1. Introduction

Figure 1. Precast concrete column – pedestal pocket foundation joint. Adapted from [2].

Figure 1. Precast concrete column – pedestal pocket foundation joint. Adapted from [2].

Figure 2. The precast socket of a pocket foundation with keyed internal walls (left), the geometry of the castellation (center) considering the most restrictive indications stated in [1,2,3,13,14,15,16,17,18,19] and the base of the prefabricated concrete columns stored in a stack (right).

Figure 2. The precast socket of a pocket foundation with keyed internal walls (left), the geometry of the castellation (center) considering the most restrictive indications stated in [1,2,3,13,14,15,16,17,18,19] and the base of the prefabricated concrete columns stored in a stack (right).

2. Materials and Design Method

2.2. The Technology of Mounting the Column in the Pocket Foundation

The sequence of stages for mounting the prefabricated reinforced concrete column in the pocket foundation [5,20,26,27,28] can be summarized as:

(a)

marking the axes of the foundation and positioning the centering device on the bottom of the pocket on a concrete layer approx. 50 mm thick (at least of the same strength class as the infill concrete);

(b)

marking the station points of the mobile crane (by beating poles or painting the ground surfaces) and parking points for the trailer (when the installation is done directly with unloading from the transport equipment), in accordance with the technological project;

(c)

transporting the columns in the vicinity of the final mounting position;

(d)

cleaning the inner surfaces of the pocket and the outer surfaces at the base of the column;

(e)

after positioning and centering the mobile crane, the handling device is fixed on its hook and mounting of the columns starts;

(f)

after placing the column horizontally on the ground, next the column is lifted again to be brought to a vertical position (by crawling, turning or tipping);

(g)

cleaning the column`s leaning areas and marking down the structural axes on the sides;

(h)

fastening the guide ropes to the column;

(i)

the slow lowering of the column into the foundation and adjusting its position by workers with the help of ropes and two crowbars, so that the axes marked on the column coincide with the position of the axes marked on the foundation;

(j)

verifying the verticality of precast column on the two orthogonal sides using a plumb line, bubble level (spirit level) or laser level, followed by the initial temporary fixation of the column at the foundation level by hammering special hardwood wedges (e.g. oak, beech, acacia, etc.) on each side (minimum 6 wedges in total, of which on two parallel faces there are at least 2 pieces) [20]. The geometry of a wedge is a quadrilateral prism type, with one of the faces chamfered under an angle of 10 sexagesimal degrees [20], with a length of about 300 mm and a width of about 100 mm (Figure 4). The geometry of the wedge is made in such a way that after hammering it remains above the pocket foundation approx. 120 mm. The use of wedges with too large angles, or wedges made of soft wood (e.g. spruce, fir, pine, etc.) or overlapping wedges to compensate for their insufficient thickness – may lead to the rotation of the columns in the foundation under the wind action or under an accidental action (e.g. the collision of heavy members with columns) and their collapse in the intermediate phases of construction execution (Figures S01-S09, Video V1 and V2). In the mentioned figures and videos one can see the domino effect following the rotation of a column in the pocket foundation caused by a gust of wind with speed of up to 80 km/h. The dynamic action of wind can cause the expulsion of the wooden wedges as a result of the cyclical/balanced movement of the precast column. From a row of 11 precast reinforced concrete columns, the first column did not rotate in the pocket foundation, the second column rotated (the wooden wedges being crushed or expelled) and the deformed shape of the column was large enough to hit the third standing column (S01). Following the impact, the third column rotated in the pocket foundation (by crushing the wooden wedges or by expelling them, S02) hitting the fourth column. After the impact, the fourth rotated at the base and deformed and broke under its own-weight. Now broken, in the fall the fourth column hit the fifth column with amplification from the shock, and the rest of the failures happened similarly, ending with a total of 8 prefabricated columns collapsing (S03-S09). It can be noticed that the first fallen columns (fourth and fifth) developed a single plastic joint each (the cross-section broke at the base), but the next 5 columns developed two plastic joints each (one at the base and one at the middle of the height as a result of the change of the static scheme: from a cantilevering beam to that of a beam hinged in the foundation and supported at the top). The causes of the failure could be one or more of the following: high wind loading in the intermediate construction phase (the precast column – pocket foundation interspace was not completed), the fixing with wooden wedges was not correctly carried out (insufficient wedges, wedges with a chamfering of only about 20°...30 °, the wooden wedges were short, the thickness of the wooden wedges was supplemented by using short ends of wooden slats or softwood was used) and the distance between the pocket foundation internal wall and the precast concrete column (thickness of the concrete infill layer) was excessively large, a situation in which after the expulsion of the wedges, the rotation of the column at the base was so great that its tip collided with the neighboring column;

