a typical case of a W14x90 encased column in 5,000psi concrete with a floor-to-floor height (h O ) of 13 feet. The

average ultimate bond stress is

The ultimate shear force that could be transferred by bond is

These results indicate that typical floor reactions on the composite column could be easily transferred by bond alone.The above discussion considered the case where axial load alone is transferred from the encased steel section to the concrete. For beam-columns where high bending moments may exist on the composite column, the need for shear con-nectors must also be evaluated. Until such time as research data is provided, the following simplistic evaluation may be made. Assume a situation where a composite column is part of a lateral load resisting frame with a point of inflection at mid-column height and a plastic neutral axis completely outside the steel cross section (similar to Fig. 4 except for plastic neutral axis location). An analogy can be made be-tween this case and that of a composite beam where shear connectors are provided uniformly across the member length

between the point of zero moment and maximum moment.The ultimate axial force to be transferred between the encased steel column and the concrete over the full column height is 2AF y where A is the steel column area and F y is its yield strength. Assuming a bond strength is available in this case similar to the case of the push test discussed above, then shear connectors would theoretically be required when 2AF y is greater than the ultimate bond force. In the previous example,assume an A36 W 1

4×90 erection column is used. Then,

This is less than the available shear transfer from bond,which was calculated as 2,895 kips

Again, it is shown that bond stress alone can transfer the shear between the encased shape and the concrete, assuming no loss in bond occurs as a result of tensile cracking at high moments.

The composite beam-column design tables presented in Part A assume a nominal flexural strength based on the plastic stress distribution of the full composite cross section. To validate this assumption, the LRFD specification commen-tary in Section 14, requires a transfer of shear from the steel to the concrete with shear connectors. Therefore, until further research is conducted on the loss of b ond between the encased steel section and the concrete, and until more comprehensive push tests are run, the following suggestions are made with regard to shear connectors on composite columns:1. Provide shear connectors on the outside flanges where space permits. Where space does not permit, provide shear connectors on the inside flange staggered either side of the web.

2. Provide shear connectors in sufficient quantity, spaced uniformly along the encased column length and around the column cross section between floors, to carry the

Fig. 4. Plastic stress distribution in composite columns.

greater of the following minimum shear transfer forces as applicable:

a. The sum of all beam reactions at the floor level.

b. Whenever the ratio of the required axial strength to the factored nominal axial strength, is less than 0.3, a force equal to F y times the area of steel on the tensile side of the plastic neutral axis in order to sustain a moment equal to the nominal flexural strength of the composite cross section. The ratio 0.3is used as an arbitrary value to distinguish a composite column subjected to predominantly axial load from one subjected to predominately moment. Considera-tion must be given to the fact that this moment is reversible.

3. The maximum spacing of shear connectors on each flange is suggested to be 32 inches.

If minimum shear connectors are provided according to the guidelines identified herein, it is reasonable to assume com-patibility of strains between concrete and encased steel to permit higher strains than 0.0018 under axial load alone. This strain level has been identified in Reference 7 and LRFD Commentary, Section 12.1, as a point where unconfined con-crete remains unspalled and stable. Therefore, a slight in-crease in the maximum usable value of reinforcing steel stress from 55 ksi, corresponding to 0.0018 axial strain, to 60 ksi,the yield point of ASTM A615 Grade 60 reinforcing steel,would seem to be justified. Such an approach has been adopted in this Guide. The use of s hear connectors also allows the full plastic moment capacity to be counted upon when

is less than 0.3 (LRFD Commentary, I4) instead of

the reduction specified in LRFD Specification, Section I4.Suggested details for shear connectors on composite col-umns are shown in Typical Details 1 and 2 of Part 2.Base Plate

Normally a base plate for the encased steel column of a composite column is specified to be the minimum dimension possible to accommodate the anchor bolts anchoring it to the foundation during the erection phase. In doing so, the base plate will interfere the least possible amount with dowels coming up from the foundation to splice with the longitudinal vertical bars of the composite column. The design engineer must provide dowels from the composite column to the foun-dation to transmit the column load in excess of the allowable bearing stress on the foundation concrete times the effective bearing area (the total composite column area less the area of the encased wide flange column base plate).In some cases, depending on the base plate size, it may be necessary to add additional foundation dowels to adequately transmit the load carried by the concrete of the composite column. A typical base plate detail is shown in Typical Detail 4, Part 2. A composite column base plate example is included as Example 5, Part 3.

