Study on the interfacial shear behavior of steel reinforced concrete (SRC) members with stud connectors after fire

Abstract

Statically push-out tests of 20 steel reinforced concrete short columns (SRCSC) with stud connectors on the surface of shape steel after fire and two SRCSC under ambient temperature were carried out, in order to study the failure mode, load-slip relationship and the interfacial shear transfer of SRC members after fire. Experimental results show that the typical failure modes and load-slip curves of SRCSC after fire are almost the same as the case under ambient temperature. The interfacial shear transfer of SRCSC declines exponentially not only with the increase of the peak temperature the specimen experienced but also with the increase of the peak temperature duration. The interfacial shear transfer of the specimens with studs arranged at the steel web is much higher than those with studs arranged at the steel flange. Empirical formulas of SRCSC interfacial shear transfer after fire are proposed, and the calculated results generally agree well with the experimental results.

Keywords

steel reinforced concrete (SRC) / short column / stud connector / after fire / interfacial shear transfer

Introduction

Steel Reinforced Concrete (SRC), combining the advantages of reinforced concrete (RC) and steel, is applied increasingly to modern architectures, especially complicated high-rise buildings. However, in SRC structures, the bond strength between shape steel and concrete is so insufficient that the interfacial failure is prone to take place. Bryson [1], Hawkins [2], Charles [3], Hamdan [4] and Wium [5] studied the bond strength and load-slip relationship of SRC based on push-out tests and short column tests respectively. To improve the interfacial performance and ensure the force transfer between shape steel and concrete, stud connectors are always arranged in specific positions, such as bottom of SRC column, steel column of SRC along the column direction, transition zone of RC column, steel beam of SRC along the beam direction and transition zone of RC beam.

The interfacial shear transfer of SRC members with stud connectors is implemented by the natural bonding between shape steel and concrete and the shear resistance of studs. A lot of research on the shear behavior of SRC members with stud connectors under ambient temperature has been published. Based on the push-out tests of SRCSC with stud connectors, Roeder et al. [6] found that confining reinforcement has little impact upon the bond stress capacity, but increased confinement increases the post slip resistance. Shear connectors combined with natural bond stress may produce smaller load transfer than bond stress acting alone. Based on the tests of twenty-two SRCSC specimens, Sun and Wang [7] analyzed the modes and occurrence conditions of different failures, and pointed out the main factors that affect the properties of force transfer. And a shear transfer formula was proposed. According to the load mechanism and force spread principles of SRCSC, Li et al. [8] proposed a force transfer formula and structural approaches to prevent the concrete splitting, and gave a support to the reasonable arrangement of stud connectors in SRC structures.

As the temperature goes up in fire, the shear capacity of studs and the natural bond strength between steel and concrete become weak gradually because of the decreased material strength of steel and concrete, leading to the interfacial shear transfer of SRC declines afterwards [9–14]. Based on our previous work [15], it is found that the interfacial shear transfer of SRCSC with stud connectors declines negative exponentially with temperature. The interfacial shear transfer of the specimens with studs arranged at the steel flange at 200°C is only 64% of those under ambient temperature, and the percentage becomes 50% at 400°C and 42% at 600°C respectively. To the case of studs arranged at the steel web, the percentage becomes 52%, 45% and 43% respectively. It can be concluded that temperature plays an important role in the interfacial shear transfer of SRCSC with stud connectors.

After fire, the material strength of steel and concrete will partially recover as the temperature goes down. Nonetheless, the degree of recovery is closely related to the peak temperature the structure experienced and its duration, but it is impossible to regain the initial strength in any case. Therefore, the bond strength between steel and concrete and the shear capacity of studs after fire are essentially different from the case of ambient temperature or fire. Unfortunately, the issues of post-fire performance of SRC columns are seldom reported.

For the sake of studying the influence of the peak temperature, including the maximum value and duration, on the shear transfer behavior of SRCSC with stud connectors, push-out tests of 20 SRCSC with stud connectors arranged on the surface of shape steel after fire and two SRCSC under ambient temperature are carried out. This paper is organized as follows. Section 2 introduces the details of experiments, including specimen dimensions, material properties, heating program, test instrument and loading procedure. Experimental results are given out and discussed in Section 3. In Section 4, parameter analysis is performed. And empirical formulas of SRCSC’s interfacial shear transfer after fire are proposed in Section 5, followed by conclusions in Section 6.