(k)

checking the verticality of the precast column using more precise equipment (with topometric devices), adjusting the position of the column by loosening/hammering the oak wedges;

(l)

for slender precast reinforced concrete columns as well as for those whose length exceeds 12 m, it is recommended to use tilt props or scaffoldings, for additional temporary fixing (Figure 5). The tilt props will be mounted in such a way that they can take compression forces (and tensile forces, if possible), their inclination should be at 30°...45° (angle measured between the longitudinal axis of the prop and the column), and their arrangement must provide stability in two orthogonal directions in the horizontal plane. In the case of exceptionally long columns, scaffolding or other temporary constructions with a similar effect will be used for temporary support;

Figure 4. Details of precast concrete column mounting in the foundation and centering device. Adapted from [29] and stylized photos with branded products [30].

Figure 4. Details of precast concrete column mounting in the foundation and centering device. Adapted from [29] and stylized photos with branded products [30].

(m)

additional topometric verification after mounting all columns in both horizontal directions (in plane) as well vertically (in elevation). At the end, the final hammering of the oak wedges is carried out, and if tilt props/scaffolding are used, they will be firmly fixed to the concrete blocks and to the columns;

(n)

spraying the concrete surfaces that come into contact with the fresh infill concrete (base of the column and inner surfaces of the pocket) with water. The infill concrete is poured until it reaches the bottom level of the oak wedges. After pouring, the fresh infill concrete is vibrated with a concrete compactor;

(o)

removing the oak wedges after the infill concrete has reached approx. 70% of the compressive strength (approx. 3 days) that the infill concrete has at 28 days. Sprinkling with water (watering) the hardened concrete surfaces that come into contact with the rest of the fresh concrete for filling the remaining column-pocket interspace;

(p)

removal of the tilt props/scaffolding after the last part of the infill concrete poured in the interspace has reached approx. 70% of the compressive strength that the infill concrete has at 28 days;

(q)

mounting the beams is carried out after the concrete filling in the column-pocket interspace is completed (Figure 5), has hardened and has reached the strength class mentioned in the project`s technical documentation (e.g. technical drawings, technical specifications etc.), unless there are other specifications from the structural engineer.

Figure 5. Temporary support of prefabricated columns belonging to a frame structure, adapted after [5,20].

Figure 5. Temporary support of prefabricated columns belonging to a frame structure, adapted after [5,20].

Images during construction of pedestal pocket foundations with precast sockets (Figures S10-S15) and monolithic sockets (Figures S16-S20) are added as supplementary materials to this article.

2.3. Pre-Dimensioning of Socket in Pedestal Pocket Foundation

Pocket`s wall thickness, b_p. The minimum thickness specified in the standard [2] is 150 mm for sockets cast in factory, respectively 200 mm for those cast on site, but not less than one third of the short side of the column’s cross-section. However, Tillmann et al. [7,11,22] recommend a minimum thickness of 250 mm. An overview of the minimum wall thickness is presented in Table 1. Sometimes, sockets are made with variable wall thicknesses and with an inclination of about 3° to 5° [2,20].

