The Specificity of Shaping and Execution of Monolithic Pocket Foundations (PF) in Hall Buildings

Abstract

Pocket foundations (PF) are usually used under precast RC columns of steel or RC flyovers and industrial halls. PF is a specific type of foundation in which, apart from standard calculations concerning the fulfilment of the limit states in the base of the footing, the problems related to the appropriate design of its walls in the pocket are also important. In addition to the analytical and design part, a great challenge for constructors is also the selection of the appropriate construction technology of PF in terms of reinforcement preparation as well as the correct shaping of the foundation. The aim of the article is, therefore, to draw attention to the specificity of pocket foundation design—which is part of the structural systems in hall buildings—and to present the main problems that may arise during their construction at individual stages, including guidelines for checking the correctness of the conducted assembly works. The following article describes the main requirements for the construction of PF with particular attention to the type of surface present inside the pocket. The main problems related to the PF construction are also presented, and the methodology of assembling the precast columns in the footings is described. At the end of the manuscript, it is also discussed how to check and control the correctness of the skeleton installation works in hall buildings in order to prevent too large deviations in the assembled structure.

1. Introduction

Modern industrial plants constitute a system of synergistically cooperating industrial facilities and special facilities. The first group includes mainly production halls and all buildings directly related to production processes, while the second group includes the following objects: storage tanks, chimneys, cooling towers, etc. [1].

The main industrial processes most often take place in halls, that is, one-storey structures with long-span roofs and a small number of supports [2]. The most common structural systems in this type of facility are flat slab-column, frame or arch connections. In static diagrams, halls most often consist of a system of columns rigidly fixed in the foundations and connected by a flexible joint (slab-column system) or rigid (frame system) [3] system in the upper part of the hall [4,5]. An exemplary static diagram and an axonometric view of a single-nave hall building are shown in Figure 1, while a cross-section of a reinforced concrete hall built on pocket foundations (PF) is shown in Figure 2. All the characteristic structural elements of the hall buildings built on the PF are marked in both figures.

The basic foundation types in the halls are cast in-situ or precast pocket foundations. This type of foundation must be designed and calculated in such a way that it is able to transfer significant bending moments as well as vertical and horizontal forces that exist at the connection of the columns with the pocket foundation [6,7,8,9,10]. These structures were considered important so that recommendations appeared in the current standard for the design of reinforced structures, i.e., Eurocode 2 [11], regarding the calculation procedures. On the other hand, the literature lacks detailed and precise guidelines describing the procedures related to the design and construction of these specific structures. Filling this gap is particularly important as PFs are components of hall buildings, which means that they may be exposed to static, dynamic and fatigue loads [12,13,14,15,16,17,18]. Therefore, inadequate procedures when constructing structural systems in the skeleton hall buildings may result in the formation of primary microcracks in the concrete structure connecting the PF with the column. As a consequence, this may lead to the initiation of destructive processes at the point of the connection of the pocket foundation with the column as well as the beginning of failure condition in the hall structure [19,20,21,22,23,24,25,26].

Therefore, when constructing PF-column arrangements, it is very important to:

• Construct PF in accordance with the guidelines;
• Properly select and finish the interior surface of PF;
• Precisely install the column in the PF;
• Precisely connect the column with the PF;
• Check the correctness of the connection and, if necessary, correct any dimensional deviations.

The aim of the article is, therefore, to draw attention to the specificity of pocket foundation design—part of the structural systems in hall buildings—and to present the main problems that may arise during their construction at individual stages, including guidelines for checking the correctness of the conducted assembly works.

2. Background and Significance of the Study

Pocket foundations are usually used under precast RC columns of steel or RC flyovers and industrial halls. The view of the frame of the hall with a mixed structure, i.e., with RC columns and steel girders, during its installation with the connection of the column to PF, is shown in Figure 3.

The pocket foundation is a specific type of foundation in which, apart from standard calculations concerning the fulfilment of the limit states in the base of the footing, the problems related to the appropriate design of its walls in the pocket are also important. The cross-sectional area of the reinforcement in the footing base is determined with regard to its bending and punching, and the pocket walls are designed according to the forces occurring in them. It is also important to check the pressure of the column to the walls of the pocket.