Experiments

Specimen dimensions and parameter design

Twenty-two specimens were prepared. Twenty of the specimens were used to conduct push-out tests after fire and cooled naturally, and the other two were used to do comparative experiments under ambient temperature. The section size of each specimen was 350 mm × 350 mm with H100 × 100 × 6 × 8 hot-rolled H-shaped steel in core zone. The section steel ratio was 1.78%. The stud connectors were arranged at steel web or steel flange in different specimens. The stud was 70 mm in length and 13 mm in diameter. A longitudinal steel reinforcement was placed at each corner with a 30 mm thick concrete cover. The ratio of reinforcement was 0.37% with stirrups of 6 mm in diameter and 100 mm in spacing. Figure 1 shows the dimensions and steel configuration of different specimens.

Test program was designed as follows, heat the specimen to target temperature and keep the peak temperature for a while at first. Then cool the specimen naturally, followed by the push-out test carried out. The key experimental parameters, including anchorage length of steel, position of studs, peak temperature and its duration, as shown in Table 1.

Material properties

The specimens were made by vertical production, and a number of thermocouples were embedded in some specimens in order to measure the inner temperature distribution while heating, as described in Fig. 2. Standard concrete cubes were produced as specimens were concreted, and the cubes and specimens were cured under the same environment. Concrete compression strength was measured under ambient temperature according to “Standard for Test Method of Mechanical Properties of Ordinary Concrete” (GB/T50081-2002), and the results are listed in Table 1. According to “Metal Materials Tensile Test at Room Temperature” (GB/T228-2002), tensile tests were carried out to measure the yield strength and ultimate tensile strength of steel, longitudinal carrying bar, stirrup and stud under ambient temperature respectively, as shown in Table 2.

Heating program

After 28-days curing, SRCSC specimens were put into a stove to be heated. Figure 3 shows the stove and its temperature control system. The heating program is described here briefly. Firstly, put the specimen at the center of furnace. Secondly, set the peak temperature and heating power, and then power the furnace on for a designed period. Thirdly, power off and open the door of furnace and cool the specimen naturally. Typical heating curves under different heating conditions are described in Fig. 4.

Figure 5 records the rise of temperature at different points in the specimen section when the peak temperature is set in 800°C. It can be seen that the temperature in the specimen falls behind that in the stove. The closer the point to the steel, the lower temperature it has. After 225 min of heating, the internal temperature is still beyond the peak temperature, therefore, it can be concluded that it will take quite a long time to make the specimen have a uniform temperature distribution. Consequently, the peak temperature is taken as a representative parameter of temperature and in-depth study on the effect of internal temperature on the interfacial shear behavior of SRCSC is underway.

Test process

The specimen was put on the compression testing machine of 500 tons after cooling, as shown in Fig. 6. Make the lower projecting shape steel meet the base plate and keep the upper shape steel free. Lift the base plate up to engender lengthways thrust in order to give force to the lower shape steel, and then the force is transferred to concrete by natural bond stress between steel and concrete and shear capacity of studs. During the testing, the value of lengthways thrust was measured by a load transducer, and the relative slip between shape steel and concrete was measured by displacement meters at the loading end and at the free end.

Experimental results

Destruction process

At the beginning of loading, no cracks could be observed on the surface of the specimen. The lengthways cracks appeared at the upper part of the specimen on the side with studs when the load reached 65% of the ultimate load. Cracks went downwards as the load rising, and the crack width increased once the crack reached the stud. Meanwhile, the crosswise cracks appeared and developed rapidly. The studs ruptured with a clear snap after a serious deformation. Then the load descended immediately and kept in a low level. A typical failure mode is shown in Fig. 7. It can be seen that no obvious crack appears on the surface of concrete where no stud is arranged.

End cracks

Figure 8 shows the cracks at the loading end and at the free end. It can be seen that cracks mainly appeared in the area of flange limb pointed of the shape steel and outside the studs. Most cracks developed outwards along the angle of 45° and a few cracks grew horizontally parallel to the flange on both sides. Although more cracks were observed at the free end, the mode of cracks was similar to that at the loading end.