Thickness of the concrete infill, f. It is recommended that the infill thickness is f = 50..75 mm at the base of the pocket, respectively f = 85..120 mm at the top [2]. In the case of pockets with vertical internal walls (0° inclination) f ≈ 75..120 mm is used. For pockets with two or more columns, when no intermediate walls are used, the concrete infill should be at least 50 mm in thickness between two neighboring columns, that is to ensure the concreting of the entire interspace [2].

Thickness of the concrete infill underneath the column, f_H, respectively the embedding depth of the column`s base in the slab`s foundation, f_stâlp. In current design practice [5] f_H and f_stâlp are considered to be 50 mm each (Figure 1). For large horizontal reaction forces, these values can be increased.

Slab`s foundation thickness, H_f. It is done after checking for punching (in the mounting and final phases) and considering H_f ≥ 250 mm.

Depth of the pocket, H_p. It is established from the conditions of embedding the column in the pocket foundation and ensuring the anchorage length (l_bd) of the column`s longitudinal reinforcement. In Table 1, the pre-dimensioning equations are centralized for the pocket`s height provided in EC2 [1,3] and are used as a structural design standard in many countries in Europe as well as in other countries worldwide. Additionally, pre-dimensioning provisions given in the design norms issued in countries with moderate and high seismicity (Romania [2,6,31] and Brazil [32]) are inserted in Table 1.

Table 1. Pre-dimensioning of the pocket foundation (b_p and H_p) using as bibliographic references the lecture notes of the pioneer professor in reinforced and prestressed concrete, Fritz [33], respectively modern design standards from Europe [1,3] , Romania [2,6,31] and Brazil [32].

Table 1. Pre-dimensioning of the pocket foundation (b_p and H_p) using as bibliographic references the lecture notes of the pioneer professor in reinforced and prestressed concrete, Fritz [33], respectively modern design standards from Europe [1,3] , Romania [2,6,31] and Brazil [32].

