鋼梁與SRC柱之梁柱接頭耐震性能之理論分析研究

行政院國家科學委員會專題研究計畫 成果報告

鋼梁與 SRC 柱之梁柱接頭耐震性能之理論分析研究

計畫類別： 個別型計畫 計畫編號： NSC94-2211-E-009-022- 執行期間： 94 年 08 月 01 日至 95 年 07 月 31 日 執行單位： 國立交通大學土木工程學系(所) 計畫主持人： 翁正強 計畫參與人員： 王暉舜、陳信賓 報告類型： 精簡報告 報告附件： 出席國際會議研究心得報告及發表論文 處理方式： 本計畫可公開查詢

中 華 民 國 95 年 10 月 24 日

行政院國家科學委員會補助專題研究計畫成果報告

鋼梁與

SRC 柱之梁柱接頭耐震性能之理論分析研究

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個別型計畫

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整合型計畫

計畫編號：NSC－94－2211－E－009－022

執行期間：94 年 8 月 1 日 至 95 年 7 月 31 日

計畫主持人： 翁正強 教授

計畫參與人員： 王暉舜、

陳信賓

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涉及專利或其他智慧財產權，

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一年後可公開查詢

執行單位：

國立交通大學土木工程學系(所)

中 華 民 國 95 年 10 月 20 日

鋼梁與 SRC 柱之梁柱接頭耐震性能之理論分析研究

Analytical Study on Seismic Performance of

Steel Beam-to-SRC Column Connections

C. C. Weng

Professor, Department of Civil Engineering, National Chiao Tung University

ABSTRACT: Presented herein is an analytical study to predict the seismic resisting

capability of steel reinforced concrete (SRC) beam-to-column connection. The main reasons of using SRC structural members are to take their advantages in strength and stiffness as well as in fire protection and corrosion resistance. In this study, the shear strength of the SRC beam-to-column connection is taken as the sum of the shear capacities of the steel section and the reinforced concrete in the connection zone. The steel section in the connection is able to develop shear yielding in ultimate state without premature local buckling due to the protection of the concrete. The strut-and-tie model is utilized to determine the shear capacity of the reinforced concrete in the connection. Furthermore, in recognition of the superior confining effect provided by the steel section in SRC column, the concrete in the connection is sub-divided into two zones, namely “the highly confined area” and “the ordinarily confined area,” so that the shear strengths of each part of the concrete can be predicted more rationally.

KEYWORDS: Steel Reinforced Concrete (SRC); Beam-to-Column Connection; Seismic

Resisting Capability; Shear Strength; Concrete Confinement; Strut-and-Tie Model; New Design Approach

1. INTRODUCTION

With the fast advance of the construction technology, it has become increasingly popular to design buildings with composite structural system. The steel reinforced concrete (SRC) structural system provides a building with advantages including the ductility of structural steel and the stiffness of reinforced concrete. Additional merits of the SRC structural members are that the concrete also protects the steel shape from corrosion, fire damage and local buckling failure [1, 2].

The SRC structural system has been successfully used in Japan for more than half century [3]. The architectural institute of Japan published its first SRC design code in 1958 and released a latest edition in 2001 [4]. In the United States, guidelines for the design of composite structural system were first introduced in the 1994 NEHRP seismic provisions [5]. Latest design provisions of composite structures can be found from the ACI building code [6], the AISC design specification [7], and the AISC seismic provisions [8]. In Europe, the design

guidelines of composite structures can be found from the Eurocode [9]. In Taiwan, buildings constructed with SRC structural system have increased sharply after the Ji-Ji earthquake in 1999. The statistics released by the housing department of Taiwan in 2004 showed that about 19 % of the newly finished buildings were SRC structures. The ministry of the interior of Taiwan published its first official edition of SRC building design code in 2004 [10].

Figure 1 shows a typical layout of a SRC beam-to-column connection. A literature survey conducted by the authors indicated that past studies on the shear strength of SRC beam-to-column connection are very limited. It was found that a majority of past researches were focused on the steel beam-to-concrete filled tubular column (CFT) connections.

2. THE ACI-318 APPROACH

In the current ACI-318 code, the nominal shear strength V_nof a reinforced concrete beam-to-column connection can be determined as follows:

(a) For joints confined on all four faces

'

n c j

V =1.67 f A (1) (b) For joints confined on three faces or on two opposite faces

' n c j V =1.25 f A (2) (c) For others ' n c j V =1.0 f A (3) where ' c

f is the compressive strength of concrete in MPa; and A is the effective shear area at _j the joint.