Erection and Temporary Wind Bracing During

Composite Frame Construction

Historically, a structural steel erector is accustomed to work-

ing with a steel framed structure that is stabilized as the frame

is constructed with moment connections or permanent cross

bracing. Composite frames many times are not stable and not

fully able to carry lateral loads until after the concrete is

poured and cured many floors behind. Because of this fact, it

is incumbent on the engineer-of-record to state the assump-

tions of bare steel frame stability in the contract documents.

Either he designs and details the necessary temporary bracing

on the drawings or requires the erector to engage a structural

engineer to provide it. The engineer-of-record is the most

appropriate person to provide this service by virtue of his

knowledge of the loads and familiarity with the overall struc-

ture. Additional discussions about the design responsibility of

steel frames during erection may be found in the AISC Code

of Standard Practice.22 A discussion of composite frames

during erection may be found in Reference 15.

Load and Resistance Factor Design (LRFD) of

Composite Columns

To qualify as a composite column under the LRFD Specifi-

cation design procedure, the following limitations must be

satisfied as defined in Section 12.1:

1. The cross sectional area of t he steel shape, pipe, or tubing

must comprise at least four percent of the total composite

cross section.

2. Concrete encasement of a steel core shall be reinforced

with longitudinal load carrying bars, longitudinal bars to

restrain concrete, and lateral ties. Longitudinal load

carrying bars shall be continuous at framed levels; lon-

gitudinal restraining bars may be interrupted at framed

levels. The spacing of ties shall be not greater than

two-thirds of the least dimension of the composite cross

section. The cross sectional area of the transverse and

longitudinal reinforcement shall be at least 0.007 in.2 per

inch of bar spacing. The encasement shall provide at

least 1?-in. of clear cover outside of both transverse and

longitudinal reinforcement.

3. Concrete shall have a specified compressive strength

' of not less than 3 ksi nor more than 8 ksi for normal

weight concrete, and not less than 4 ksi for lightweight

concrete.

4. The specified minimum yield stress of structural steel

and reinforcing bars used in calculating the strength of

a composite column shall not exceed 55 ksi.

The required design strength P u of axially loaded composite

columns is defined in the LRFD Specification, Section E2,

with modification of certain terms according to Section I2.2.

These rules are summarized as follows:

required axial strength

(E2-1 modified)

(E2-2 modified)

(E2-3 modified)

(E2-4 modified)

= resistance factor for compression = 0.85

= gross area of steel shape

= modified yield stress

(I2-1)

= modified modulus of elasticity

(I2-2)

= specified yield stress of structural steel column, ksi

= modulus of elasticity of steel, ksi

= effective length factor

= unbraced length of column, in.

= radius of gyration of steel shape in plane of buckling,

except that it shall not be less than 0.3 times the

overall thickness of the composite cross section in

the plane of buckling, in.

= net concrete area

= gross area of composite section, in.2

= area of longitudinal reinforcing bars, in.2

= modulus of elasticity of concrete

= unit weight of concrete, lbs./ft3

= specified compressive strength of concrete, ksi

= specified minimum yield stress of longitudinal rein-

forcing bars, ksi

= 0.7

= 0.6

= 0.2

The interaction of axial compression and flexure in the

plane of symmetry on composite members is defined in

Section H1.1, H1.2, and I4 as follows:

(H1-1a)

(H1-1b)

= required compressive strength, kips

= nominal compressive strength, kips

= required flexural strength, kip-in.

= nominal flexural strength determined from plastic

stress distribution on the composite cross section,kip-in.

= resistance factor for compression = 0.85= resistance factor for flexure = 0.90

The following information on the determination of the

required flexural strength, M u , is quoted from Section H1.2 of the LRFD Specification, with minor changes in symbols as prescribed in Section I2.

"In structures designed on the basis of elastic analysis,M u may be determined from a second order elastic analysis using factored loads. In structures designed on the basis of plastic analysis, M u shall be determined from a plastic analy-

sis that satisfies the requirements of Sects. C1 and C2. In structures designed on the basis of elastic first order analysis the following procedure for the determination of M u may be used in lieu of a second order analysis:

(H1-2)

where

= required flexural strength in member assuming there is no lateral translation of the frame, kip-in.