Load-slip curves

The load-slip curves after fire are drawn in Fig. 9. It demonstrates that the curve of the loading end agrees well with that of the free end. And the load-slip process can be divided into five stages:

1) No slip stage

When the load is less than 20% of the ultimate value, no obvious slip can be observed at the loading end or at the free end. Due to the action of chemical bonding between shape steel and concrete, the shear capacity of studs is inconspicuous.

2) Rising straight line stage

When the load is between 20% and 80% of the ultimate value, the slip at the loading end can be observed but the slip at the free end is still inconspicuous. The chemical bonding between shape steel and concrete is destroyed by relative slippage. The longitudinal load is transferred through studs and friction between shape steel and concrete. The load-slip relationship can be expressed linearly.

3) Rising curve stage

When the load is over 80% of the ultimate value, the slip at the loading end develops rapidly and the slip at the free end appears. There is no chemical bonding between shape steel and concrete at this stage. When the load approaches the ultimate value, splitting cracks appear in the concrete outside the studs and the slip stiffness drops significantly.

4) Decreasing curve stage

After the ultimate load, the slip distance and the width of cracks develop sharply not only at the loading end but also at the free end. The studs are snipped one by one when the slip distance reaches a certain value, and the load-slip curve decreases in a stair-step shape.

5) Residual stage

Finally, the load-slip curve reaches a plateau when the slip distance exceeds a certain value.

Parameter analysis

The peak temperature

Figure 10 shows the effect of the peak temperature on the load-slip relationship with the anchorage length of steel, position of studs and the peak temperature duration unchanged. It can be seen that the interfacial shear transfer declines with the increase of the peak temperature.

The peak temperature duration

If the anchorage length of steel, position of studs and the peak temperature remain the same, the effect of the peak temperature duration on load-slip curves is describes in Fig. 11. Overall, the interfacial shear transfer decreases with the increase of the peak temperature duration as well.

Position of studs

Figure 12 shows the load-slip curves with studs arranged at the steel web and at the steel flange respectively. It can be observed that, with other conditions unchanged, the interfacial ultimate shear transfer and the residual strength of the specimens with studs arranged at the steel web are higher than those with studs arranged at the steel flange because the concrete cover outside the steel web is thicker.

Anchorage length of steel

Figure 13 shows load-slip curves with different anchorage length of steel. When other conditions are kept the same, the interfacial shear transfer of SRCSC goes up with the increase of anchorage length of steel.

Calculation of the interfacial shear transfer

Influence coefficient of the interfacial shear transfer after fire

Figure 14 demonstrates the relationship between the interfacial shear transfer and the peak temperature, which is generally in exponential distribution.

k_T is defined as the influence coefficient of the peak temperature on the interfacial shear transfer of SRCSC with stud connectors, and calculated by

(1)k_{\mathrm{T}}={\displaystyle \frac{P_{\mathrm{u}}(T)}{P_{\mathrm{u}}}}.

where P_u is the interfacial shear transfer of SRCSC with stud connectors under ambient temperature, Pu(T) is the interfacial shear transfer of SRCSC with stud connectors suffering a period of fire. Through a statistical analysis of the test results, when the duration is 30 min, k_T of the specimen with studs arranged at the steel flange and at the steel web can be estimated by Eq. (2) and Eq. (3) respectively.

(2)\mathrm{A}\mathrm{t} \mathrm{f}\mathrm{l}\mathrm{a}\mathrm{n}\mathrm{g}\mathrm{e}:k_{\mathrm{T}}=\mathrm{1.22}{e}^{\frac{T}{-\mathrm{1388.01}}}-\mathrm{0.20}(\mathrm{20}℃\le T\le \mathrm{800}℃),

(3)\mathrm{A}\mathrm{t} \mathrm{w}\mathrm{e}\mathrm{b}:k_{\mathrm{T}}=\mathrm{0.52}{e}^{\frac{T}{-\mathrm{781.48}}}+\mathrm{0.49}(\mathrm{20}℃\le T\le \mathrm{800}℃).