It has been known that the above equations are relatively conservative and maybe over-simplified. Furthermore, due to the existence of a steel section in the SRC column, it would not be appropriate to directly adopt these equations to predict the shear capacity of the SRC beam-to-column connection.

3. AN ALTERNATIVE APPROACH

This study develops a new analytical approach to estimate the shear strength of SRC beam-to-column connection. In general, the design shear strength φvVnof the SRC beam-to-column

connection is taken as the sum of the shear capacities of the steel section φ_vsV_ns and the accompanying reinforced concrete φvrc nrcV in the connection zone. That is

vV =n vsVns vrcVnrc

φ φ + φ (4)

where φ_vs=0.85 and φ_vrc=0.75.

The mutual beneficial correlation between the steel section and the reinforced concrete in the SRC joint is one of the important factors in developing the new design approach. It is noted that due to the protection of the reinforced concrete, the steel section embedded in the SRC connection is able to develop “shear yielding” in the ultimate state without premature failure caused by local buckling. Thus, the nominal shear strength of the steel section can be taken as

ns ys w

V =0.6f A (5) where fys is the yield strength of the steel section; and Awis the area of the web of the steel

section.

In addition, as shown in Fig. 2, in recognition of the superior confining effect provided by the steel section in the SRC column, the concrete within the column is sub-divided into two different zones, namely “the highly confined area” and “the ordinarily confined area,” so that the shear strengths of each part of the concrete can be predicted more rationally.

In this study, the strut-and-tie model is utilized to estimate the shear capacity of the reinforced concrete in the connection. Figures 3 (a) and (b) show the possible diagonal compressive struts in the SRC beam-to-column joint. The shear capacity of the reinforced concrete in SRC joint, V_nrc, is taken as the sum of the horizontal components of the capacities of the diagonal compressive struts of the “highly confined concrete” and the “ordinarily confined concrete” in the connection zone. That is

nrc hc oc

V =V +V (6) whereVhc and Vocare the shear capacities of the “highly confined concrete” and the “ordinarily

confined concrete” in the connection, respectively.

As shown in Fig. 4, in the highly confined area, it is possible for this part of concrete to remain sound without major cracking due to the protection the steel section in the SRC joint. Thus, by using the strut-and-tie model, the shear strength of the “highly confined concrete” can be approximated as the horizontal component of the maximum capacity of the compressive strut. It is further simplified as

( )

'

( )

hc cc _{hc eff hc}

V =0.85 f × A (7)

where

( )

' cc hc

f is the compressive strength of the highly confined concrete considering the

beneficial confinement effect provide by the steel section and the hoop reinforcements; and

(Aeff)hcis the effective area of the highly confined concrete in the connection.

As shown in Fig.5, in the ordinarily confined area, it is more difficult for this part of concrete to remain sound in the ultimate state due to the absence of the confinement provided by steel section. However, it is noted that the shear strength of the “ordinarily confined concrete” is significantly influenced by the constraint provided by the surrounding SRC beams at the joint. Thus, by using the strut-and-tie model, the shear strength of the “ordinarily confined concrete” can be approximated as the horizontal component of the maximum capacity of the compressive strut. It is further simplified as

(a) For joints confined on all four faces

( )

'

(

)

oc cc _{oc eff oc}

V =0.85 f × A (8) (b) For joints confined on three faces or on two opposite faces

( )

'

(

)

oc cc _{oc eff oc} V =0.63 f × A (9) (c) For others

( )

'

(

)

= × (10)

where

( )

' cc oc

f is the compressive strength of the ordinarily confined concrete considering the beneficial confinement effect provide by the hoop reinforcements; and

(

A_{eff oc}

)

is the effective area of the ordinarily confined concrete in the connection.

It is found that the effective area of the “highly confined concrete”

(

A_{eff hc}

)

can be expressed as

(

Aeff

)

_hc=bcf×ahe (11)

where b_cf is the flange width of the steel column perpendicular to the shear direction; and

he

a is the effective depth of the highly confined concrete parallel to the shear direction.

hc hc

he ' ' hc hc hc

c cf cs c cf cs

M P

a 1 cos (cos sin ) 0.85f b h 0.85f b h

⎛ ⎞

= ⎜_⎜ + ⎟_⎟× θ θ + θ

⎝ ⎠ (12)

where M_hcand P_hcare the bending moment and the axial force shared by the highly confined concrete, respectively; h_csis the depth of the steel column parallel to shear direction; and θ_hcis the angle between the horizontal shear force and the diagonal strut of the highly confined concrete.