= required flexural strength in member as a result of lateral translation of the frame only, kip-in.

(H1-3)

where is defined by Formula

E2-4 with in the plane of bending.

= a coefficient whose value shall be taken as follows:i. For restrained compression members in frames braced against joint translation and not subject to transverse

loading between their supports in the plane of bending,

(H1-4)

where M 1 / M 2 is the ratio of the smaller to larger moments at the ends of that portion of the member unbraced in the plane of bending under consideration.M 1 / M 2 is positive when the member is bent in reverse curvature, negative when bent in single curvature.ii. For compression members in frames braced against j oint

translation in the plane of loading and subjected to transverse loading between their supports, the value of C m can be determined by rational analysis. In lieu of such analysis, the following values may be used:

for members whose ends are restrained, C m = 0.85for members whose ends are unrestrained, C m = 1.0

(H1-5)

(H1-6)

= required axial load strength of all columns in a story, kips

= translation deflection of the story under considera-

tion, in.

= sum of all story horizontal forces producing kips

= story height, in.

kips, where is the slenderness para-meter defined by Formula E2-4, in which the effective length factor K in the plane of bending shall be determined in accordance with Sect.C2.2, but shall not be less than unity."

The nominal flexural strength M n is determined for the plastic stress distribution on the composite cross section as shown in Fig. 4. The plastic neutral axis is first determined

such that there is equilibrium of axial forces in the concrete,reinforcing steel and embedded steel column. The nominal flexural strength M n is determined as the summation of the first moment of axial forces about the neutral axis. See Example 2, Part 3.

In the determination of the concrete compressive axial force, a concrete compressive stress of 0.85f c ' is assumed uniformly distributed over an equivalent stress block bounded

by the edges of the cross section and a straight line parallel to the plastic neutral axis at a distance where c is the distance from the edge of the cross section to the plastic neutral axis, and,

These assumptions are contained in the ACI 318-89 Code (Section 10.2.7.3).

Comparison Between LRFD and Strain Compatibility M ethods

Guidelines for the design of composite columns were first introduced into the ACI Building Code in 1971 (ACI 318-71).With the widespread use and popularity of composite col-umns in the 1970s and 1980s, many engineers designed composite columns according to these principles, which are essentially the same ones used for conventional reinforced concrete columns.

The current rules for designing composite columns by the

ACI approach are found in ACI 318-89, Chapter 10. The method essentially is one based on the assumption of a linear strain diagram across the composite cross section with the maximum failure strain at ultimate load defined as 0.003.With these assumptions, it is possible to generate strength capacities of the cross section for successive assumed loca-tions of the neutral axis. Strains at each location of the cross section are converted to stress for the usual assumption of a linear stress-strain curve for reinforcing steel and structural steel. The first moment of f o r c e s in each element of concrete,structural steel, and reinforcing steel is taken about the neutral axis to generate a point (axial load and moment) on an interaction curve.

A comparison between the strain compatibility approach and the LRFD approach is shown in Figs. 5 through 7.Interaction curves (axial load vs. moment) are plotted cover-ing the wide range of composite column sizes (28×28 in.,36×36 in., 48×48 in.) steel column sizes (minimum of four percent of the composite column cross section to maximum W 14×730) and reinforcing steel percentages (one percent to four percent) that are likely to be found in practice. Examina-tion of these figures reveals the following comparison:1. The ACI approach yields curves that are parabolic in nature while the AISC curves are essentially bilinear.2. The two methods yield pure moment capacities that are very close to each other. The maximum difference is approximately 15 percent with most values much closer than that. LRFD in all cases predicts higher moment values.

3. The two methods yield pure axial load capacities that are reasonably close when the steel column constitutes a small part of the total column capacity, but are signifi-cantly different as the steel column becomes larger. With larger steel column sizes, the LRFD approach yields axial capacities as much as 30 percent larger than ACI.This comparison, however, is not very meaningful be-cause the ACI approach essentially does not recognize pure axially loaded columns with its minimum eccen-tricity provisions.

4. Large differences in capacity are predicted (as much as 50 percent) for composite columns having small steel columns. The ACI method yields significantly larger axial loads for a given moment than the LRFD method.This difference is most striking in the intermediate range of the curve.

5. With larger steel columns, the LRFD curve is mostly above (predicts higher values) the ACI curve. As the steel column section becomes lighter, the ACI curve tends to be above the LRFD curve, particularly in the middle ranges of eccentricity.