Figure 15 shows the relationship between the interfacial shear transfer and the peak temperature duration. It is found that the interfacial shear transfer declines exponentially with the increase of the peak temperature duration.

k_t is defined as the influence coefficient of the peak temperature duration on the interfacial shear transfer of SRCSC with stud connectors, and calculated by

(4)k_{\mathrm{t}}={\displaystyle \frac{P_{\mathrm{u}}(T,t)}{P_{\mathrm{u}}(T)}},

where P_u (T, t) is the interfacial shear transfer of SRCSC with stud connectors when the peak temperature is T and the duration is t. P_u (T) is the corresponding value assuming the duration is 30 min. Also through a statistical analysis, k_t can be estimated by Eq. (5) and Eq. (6) respectively.

(5)\mathrm{A}\mathrm{t} \mathrm{f}\mathrm{l}\mathrm{a}\mathrm{n}\mathrm{g}\mathrm{e}:k_t=\mathrm{0.102}{e}^{\frac{t}{-\mathrm{26.68}}}+\mathrm{0.97}(\mathrm{0}\min \le t\le \mathrm{90}\min ),

(6)\mathrm{A}\mathrm{t} \mathrm{w}\mathrm{e}\mathrm{b}:k_t=\mathrm{0.27}{e}^{\frac{t}{-\mathrm{73.98}}}+\mathrm{0.82}(\mathrm{0}\min \le t\le \mathrm{90}\min ).

Calculation of the interfacial shear transfer after fire

Taking the peak temperature and its duration into account, the interfacial shear transfer of SRCSC with stud connectors after fire is expressed as

(7)P_{\mathrm{u}}(T,t)=k_{\mathrm{T}}{k}_{\mathrm{t}}{P}_{\mathrm{u}},

where P_u is the interfacial shear transfer of SRCSC with stud connectors under ambient temperature. According to our previous work, for the case of studs arranged at the steel web, P_u can be calculated by

(8)P_{\mathrm{u}}=P_{\mathrm{1}}+P_{\mathrm{2}}+P_{\mathrm{3}},

(9)P_{\mathrm{1}}=n\cdot \min (\mathrm{0.7}{A}_{\mathrm{s}}\gamma f; \mathrm{0.43}{A}_{\mathrm{s}}\sqrt{f_{\mathrm{c}}{E}_{\mathrm{c}}}),

(10)P_{\mathrm{2}}=(-\mathrm{0.0468}+\mathrm{1.47}\times C_{\mathrm{s}\mathrm{s}}/d+\mathrm{1.5708}{\rho }_{\mathrm{s}\mathrm{v}})f_{\mathrm{t}}{b}_{\mathrm{f}}{L}_{\mathrm{e}},

(11)P_{\mathrm{3}}={\displaystyle \frac{n_{\mathrm{1}}}{\mathrm{2}n}}\mu P_{\mathrm{1}},

where n is the number of studs, A_s is the section area of stud, f is the tensile strength of stud, γ is the ratio of minimum tensile strength and the yield strength of stud, f_c is the compression strength of concrete, E_c is the Young’s modulus of concrete, C_ss is the thickness of concrete cover outside the shape steel, d is the section depth of shape steel, ρ_sv is stirrup ratio, f_t is the tensile strength of concrete, b_f is the width of steel flange, L_e is the anchorage length of shape steel, n₁ is the number of studs near the steel flange, μ is the friction coefficient between steel flange and concrete, which is 0.5 here.

For the case of studs arranged at the steel flange, P_u can be calculated by

(12)P_{\mathrm{u}}=P_{\mathrm{1}}+P_{\mathrm{4}},

(13)P_{\mathrm{4}}=(-\mathrm{0.0234}+\mathrm{0.735}\times C_{\mathrm{s}\mathrm{s}}/d+\mathrm{0.7854}{\rho }_{\mathrm{s}\mathrm{v}})(b_{\mathrm{f}}+h)f_{\mathrm{t}}{L}_{\mathrm{e}},

where h is the height of steel web.

Comparison of experimental results with calculated results

Table 3 compares the experimental results with the results calculated by Eq. (7). The material strength is measured under ambient temperature. Overall, it can be observed that the calculated interfacial shear transfer agree well with the experimental data so that the fitting formulas are proved to be safe.