The recommendations included in [11] should be taken into account when designing the reinforcement of the pocket footing of this type. In this standard, two cases of pocket foundation design are considered, i.e.:

• With keyed contact surface;
• With smooth contact surface.

The theoretical and experimental study of the pocket connections with a smooth surface interface of precast concrete structures were presented in several papers, for example, [27,28]. However, in the literature, there are no reports or design recommendations regarding PF with a more favourable keyed surface. Therefore, in order to fill this gap, these guidelines will be included in this article.

In calculations of PF, the tangential forces acting on the pocket’s wall should be taken into account. For this purpose, a computational model was assumed in [11], in which the external forces from the column: M_Sd, V_Sd and N_Sd are transferred to the foundation by three groups of compressive forces: F₁, F₂ and F₃, on both sides of the pocket and directly below it, taking into account the friction of concrete, μ (Figure 4).

The distribution of forces in a connection depends on the dimensions of its elements, the design solution and the proportion of forces. For the stress distribution shown in Figure 4, a calculation model has been formulated that includes three equilibrium conditions:

$$\begin{gathered} V_{Sd}\le V_{Rd}=F_1-F_2-\mu F_3 \\ {N}_{Sd}\le N_{Rd}=\mu F_1-\mu F_2+F_3 \\ {M}_{Sd}\le M_{Rd}=-V_{Rd}y+0.5N_{Rd}h+\mu F_{2}h+0.9y{F}_1-0.1y{F}_2-0.5h{F}_3 \end{gathered}$$

(1)

When using the presented computational model, attention should be paid to the:

• Design of reinforcement for force F₁;
• Transfer of force F₁ along the vertical walls of the footing;
• Correct anchor installation of the main reinforcement of the column and the footing’s walls;
• Ensuring the shear load capacity of the column;
• Checking punching load capacity of PF.

The solution of the above system of three equations (1) contained in [11] allows for the determination of the cross-sectional area of the reinforcement in the PF walls. The reinforcement, shown in Figure 5, for a footing with an associated reinforcement skeleton consists of:

• Vertical bars working in bending (Figure 5—1);
• Horizontal reinforcement in the form of closed frames working in tension, placed in the upper part of the pocket (Figure 5—2);
• Structural reinforcement located at the height of PF (Figure 5—3).

Additionally, it should be clearly emphasised that with incorrectly designed tensile reinforcement (Figure 5—2), the upper part of the PF may tear.

Moreover, the determination of the reinforcement in the PF base is also important. Such reinforcement is most often in the form of orthogonal meshes (Figure 5—4).

Apart from the rules for calculating and selecting reinforcement in PF in terms of the review of the current achievements in the field of PF design, the latest guidelines for the selection of components for concrete in these structural elements were also presented. It has been established that the advantageous solution is the use of crushed or recycled aggregates for concrete [29,30] and binders modified with mineral additives [31,32,33,34,35,36,37,38,39], admixtures [40,41,42,43,44] and micro-fillers [45,46,47,48,49,50,51,52]. Such treatments allow limiting the adverse damage to PF, which is the result of dynamic loads occurring during the operation of machines located in the hall buildings [12,53,54,55,56,57,58,59,60,61,62].

In addition to the analytical and design parts, a great challenge for constructors is also the selection of the appropriate construction technology of PF in terms of preparation of reinforcement as well as the correct shaping of the foundation. The issue of the correct construction of the footing is very important as the upper pocket of the foundation is the place where the skeleton columns of the hall building are placed. Hence, the precise construction of the foundation also guarantees smaller deviations of the columns from the geodetic axes of the hall, and a more precise location of the remaining elements of the structural skeleton [63].

Since the problems related to the construction and realisation of PF have not been fully presented in the literature so far, the following article describes the main requirements for the construction of PF with particular attention to the type of surface present inside the pocket. The guidelines for PF with both smooth and wrought surfaces are presented in this article. The main problems related to PF construction are also presented, and the methodology of assembling the precast columns in the footings is described. At the end of the manuscript, we also discuss how to check and control the correctness of the skeleton installation in hall buildings to prevent large deviations in the assembled structure. All the issues described in the article are supplemented with photos and diagrams from the various stages of construction in the scope of the presented problem (from the author’s collection).