6. It can generally be stated that, as the steel column becomes a larger portion of the total column capacity,design economy can be realized by designing using the LRFD approach. When the steel column becomes

Fig. 5. Interaction curve comparisons ACI vs. LRFD.Fig. 6. Interaction curve comparisons ACI vs. LRFD.

smaller (the column is more like a conventional concrete column), the ACI method is more economical in design.

Reference 23 also presents a comparison of design methods.

Description of the Composite Beam-Column Load Tables Design tables are presented in Part 4 of this Guide to assist the engineer in the rapid selection of the most economical composite column to resist factored values of axial load and moment. The tables are based on the LRFD Specification requirements outlined in the previous sections. The tables have been set up to follow the general format of the LRFD

Manual,4

including the column tables in Part 2 (Axial Loaded Steel Columns) and Part 4 (Axially Loaded Composite Col-umns) of the Manual, because these are already familiar to most design engineers. The tables indicate the following parameters from which the engineer can select a design (Refer to sample table at beginning of Part 4 of this Guide):

Item 1: Composite Column Size (b × h, in.). The composite

column size (b × h) is indicated in inches in the upper right comer of the table. Note that the x- x axis is always the strong axis of the steel column and is in the direction of b. The y-y axis is always the weak axis of the steel column and is in the direction of h. The table covers square and rectangular sizes varying from 16 inches to 36 inches in four-inch increments.

Fig. 7. Interaction curve comparisons ACI vs. LRFD.

Item 2: Concrete Strength (f 'c ,ksi). Concrete compression strength is indicated in the top right corner for 3 and 8ksi. All concrete is assumed to be normal weight concrete weighing 145 pcf. Linear interpolation can be used for con-crete strengths between 3 and 8 ksi.

Item 3: Reinforcing Bar Yield Strength (F yr , ksi). All longitu-dinal and transverse reinforcing steel in the table is based on ASTM A615 Grade 60 reinforcing steel.

Item 4: Steel Column Size. Steel column size is listed across the top of the table. Sizes tabulated include all W8, W10,W12, and W14 wide flange shapes that are listed in the steel column tables in Part 4 of the LRFD manual. They include W8 (35 to 67), W10 (39 to 112), W12 (50 to 336), and W14(43 to 426).

Item 5: Steel Grade (F y , ksi). Steel grade is presented across the top of the page for both A36 and Grade 50 steel.

Item 6: Reinforcement. Information on column reinforce-ment is indicated in the extreme left column and includes the

percentage of vertical steel, area of steel (A r , in.2

) number,size of bar, pattern of vertical steel, and lateral tie size and spacing (see Fig. 2 for notation). The table covers steel percentages as close as practical to 0.5 percent, 1 percent, 2percent, 3 percent, and 4 percent steel. If zeroes are tabulated,it indicates steel cover or spacing requirements could not be satisfied for the steel percentage indicated. Bar arrangements and their designations are shown in Fig. 2.

Item 7: Unbraced Length (KL, ft). Axial load capacities are tabulated for unbraced lengths of 0, 11, 13, 17, 21, 25, and 40feet.

Item 8: Axial Design Strength (Nominal Axial Strength times Resistance Factor, kips). For each unbraced length,KL, equations E2-1, E2-2, E2-3, and E2-4 are used to

calculate the nominal axial strength which is multiplied by

and tabulated in the column marked 8.

Item 9, 10, and 11: Available Required Flexural Strength (Uniaxial Moment Capacity, ft-kips). For each ratio of applied factored axial load to times the nominal axial capacity, available uniaxial moment capacity is tabu-lated by solving equation H1-1a or H1-1b as applicable. Note that these moment capacities are uniaxial capacities and are applied independently. Biaxial moment capacities are not tabulated.

Item 12: Euler Buckling Term ( kip-ft 2). The second order moment, M u , can be taken directly from a second order elastic analysis, or it can be calculated from a first order elastic analysis by using LRFD equations H1-1 through H1-6.To aid the designer in such a calculation, the terms and are tabulated for each column configuration. The follow-ing definitions apply.

(f'c )

Thus, the Euler buckling load needed for the calculation is

simply

Item 13: Radius of Gyration ( in.). To compare the

axial design strength for buckling about each axis, and to

assist the designer in determining column capacity for un-braced lengths not shown in the table, values of and are tabulated for each column configuration.