Type of surface	Pocket`s wall thickness bp	Depth of the pocket Hp	Bibliographic source
smooth	$\ge max\left\{\begin{array}{c}min\left[\left(l_{st}+2·f\right);\left(b_{st}+2·f\right)\right]/3\\ 100 mm\end{array}\right.$	$\begin{array}{c}\ge f_H+max\left(l_{st};b_{st}\right)·\mathrm{1,2}·\mathrm{1,4}\\ dacă M_{Rd.st}/N_{Ed.\mathrm{s}\mathrm{t}}\le \mathrm{0,15}\end{array}$ $\begin{array}{c}\ge f_H+max\left(l_{st};b_{st}\right)·\mathrm{2,0}·\mathrm{1,4}\\ dacă M_{Rd.st}/N_{Ed.\mathrm{s}\mathrm{t}}\ge \mathrm{2,00}\end{array}$	Leonhardt & Mönnig (1975) [33]
rough		$\begin{array}{c}\ge f_H+max\left(l_{st};b_{st}\right)·\mathrm{1,2}\\ dacă M_{Rd.st}/N_{Ed.\mathrm{s}\mathrm{t}}\le \mathrm{0,15}\end{array}$ $\begin{array}{c}\ge f_H+max\left(l_{st};b_{st}\right)·\mathrm{2,0}\\ dacă M_{Rd.st}/N_{Ed.\mathrm{s}\mathrm{t}}\ge \mathrm{2,00}\end{array}$
For intermediate values ​​of the ratio between the value of the bending moment and that of the axial force, a linear interpolation is to be used.
smooth rough keyed	$\ge max\left\{\begin{array}{c}\mathrm{1,5}·\left[M_{Rd.1}/H_P+V_{Ed.st}\right]/\left(H_P·f_{ctd}\right)\\ 200 mm \left(monolit\right) sau\\ 150 mm \left(prefabricat\right)\end{array}\right.$where, correlated with the direction of VEd.st , we have: $M_{Rd.1}=max\left(M_{Rd.st}-N_{Ed.st}·l_0/3;M_{Rd.st}/2\right)$ or $M_{Rd.1}=max\left(M_{Rd.st}-N_{Ed.st}·b_0/3;M_{Rd.st}/2\right)$	$\ge max\left\{\begin{array}{c}max\left(l_{st};b_{st}\right)·\mathrm{1,2}\\ 500 mm\\ \begin{array}{c}Hs/11 \left(**\right)\\ l_{bd,st}+250 mm\\ M_{Rd.st}/\left(3·l_{st}·b_{st}·f_{ctd,st}\right)\end{array}\end{array}\right.$	NP 112-*04 (2004) [31]
smooth	No specification.	$\ge f_H+max\left(l_{st};b_{st}\right)·\mathrm{1,2}$	EN 1992-1-1:2004 (2004) [1]
keyed		$\ge l_{0,pv}+s+\left(f_H+c_{nom.st}+c_{nom.pv}\right)$
smooth rough	$\ge max\left\{\begin{array}{c}min\left(l_{st};b_{st}\right)/3\\ 200 mm \left(monolit\right) sau\\ 150 mm \left(prefabricat\right)\end{array}\right.$	$\ge max\left\{\begin{array}{c}max\left(l_{st};b_{st}\right)·\mathrm{1,2}\\ 500 mm\\ \begin{array}{c}Hs/8 \left(*\right)\\ l_{bd,stv}+100 mm\end{array}\end{array}\right.$	NP 112-2014 (2014) [2]P100-1/2013+2019 [6]
keyed		$\ge max\left\{\begin{array}{c}max\left(l_{st};b_{st}\right)·\mathrm{1,2}\\ 500 mm\\ \begin{array}{c}Hs/8 \left(*\right)\\ l_{bd,st}+100 mm\\ l_{0,pv}+s+\left(f_H+c_{nom.st}+c_{nom.pv}\right)\end{array}\end{array}\right.$
For columns subjected to tensile axial force, the use of column-pocket foundation joint with smooth internal walls is not permitted. The longitudinal reinforcements of the columns subjected to tensile axial force in the seismic combination must be anchored with an anchorage length of 1,50·lbd. (par. 5.7.2.2(1)) [6] and (par. 5.6.2.1(2)) [12]. The longitudinal reinforcement in the critical areas of the columns belonging to structures designed for ductility class high (DCH), require anchoring starting at 5·ϕsl inside the element in which the anchoring is made (in the pocket). In addition, the anchorage length for the rebars in tension will be 1,20·lbd, where ϕsl is the diameter of the longitudinal rebars. (par. 5.7.1(4)) [6] (par. 5.6.1(3)) [12].
smooth rough	$\ge 150 mm$ for pedestal pocket foundations $\ge 400 mm$ for pad foundations with pocket	$\begin{array}{c}\ge f_H+max\left\{400\mathrm{m}\mathrm{m};\left[max\left(l_{st};b_{st}\right)·\mathrm{1,5}\right]\right\}\\ dacă M_{Rd.st}/N_{Ed.\mathrm{s}\mathrm{t}}\le \mathrm{0,15}\end{array}$ $\begin{array}{c}\ge f_H+max\left\{400\mathrm{m}\mathrm{m};\left[max\left(l_{st};b_{st}\right)·\mathrm{2,0}\right]\right\}\\ dacă M_{Rd.st}/N_{Ed.\mathrm{s}\mathrm{t}}\ge \mathrm{2,00}\end{array}$	ABNT NBR -9062:2017 (2017) [32]
keyed		$\begin{array}{c}\ge f_H+max\left\{400\mathrm{m}\mathrm{m};\left[max\left(l_{st};b_{st}\right)·\mathrm{1,2}\right]\right\}\\ dacă M_{Rd.st}/N_{Ed.\mathrm{s}\mathrm{t}}\le \mathrm{0,15}\end{array}$ $\begin{array}{c}\ge f_H+max\left\{400\mathrm{m}\mathrm{m};\left[max\left(l_{st};b_{st}\right)·\mathrm{1,6}\right]\right\}\\ dacă M_{Rd.st}/N_{Ed.\mathrm{s}\mathrm{t}}\ge \mathrm{2,00}\end{array}$
For intermediate values of the ratio between the value of the bending moment and that of the axial force, a linear interpolation is to be used. In the dimensioning of the pocket foundation an overstrength factor for the column resistance must be used: ${\gamma }_{Rd}=\mathrm{1,2}$. When rough interfaces are used, lower values ​​of Hp can be used as long as they are validated experimentally. For columns subjected to tensile axial force it is mandatory to use keyed interfaces and $H_p\ge f_H+max\left\{400\mathrm{m}\mathrm{m};\left[max\left(l_{st};b_{st}\right)·\mathrm{2,0}\right]\right\}$
smooth rough	No specification.	$\begin{array}{c}\ge f_H+max\left(l_{st};b_{st}\right)·\mathrm{1,2}\\ dacă M_{Rd.st}/N_{Ed.\mathrm{s}\mathrm{t}}\le \mathrm{0,15}\end{array}$ $\begin{array}{c}\ge f_H+max\left(l_{st};b_{st}\right)·\mathrm{2,0}\\ dacă M_{Rd.st}/N_{Ed.\mathrm{s}\mathrm{t}}\ge \mathrm{2,00}\end{array}$	EN 1992-1-1:2023 (2023) [3]
keyed		$\ge l_{0,pv}+s+\left(f_H+c_{nom.st}+c_{nom.pv}\right)$
For intermediate values ​​of the ratio between the value of the bending moment and that of the axial force, a linear interpolation is to be used.