3. Structure of Pocket Foundations

3.1. Foundation Requirements

In practice, PFs are constructed in formwork as monolithic or precast, and then columns are placed in the pockets of these footings. It is also possible in certain special situations to use simpler solutions such as the so-called column footings (precast elements in the form of foundations directly connected to the columns), which reduce the problematic assembly works on the construction site. However, such structures are not suitable for use in all types of halls, and therefore the most common solution in the case of this type of foundation is monolithic pocket footings.

Such foundations, depending on the loads, are constructed with one, two or three offsets (Figure 6), and their number depends primarily on the height of the footing, h_PF. The number of offsets in PF is determined on the basis of the relationships given in

Table 1. The number of offsets of PF depending on its h_PF, adapted from Ref. [64].

Footing Height hPF (mm)	Number of Offsets
$h_{PF}\le 350$	1
$350\le h_{PF}\le 850$	2
$h_{PF}>850$	3

[64].

When selecting the remaining PF dimensions, the most important parameters are (Figure 6):

• Depth and width of the pocket, i.e., h_p and b_p;
• Number of offsets, i.e., h_1, h_2, h_3, b_1, b₂.

Due to punching, part of the foundation should be constructed sufficiently deep, below the bottom of the pocket (not less than 200 mm), and the space between the column and pocket should be left, which will be filled with monolithic concrete in the final stage of the installation of the columns. It is required that the distance between the column and the pocket is at least 75 mm in the upper part and 50 mm in the lower part (Figure 6).

The requirements for the dimensions of PF h_p and b_p are mainly related to the appropriate anchoring of the column reinforcement within the pocket and the correct transfer of all internal forces from the column to the foundation so that it does not suffer unforeseen damage during operation. The pocket width is selected as the largest of the values according to Formula (2), while the required pocket depth depends on several parameters such as the dimensions of the column (cross-section height h_col and cross-section width b_col) and the value of the eccentricity on which the vertical force acts, e₀ and b_p [65].

$$b_p\ge max\left\{\begin{array}{c}250 \mathrm{mm}\\ 0.75·h_3\\ \frac{h_{col}+b_{col}+4·75 \mathrm{mm}}{6}\\ 0.5·h_{col}\end{array}\right\}$$

(2)

3.2. Design of the Inside of the Pocket

In order to ensure the correct transfer of forces from the column to the walls of the pocket, their internal surfaces should be carefully prepared. Most often, they are smooth (Figure 7a); however, a more advantageous solution is to shape the internal surfaces of the pocket as keyed with a special geometry (Figure 7b). This technological solution ensures better adherence between the surface of the pocket and the monolithic concrete and also allows for the transfer of tangential forces to a better degree.

The keyed surface inside the pocket footing is obtained by using metal forming inserts during the construction of the foundation, constituting the internal formwork of the pocket (Figure 8). When setting up the footing formwork, such an insert can be placed in the PF according to one of the following two methods, i.e.:

• Special composite with its structure—in the case of concreting the foundation base together with the pocket at one time;
• Anchored in the concrete of the slab foundation to a depth of several dozen mm (Figure 8c)—in the case of two-stage concreting; slab of PF and pocket separately.

The view of PF with the forming insert is shown in Figure 9a. For comparison, Figure 9b shows the foundation, the pocket of which is finished with a smooth surface, i.e., without the use of an insert forming keyed surface.

If RC columns are to be placed in the PF, it is also possible to make a keyed surface (while forming the columns) at their ends, along the length of the column equal to the pocket depth h_p. The required wrought shapes, in this case, are obtained by a special design of the shape of the columns in their lower zone. An example of a column with a shaped keyed surface is shown in Figure 10.

4. Stages in the Process of Construction of Monolithic PF and Installing Columns in Them

4.1. Construction of a Footing

In the construction of monolithic PFs, several main stages can be distinguished that are directly related to each other. In the first phase, the work consists of proper preparation of the base for the foundation so that it is easy to set up formwork for both the footing and the pocket. For this purpose, a sand bed with an appropriate degree of compaction should be prepared, and then a 10 cm layer of base concrete with a strength class of at least C8/10 should be laid, according to Eurocode 2 [11].

After this stage, the formwork of the footing base should be set up in the shape that results from the dimensions of the lowest offset, and then the bottom reinforcement of the PF should be laid, resulting from its calculations for bending. If the foundation is planned to be constructed in several stages, it is possible to fill the boarded part of the footing with concrete mix; if it is to be made during one concreting, the next step is to set the pocket formwork and other offsets.