Note that the development of the moment capacities listed in the tables is based on a numerical calculation of t he contribution

of the encased shape, the precise number and location of rein-forcing bars as prescribed in the bar arrangements of Fig. 2, and the concrete. This is in lieu of the approximate plastic moment capacity expression p rescribed b y the LRFD Commentary e qua-tion C-I4-1. The approximate expression was used in the mo-ment capacities tabulated in the composite column tables pres-ently in the LRFD Manual and will result in some differences when compared to the more precise method used in the new composite beam-column tables in this Guide.

The following factors should be considered in the use of the tables:

1. Where zeroes exist in the tables, no bar pattern from the configurations considered in Fig. 2 exists that would satisfy bar cover and spacing requirements between bars, or between bars and the surface of the encased steel column (Refer to Fig. 3).

2. Moment capacity tabulated is the uniaxial moment ca-

pacity considering each axis separately.

3. Only column configurations conforming to all the limi-tations in the LRFD Specification (Section I2.1) are tabulated.

4. Capacities shown are only applicable to the bar arrange-ments shown in Fig. 2.

5. The designer must determine in each case that necessary clearances are available for beams framing into the steel column without interrupting the vertical bars.

6. Linear interpolation can be used to determine table

values for concrete strengths between 3 and 8 ksi.

Specific instruction for using the tables are given at the beginning of the tables, Part 4 of this Guide. The background for the development of the tables is presented in Examples 1and 2, Part 3 of this Guide.

REFERENCES

1. Talbot, A. N. and Lord, A. R., "Tests of Columns: An Investigation of the Value of Concrete as Reinforcement

for Structural Steel Columns," Engineering Station Bul-letin, No. 56, 1912, University of Illinois, Urbana, Ill.2. Joint Committee Report on Standard Specifications for Concrete and Reinforced Concrete, August 1924.

3. American Institute of Steel Construction, Inc., Load and Resistance Factor Design Specification for Structural Steel Buildings, Sept. 1, 1986, Chicago, Ill.

4. American Institute of Steel Construction, Inc., Load and Resistance Factor Design (LRFD) Manual of Steel Con-struction, 1st Ed., 1986, Chicago, Ill.

5. American Institute of Steel Construction, Inc., Commen-tary on the Load and Resistance Factor Design Specifi-cation for Structural Steel Buildings, Sept. 1, 1986, Chi-cago, Ill.

6. Galambos, T. V. and J. Chapuis, LRFD Criteria for Com-posite Columns and Beam-Columns, Revised Draft, De-cember 1980, Washington University, St. Louis, Mo.

7. SSRC Task Group 20, "A Specification for the Design of Steel-Concrete Composite Columns," AISC Engineering Journal, 4th Qtr., 1979, Chicago, Ill.

8. American Institute of Steel Construction, Inc., Specifica-tion for the Design, Fabrication, and Erection of Struc-tural Steel f or Buildings, Nov. 1, 1978, Chicago, Ill.9. Belford, Don, "Composite Steel Concrete Building Frame," Civil Engineering, July 1972.

10. Kahn, Fazlur R., "Recent Structural Systems in Steel for High Rise Buildings," BCSA Conference on Steel in Architecture, Nov. 24-26, 1969.

11. Iyengar, Hal, Recent Developments in Mixed Steel Con-crete Systems, High Rise Buildings: Recent Progress,Council on Tall Building and Urban Habitat, 1986.

12. Moore, Walter P. and Narendra R. Gosain, Mixed Systems:Past Practices, Recent Experience, and Future Direction,High Rise Buildings: Recent Progress, Council on Tall Buildings and Urban Habitat, 1986.

13. Winter, George, Proposed New Design Methods f or Com-posite Columns, Developments in Tall Buildings 1983,Council on Tall Buildings and Urban Habitat, 1983.14. Iyengar, Hal, Recent Developments in Composite High Rise Systems, Advances in Tall Building, Council on Tall Buildings and Urban Habitat, 1986.

15. Griffis, Lawrence G., "Some Design Considerations for Composite Frame Structures," AISC Engineering Jour-nal, 2nd Qtr. 1986, Chicago, Ill.