Notes:

(*) Hs is the clear height of the column from the top face of the foundation to the roof purlin or to the runway beam. It is applicable only for columns shorter than 10 m.

(**) Hs is the clear height of the column from the upper face of the foundation to the roof purlin. It only applies to the columns of single-story buildings with overhead bridge cranes and flyovers.

Notations:

• l_bd,st is the anchorage length of the column`s longitudinal rebars. It is to be determined according to [1,6,12] and considering the maximum diameter of the rebar in tension.
• f_ctd,st is the design value of the tensile strength of concrete used for the column calculated according to [1,6,12].
• l_0,pv is the overlapping length of the vertical rebars in the pocket with the ones in the column calculated according to [1,6,12].
• c_nom.st is the concrete cover for longitudinal rebars in the column.
• c_nom.pv is the concrete cover for vertical rebars in the pocket foundation.
• M_Rd.st is the resisting bending moment of the column associated with the axial force at the base of the column N_Ed.st.
• V_Ed.st is the shear force of the column (in the cross-section at the base of the column) associated with the occurrence of the resisting bending moment M_Rd.st.

Conclusions are to be drawn based on the study in Table 1. The most numerous and at the same time restrictive provisions regarding the pocket`s wall thickness are those in NP 112-2014 [2] and NP 112-04* [31] The maximum pocket height results by applying the pre-dimensioning mathematical expressions from Leonhardt & Mönnig (1975) [33], where unlike the calculation models in the design rules [1,2,3,31,32], when transmitting reaction forces from the column`s base to the pocket foundation, the friction at the column – pocket foundation interfaces is neglected [34]. Following several experimental investigations on columns embedded in pocket foundations [35,36,37,38,39], it was observed that not taking into account the friction that occurs on these interfaces, load-bearing capacity of the pocket foundation determined experimentally is equal to 3 times the bearing capacity of the pocket foundation calculated using a calculation model that neglects friction [34]. Consequently, friction is especially important and must be considered in the calculation model of the column-pocket foundation joint in order to reach a more rational design [34]. Considering these observations, in the case study undertaken (chapter 3) the mathematical expressions from [1,2,6] will be used, as they are the most recent versions of the studied design standards enforced, and considered together are applicable for designing structures located in seismic areas.

3. Case study. Results

Table 4. Results of the case study, using [1,2,6].

4. Discussion

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