Comparison of the interfacial shear transfer

Comparing the experimental results in Table 3 with the case in fire (see Ref. [15]), it can be found that the interfacial shear transfer of SRCSC with stud connectors after fire is higher than that in fire. The ratios of interfacial shear transfer of 20 specimens after fire and in fire are figured out. The maximum, the minimum and the average value are 2.01, 1.48 and 1.77 respectively. It can be inferred that the shear strength of stud gradually recovers after fire thus the interfacial shear transfer of SRCSC with stud connectors is enhanced.

Conclusion

Statically push-out tests of twenty SRCSC with stud connectors after fire and two SRCSC under ambient temperature were carried out. Based on the experimental results, it can be seen that the failure mode of SRCSC with studs after fire is similar to that under ambient temperature. The splitting cracks appear outside the studs when the specimen fails and cracks prone to appear at the free end. But the developing patterns of cracks are the same at the free end and at the loading end. The load-slip curve of SRCSC with studs after fire can be divided into five stages, including no slip stage, rising straight line stage, rising curve stage, decreasing curve stage and residual stage. The interfacial shear transfer of SRCSC declines exponentially with the increase of the peak temperature when the peak temperature duration is unchanged. The interfacial shear transfer of SRCSC also declines exponentially with the increase of the peak temperature duration when the peak temperature is a constant. The interfacial shear transfer of the specimen with studs arranged at the steel web is higher than that with studs at the steel flange when the peak temperature and the peak temperature duration are unchangeable. The calculation method of the interfacial shear transfer of SRCSC with stud connectors after fire is given out through a statistical analysis based on the experimental results. And the calculation results generally agree well with the experimental data. The interfacial shear transfer of SRCSC with stud connectors after fire is higher than the case in fire when other experimental conditions are the same.

References

[1] BrysonJ O, MatheyR G. Surface condition effect on bond strength of steel beams in concrete. Journal of ACI, 1962, 59(3): 397–406

[2] HawkinsN M. Strength of concrete encased steel beams. Civil Engineering Transaction of the Institution of Australia Engineer, 1973, CE15(1): 39–46

[3] CharlesC W. Bond stress in embedded steel shapes in concrete. Composite and Mixed Construction. New york: ASCE, 1984

[4] HamdanM, HunaitiY. Factors effecting bond Strength in composite columns. In: Proceedings of the 3rd International Conference on Steel-Concrete Composite Structures. Fukuoka, Japan, 1991: 213–218

[5] WiumJ A, LebetJ P. Simplified calculation method for force transfer in composite columns. Journal of the Structural Division, 1994, 120(3): 728–746

[6] RoederC W, ChmielowskiR, BrownC B. Shear connector requirements for embedded steel sections. Journal of Structural Engineering, 1999, 125(2): 142–151

[7] SunG L, WangY J. Experimental study and calculation of axial load transmission in the top section of encased columns. Journal of Building Structures, 1989, 10(6): 40–49 (in Chinese)

[8] LiJ H, WangG F, QiuD L, YuK. Study on the force transfer behavior of SRC members with stud shear connectors. China Civil Engineering Journal, 2012, 45(12): 74–82 (in Chinese)

[9] KatzA, BermanN, BankL C. Effect of high temperature on bond strength of FRP rebars. Journal of Composites for Construction, 1999, 3(2): 73–81

[10] HunaitiY M. Bond strength in battened composite columns. Journal of Structural Engineering, 1991, 117(3): 699–714

[11] JiangS C, LiG Q, LiM F. Experimental research on bond strength between profiled steel sheet and concrete at elevated temperature. Journal of Tongji University, 2003, 31(3): 273–276 (in Chinese)

[12] ChoiS K, HanS H. Performance of shear studs in fire. Proceedings of International Conference Application of Structural Fire Engineering, Prague: Czech Technical University, 2009, 490–495

[13] MirzaO, UyB. Behaviour of headed stud shear conncetors for composite steel concrete beams at elevated temperature. Journal of Constructional Steel Research, 2009, 65(3): 662–674

[14] ChenL Z, JiangS C, LiG Q. Numerical simulation of structural behavior of stud shear connectors at elevated temperature. Journal of Disaster Prevention and Mitigation Engineering, 2012, 32(1): 77–83 (in Chinese)

[15] LiJ H, YuK, WangG F, QiuD L. Study on the force transfer behavior of SRC members with stud shear connectors at elevated temperatures. Journal of Building Structures, 2013, 34(8): 37–45 (in Chinese)

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