16. Roeder, Charles W, "Bond Stress of E mbedded Steel Shapes in Concrete," Composite and Mixed Construction, Ameri-can Society of Civil Engineers, 1985, New York, NY.17. Furlong, Richard W, "Binding and Bonding Concrete to Composite Columns," Composite and Mixed Construc-tion, American Society of Civil Engineers, 1985, New York, NY.

18. American Concrete Institute, Building Code Require-ments for Reinforced Concrete, ACI 318-89, 1989, De-troit, Mich.

19. American Iron and Steel Institute, Washington, D.C., Fire Resistant Steel Frame Construction.

20. American Iron and Steel Institute, Washington, D.C.,Designing Fire Protection for Steel Columns.

21. American Iron and Steel Institute, Washington, D.C.,Designing Fire Protection for Steel Trusses.

22. American Institute of Steel Construction, Inc., Code of Standard Practice for Steel Buildings and Bridges, Sept.1, 1986, Chicago, Ill.

23. Furlong, Richard W, "Column Rules of ACI, SSRC, and LRFD Compared," ASCE Journal of the Structural Divi-sion, Vol. 109, No. 10, (pp. 2375-2386) New York, NY.24. Deierlein, Gregory G., Joseph A. Yura, and James O. Jirsa,Design of Moment Connections for Composite Framed Structures, Phil M. Ferguson Structural Engineering Laboratory, Bureau of Engineering Research, the Univer-sity of Texas at Austin, May 1988.

NOMENCLATURE

= Area of base plate, in.

= Full cross sectional area of concrete support, in.2

= Net concrete area, in.2

= Gross area of composite section, in.2

= Area of H-shaped portion of base plate, in.

= Area of reinforcing bars, in.2

= Gross area of steel shape, in.2= Base plate width, in.

= Factors used in determining M u for combined bending and axial forces when first order analy-sis is employed

= Compression force in reinforcing bar, kips = Compressive force in concrete, kips

= Factor for calculating Euler buckling strength,kip-ft 2

= Coefficient applied to bending term in interaction formula

= Modulus of elasticity of steel (29,000 ksi)= Modulus of elasticity of concrete, ksi = Modified modulus of elasticity, ksi = Critical stress, ksi

= Modified yield stress, ksi

= Specified minimum yield stress of the type of steel being used, ksi

= Specified minimum yield stress of reinforcing bars, ksi

= Horizontal force, kips

= Effective length factor for prismatic member

= Unbraced length of member measured between the center of gravity of the bracing members, in.= Story height, in.

= Smaller moment at end of unbraced length of beam column, kip-in.

= Larger moment at end of unbraced length of beam column, kip-in.

= Required flexural strength in member due to lateral frame translation, kip-in.= Nominal flexural strength, kip-in.

= Required flexural strength in member assuming there is no lateral translation of the frame, kip-in.= Required flexural strength, kip-in.= Base plate length, in.

= Euler buckling strength, kips = Nominal axial strength, kips

= Factored load contributory to area enclosed by steel shape, kips

= Factored axial load resisted by steel shape, kips = Service load for encased shape limited by bond stress, lbs

= Required axial strength, kips

= Ratio of required axial strength to factored nominal axial strength

= Tension force in reinforcing bar, kips = Tension force in steel shape, kips

= Depth of compression block of concrete in com-posite column, in.

= Overall width of composite column, in.= Flange width, in.

= Distance to outer fiber from plastic neutral axis, in.= Numerical coefficients for calculating modified properties

= Overall depth of member, in.

= Concrete compressive stress, psi or ksi, as applicable

= Overall depth of composite column, in.= Floor-to-floor height, ft

= Factor in bond strength calculation

= Unbraced length of column, in.= Encased length of steel shape, in.

= Cantilever distance in base plate analysis, in.= Cantilever distance in base plate analysis, in.= Radius of gyration, in.

= Radius of gyration of steel shape in composite

column, in.

= Spacing (clear distance), in.= Flange thickness, in.

= Thickness of base plate, in.= Web thickness, in.

= Unit weight of concrete, lbs/ft 3

= Factor for determining depth of concrete in compression

= Translation deflection of story, in.= Column slenderness parameter = Resistance factor for flexure

= Resistance factor for axially loaded composite column

PART 2: SUGGESTED DETAILS FOR COMPOSITE COLUMNS

Typical Detail 1: Composite column elevation.

Typical Detail 2: Composite column cross section.

Typical Detail 3: Composite column joint.