Also, the effective area of the “ordinarily confined concrete”

( )

A_{eff oc}can be expressed as

(

Aeff

)

oc =boe×aoe (13)

where b is the effective width perpendicular to the shear direction; and_oe a is the effective _oe depth of the ordinarily confined concrete parallel to the shear direction. b and _oe a can be _oe found as follows:

(

)

(

)

oe bf y bf ocx cf

b =min b⎡_⎣ +2b , b +h ⎤_⎦−b (14) where b is the flange width of steel beam; and _bf b is the distance from the edge of _y steel beam flange to the center of hoop reinforcement parallel to the shear direction.

oc oc

oe ' ' oc oc oc

c oc ocx c oc ocx

M P

a 1 cos (cos sin )

0.85f b h 0.85f b h

⎛ ⎞

= _⎜ + _⎟× θ θ + θ

⎝ ⎠ (15)

where M and _oc P are the bending moment and the axial force shared by the _oc ordinarily confined concrete, respectively;b is the width of the ordinarily confined _oc concrete perpendicular to shear direction; h is the depth of the ordinarily confined _ocx concrete parallel to shear direction; and θ is the angle between the horizontal shear _oc force and the diagonal strut of the ordinarily confined concrete.

4. CONCLUDING REMARKS

The proposed new approach is intended to provide an alternative method to predict the shear strength of the SRC beam-to-column connection. This study takes into account the composite nature of the SRC joint so that the interaction between the steel section and the reinforced

concrete in the connection zone can be treated more rationally. The mutual beneficial correlation between the steel section and the reinforced concrete in the SRC joint is one of the important factors in developing the new design approach. It is also hoped that this investigation will provide further insight on the understanding on the true mechanical behavior of the SRC beam-to-column connection.

5. REFERENCES

1. Weng, C.C., Yen, S.I. and Jiang, M.H.( 2002), Experimental Study on Shear Splitting Failure of Full-Scale Composite Concrete Encased Steel Beams. Journal of Structural

Engineering, ASCE, Vol. 128, No. 9, pp. 1186-1194.

2. Weng, C.C. and Yen, S.I. (2002), Comparisons of Concrete-Encased Composite Column Strength Provisions of ACI Code and AISC Specification. Engineering Structures, Vol. 24, No.1, pp. 59-72.

3. Wakabayashi, M.A. (1987), Historical Study of Research on Composite Construction in Japan. Proc. of Conf. on Composite Construction in Steel and Concrete, ASCE, New York, pp. 400-427.

4. AIJ (2001), Standards for Structural Calculation of Steel Reinforced Concrete Structures. Architectural Institute of Japan, Tokyo, Japan.

5. FEMA (1994), Recommended Provisions for the Development of Seismic Regulations for

New Buildings. FEMA-222A, National Earthquake Hazards Reduction Program, Building

Seismic Safety Council, Washington, D.C.

6. ACI (2005), Building Code Requirements for Structural Concrete (ACI 318) and

Commentary (ACI 318R). American Concrete Institute, Farmington Hills, Michigan.

7. AISC (2005), Load and Resistance Factor Design Specification for Structural Steel

Buildings. 4th Ed., American Institute of Steel Construction, Chicago, Illinois.

8. AISC (2002), Seismic Provisions for Structural Steel Buildings, American Institute of

Steel Construction, Chicago, Illinois.

9. ECCS (1994), Design of Composite Steel and Concrete Structures, Part 1.1: General

Rules and Rules for Buildings, ENV 1994-1-1: Eurocode 4, European Convention for

Constructional Steelwork.

10. MOI (2004), Building Code for Design of Steel Reinforced Concrete (SRC) Structures. Ministry of the Interior, Taiwan, Taipei (in Chinese)

Figure 2 “Highly confined concrete” and “ordinarily confined concrete” in SRC columns

Highly Confined Area Ordinarily Confined Area

Unconfined Area

Highly Confined Area

Ordinarily Confined Area

Unconfined Area

M P Strut Node M M V P Strut Node

Figure 3 Compressive strut of concrete in SRC beam-to-column joint

(a) Strut of ordinarily confined concrete (b) Strut of highly confined concrete

Ordinarily Confined Area Highly Confined Area

Figure 4 Highly confined area of concrete in the SRC joint

Highly Confined Concrete

V V