Estimation of the cyclic capacity of beam-to-column dowel connections in precast industrial buildings

Abstract

The behaviour of precast systems depends on the performance of the specific connections between the precast elements. In European precast design practice, the most common type of connection between beams and columns is a dowel connection. Such connections are subject to the following types of potential failure mechanism: (a) local failure characterized by the simultaneous yielding of the dowel and crushing of the surrounding concrete, and (b) global failure, characterized by spalling of the concrete between the dowel and the edge of the column or the beam. In this paper both types of failure of dowel connections are studied, although somewhat more attention is paid to the less investigated global failure. The local failure mechanism has been relatively well investigated, and the results have been presented in several studies. Thus only some minor changes are proposed in connection with the prediction of the related strength. On the other hand, the majority of existing procedures for the estimation of global strength are over-conservative since they neglect the influence of stirrups, or else only take them into account implicitly. None of these methods explicitly take into account the fact that the global failure of the dowel connection is changed by the presence of stirrups from brittle to ductile. In the paper, a new procedure for the estimation of resistance against global failure is proposed. Taking into account an appropriate strut and tie model of the connections, the influence of stirrups on this resistance as well as on the type of the failure is taken into account explicitly. Comparisons that were performed between the analytically calculated strength and the experimental results obtained have clearly shown that both of the proposed procedures for the estimation of resistance against local and global failure agree very well with the experimental results.

Appendix: Application of the proposed procedures to the example of a one-story precast industrial building; comparison with existing procedures

In order to illustrate the differences between the proposed and existing procedures, the strength of a typical connection between a column and a beam is analysed. It is supposed that the column and beam are part of a one-storey precast industrial building, which is 60 m long, 40 m wide and 7 m high (see Fig. 15). The structural system of this building includes 27 evenly distributed cantilever columns (Fig. 16) tied at the top with beam elements by means of dowel connections.

Fig. 15

Plan and front view of the analysed one-storey industrial building

Fig. 16

Typical column section and column section at the top of the column (at the dowel connection) for the analysed industrial building

Taking into account the weight of the roof elements, waterproofing, I beam elements and half of the cladding panels, the weight per square meter distributed over the whole roof area is considered to be $6\,\hbox {kN/m}^{2}$. Thus the total mass is $m_{tot}=1{,}400\,\hbox {t}$, or $m_{1}=52\,\hbox {t}$ per column.

Taking into account properties of cracked columns, the stiffness of a single column is:

The period of vibration T is:

$$\begin{aligned} T=2\pi \sqrt{\frac{m_1 }{k_{col,cr} }}=1.09s \end{aligned}$$

Assuming that the building is founded on soil class B, taking into account Eurocode 8 (CEN 2004a) a design spectrum corresponding to a peak ground motion of 0.25 g, and taking into account a behaviour factor of $q_{p}=3.0$ (ductility class medium) the base shear of a single column is:

$$\begin{aligned} Q_{Ed} =S_a \;m_1 =0.115\cdot 9.81\cdot 52=59\,\hbox {kN}, \end{aligned}$$

Considering second order effects this force is increased to:

$$\begin{aligned} \theta&=\frac{N_{Ed} }{Q_{Ed} }\frac{m_1 }{k_{col,cr} }=\frac{510}{59}\frac{52}{1{,}744}=0.26\\ Q_{Ed,P\Delta }&=Q_{Ed} \frac{1}{(1-\theta )}=59\frac{1}{(1-0.26)}=80\,\hbox {kN} \end{aligned}$$

The corresponding design moment $M_{Ed}$ at the bottom of the column is then

$$\begin{aligned} M_{Ed} =Q_{Ed,P\Delta } \;H=80\cdot 7=560\,\hbox {kNm} \end{aligned}$$

The design flexural resistance of the column (see Fig. 16), corresponding to axial force $N_{Ed}=510\,\hbox {kN}$ is:

$$\begin{aligned} M_{Rd} =565\,\,\hbox {kNm}>M_{Ed} =560\,\hbox {kNm} \end{aligned}$$

The dowel connections should be designed according to the capacity design approach used in Eurocode 8 (CEN 2004a). Thus the design shear load acting on the dowel connection can be estimated as:

$$\begin{aligned} F_{Ed} =\gamma _{Rd} M_{Rd} /h=89\,\hbox {kN} \end{aligned}$$

where $h$ is the height of the column and $\gamma _{Rd}$ is 1.1.

The design strength of the dowel corresponding to local failure, and calculated using Eq. (31) is:

$$\begin{aligned} R_{d,Rd} =2\;d_d ^{2}\;\sqrt{f_{cd} f_{yd} }=135\,\hbox {kN}>F_{Ed} \end{aligned}$$

One gets a similar but somewhat larger strength if Eq. (33) is used:

$$\begin{aligned} R_{d,Rd} =2\cdot 1.03\;d_d ^{2}\;\sqrt{f_{cd} f_{yd} }=149\,\hbox {kN}>F_{Ed} \end{aligned}$$

The design strength of the dowel corresponding to global failure, and calculated according to the procedure proposed in Sect. 3 (see Fig. 5) is:

$$\begin{aligned} R_{d,Rd} =n\;A_{s1} \;f_{yd} =\left[ {(2.5\;d_d +c-a)/s+1\;} \right] A_{s1} f_{yd} =111\,\hbox {kN}>F_{Ed} \end{aligned}$$

If Eq. (2) is taken into account, the global strength is equal to:

$$\begin{aligned} R_{d,Rd}&=(A_{v\;} /A_{v\;0} )\;\psi _s \;\psi _h \;\psi _{ec} \;\psi _\alpha \;\psi _{re} \;R_{n0} =54\,\hbox {kN}<F_{Ed} \nonumber \\ R_{n0}&=1.6\;d_d ^{\alpha }\;l_f ^{\beta }\;f_c ^{0.5}\;c_1 ^{1.5}=29\,\hbox {kN} \nonumber \\ \alpha&=0.1\; (l_f /c_1 )^{0.5}=0.126;\;\beta =0.1\; (d_d /c_1 )^{0.2}=0.072, \nonumber \\ \psi _s&=0.86 \nonumber \\ \psi _h&=\psi _{ec} =\psi _\alpha =1 \nonumber \\ \psi _{re}&=1.4 \nonumber \\ A_V&=(2c_2 +2\cdot 1.5c_1 )\cdot 1.5c_1 =107{,}813\,\hbox {mm}^{2} \nonumber \\ A_{V0}&=3c_1 \cdot 1.5c_1 =70{,}313\,\hbox {mm}^{2} \end{aligned}$$

(37)

The strength according to Eq. (2) is half the size of the value estimated by the proposed procedure. It is also considerably smaller than the design shear load on the connection (the resistance is 61 % of the design shear load).

The strength of the connection can be increased by increasing the distance $c_{1}$, the diameter of the dowel, and the quality of the concrete. If $c_{1}$ is increased to the maximum possible distance $(c_{1}=300\,\hbox {mm})$, the strength of the connection will be increased insignificantly to only 63 kN, which is still only 70 % of the design shear load.

In fact, with the given diameter of the dowels and the concrete strength, design of the connection is not possible. If the dowel diameter is increased to 32 mm instead of increasing the edge distance, the resistance grows to 61 kN. If the design concrete strength is 50 MPa instead of 35 MPa, the resistance is 64 kN. Exploiting all of the above three measures, i.e. increasing the distance $c_{1}$, the dowel diameter, and the concrete strength, the resistance is 83 kN, which is still less than the design load.