If the design includes a keyed-shaped surface of the pocket, it is important (in the case of multi-stage concreting) to place a metal forming insert in the hardening concrete of the lower layer. Then, the reinforcement skeleton of the pocket should be prepared, remembering the correct placement of the vertical and horizontal reinforcement bars as well as the structural reinforcement, in accordance with the diagram in Figure 5. The last steps in the process of construction of PF are the concreting of boarded parts of the foundation and proper care of the concrete once concreted. Examples of PF formworks and reinforcements in the case of one- and two-stage formations are shown in Figure 11.

4.2. Installation of the Columns in the Pocket Footings

In typical situations, columns can be placed in PF after 28 days from the concreting; however, in the current realities of building with a continuous shortage of time, it is permissible to start assembly works after 14 days from the concreting of the footings. The columns should be installed very carefully, with detailed guidelines, and the procedure path according to the following stages:

• Stage 1: pouring a 5 cm layer of levelling base concrete on the bottom of the pocket, on which the columns will be placed;
• Stage 2: installation of columns in the footings with the use of cranes (usually self-propelled) with the use of special slings; this operation should be conducted very precisely in order to minimise the possible damage of the pocket in the event of the column hitting the wall; in order to ensure adequate comfort of installation works, it should be borne in mind that the installation of columns should be suspended in bad weather conditions, i.e. during strong winds, the speed of which exceeds 5.0 m/s;
• Stage 3: vertical alignment of the columns with the use of geodetic measurements; for this purpose, specially prepared oak wedges are hammered to the appropriate depth between the edge of the pocket and the column (at least six wedges are required); an example of column wedging in PF is shown in Figure 12;
• Stage 4: temporary support of the columns and release of the crane hooks in their upper part (Figure 13);
• Stage 5: geodetic checks of the correct installation of columns and possible corrections of their positioning;
• Stage 6: placing monolithic concrete inside the pocket; due to the narrow space between the column and the pocket, it is recommended to use concrete with smaller grain size (preferably with a maximum grain size of D_max = 8 mm) and a strength class higher than the structural concrete PF;
• Stage 7: precise compaction of monolithic concrete in the pocket space, preferably with an internal vibrating poker with a thin tip, e.g., needle.

5. Control of the Correctness of the Conducted Assembly Works

After installing all the columns in PF, geodetic measurements of the correctness of the works performed should be carried out before proceeding to further assembly works. Their purpose is to capture all the deviations of the columns in relation to the hall axes and to show any differences in height between the individual columns.

An example of the geodetic map, including control measurements of the columns after assembly, is shown in Figure 14. The sketch from measurements should always include:

• Values of the upper and lower deviations of the columns (mm);
• Directions of column deviations marked with arrows (right or left side);
• Differences in the height of columns (mm) determined from the highest point in relation to the zero of the building.

Thanks to the sketches made in this way, it is possible to correct further construction works and avoid overlapping of assembly deviations in the subsequent stages of erecting the skeleton of the hall building.

6. Summary and Conclusions

Reinforced concrete PFs are used mainly as elements for transferring loads to the ground in the structural systems of industrial halls or loading flyovers. In this type of facility, apart from the typical static loads, there are often dynamic effects (e.g., operation of overhead cranes), which require that the foundation of the building is able to prevent the transmission of harmful vibrations. The literature studies and experience from construction practice allow us to conclude that it may be helpful in the construction and implementation of this type of foundation in hall buildings.

Therefore, when determining the shape and dimensions of the foundations, attention should be paid to:

• Proper selection of the depth and width of the pocket and the number of offsets, which mainly depend on the height of PF and the dimensions of the column’s cross-section (Figure 6);
• Leaving enough space between the walls of the pocket and the column (Figure 6) and use of a keyed-shaped inner surface of the pocket (Figure 7b and Figure 9a).
• However, during the construction of PF, attention should be paid to the following:
• Correct execution of the reinforcement of the pocket’s walls (Figure 5) as well as correct and precise conduct of works related to the assembly of columns in the pocket (Figure 12 and Figure 13);
• Preparing a geodetic sketch from the control measurements of columns after installation (Figure 14).