An Assessment of the Impact of Protective Lifeline Safety Systems on Formwork Systems

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Results of tests of safety systems for work at height during the installation of formwork systems. The tests used real formwork systems and climber protection systems.

Abstract

Work related to the installation of formwork and the pouring of concrete involves the need to move on the formwork walls. There is a high risk of falling off the wall and falling during work. It is therefore necessary to use systems that allow for safe work at heights, but also the safety of the formwork systems themselves. This article presents the results of tests under real conditions of a person falling from the formwork. Two safety systems were tested: one based on a pole–rope system with a flexible joint and one based on climbing arrester hooks. The test results showed that the additional forces occurring in the system do not exceed 6.4 kN and that the stresses reach the highest value only at the site of installation of the system and do not exceed the yield strength of the steel used in the formwork structure. The results obtained indicate that during a fall, the fall energy is absorbed so much that there is no damage to the formwork elements, and the highest effort is observed for the formwork arrester hook and reaches 87% of the structure effort. In the case of systems with flexible posts, the maximum load values do not exceed 30% of the structure. The presented results may be useful in designing and planning assembly works and using formwork systems in concrete pouring conditions.

1. Introduction

Formwork systems are widely used in construction to make concrete elements [1,2]. The height of the formworks can often be several dozen meters above the ground, which requires the involvement of the employees to work at heights and the need to use fall protection systems [3]. The structure of the formwork panels is based on a concrete retaining plate and a steel frame with assembly holes for connecting individual elements to a whole. The presence of assembly holes in the steel frame allows them to be used for the movement of assembly personnel without the need to use additional external scaffolding. These works are performed during both the assembly of the formwork and the inspection of its assembly, as well as during its dismantling and while pouring concrete. This makes it necessary to ensure full access to system connectors by carpenters and installers [4], as well as to ensure their mobility and the ability to move relatively quickly and freely [5,6] and high ergonomics of work to reduce energy expenditure [7,8,9,10,11,12]. Horizontal lifeline systems and formwork hooks are perfect for this purpose [13]. Horizontal lifeline systems are based on a system of fixed points and a steel or textile rope stretched between them, to which the carpenter connects. Such systems are also used on roofs, allowing free movement on their surface, and in formwork systems they can be mounted on top of the system [14,15]. Mounting points are most often rigid elements, which means that the entire fall energy is transferred directly to the structural elements and on the climber’s side to the absorbers. Another solution are posts with a plastic joint, which undergo plastic deformation due to the force of falling. During deformation, the energy of the fall is absorbed, which is beneficial for the climber because the free fall time is extended and the fall is gently slowed [16,17,18]. However, there are no reports in the literature that report on the values of forces acting on the formwork, as well as the stresses occurring at the place of mounting the horizontal fall arrest systems.

However, carpenters performing assembly and inspection work must have free access to each of the connections of the assembly system. It is possible to use a scaffolding platform and a passenger lift system, which are energy-intensive and cost-intensive, or move directly onto formwork.

For moving purposes, it is possible to use hooks, which are a universal solution, but formwork systems often have limitations in their easy attachment and detachment. Hooks are equipment dedicated to moving around the formwork, as well as for working with it. They constitute a rigid attachment to the formwork structure, which means that all of the fall energy is transferred directly to the structure. Data from the literature also lack the actual impact of the hook on the structure of formwork elements [19]. Therefore, in this article, an attempt was made to indicate the key force-stress factors that occur during a fall in formwork assembly. To reflect real working conditions, the tests were carried out under field conditions using the Hunnebeck, Peri, Uhlma and Doka formwork system and a mannequin with a typical human body structure, mass distribution and joints. These tests were preceded by a series of tests under laboratory conditions using a steel weight and dynamic falls. Since the test results are similar for different types of formworks, for the purposes of this article, the description of the Hunnebeck formwork system and the Manto type was limited.

2. Materials and Methods

The tests were carried out on the Hunnebeck formwork system equipped with a system of SAS posts connected to the safety tape (fine-line type) with a fall factor (FI) of 1.0 and 2.0. For the tests, a rigid formwork system consisting of three interconnected boards connected sideways and with oblique supports that prevent the formwork from bending in the direction of force was used. The tests used a 100 kg freely falling mannequin equipped with a personal safety system consisting of carabiners, a strap and a shock absorber. The mass energy absorption system is based on the action of the shock absorber and the plastic deformation of the joints of the belay system posts. To determine the impact of the formwork protection system on the formwork structure, strain gauge measurements were performed. The strain gauges were placed in the immediate vicinity of the installation of the safety system posts, i.e., at the point of energy transfer from the safety system to the formwork. The tests used double strain gauge systems placed obliquely relative to the plane of the transverse stiffening beams to which the safety systems were mounted. The strain gauge systems were placed on two formwork boards in the area of tensile stresses—two for testing the formwork posts and two for testing the hooks (Figure 1 and Figure 2).

The tests were carried out under field conditions that reflect the actual working conditions on construction sites in winter. The test period resulted from unfavorable temperature conditions in which, due to low temperatures, the plasticity of the steel is reduced and the risk of brittle cracking in the structural elements increases. The air temperature during the test was in the range of 1–3 °C with high air humidity. Acoustic emission was used to reveal cracking phenomena, which did not reveal any changes in the formwork frame material or the appearance of new, active cracks.

The tests included a simulated fall factor while working at height using a 100 kg dummy. The tests used a fall factor typical for the conditions of use of the belay system. The fall factor (FI) is the ratio of the height (H) a climber falls before the climber’s rope begins to stretch and the rope length (L) available to absorb the energy of the fall (FI = H/L). FI is the main factor determining the violence of the forces acting on the climber and the gear. The tests were carried out in the following system:

• Two posts with a flexible joint fastened with a tape to which the mannequin was attached. There was extreme mounting in relation to the formwork boards, with the distance between the posts approximately 2 m and the fall factor FI = 2.0 (test 1)—Figure 1a.
• A single post mounted in the central part of the formwork board, where the distance from the vertical supports of the formwork system was approximately 0.7/0.5 m and the fall factor FI = 1.0 (test 2)—Figure 1b.
• A formwork hook mounted in the central part of the formwork board. The distance from the vertical supports of the formwork system was approx. 0.7/0.5 m; fall factor FI = 1.3 (test 3) and FI = 1.0 (test 4)—Figure 1c.

In addition, calculations were made of the impact of the designed anchor devices on the structural element of the formwork (Figure 2). The calculations are of a verification nature based on measurement data, allowing the determination of the effort of the formwork structure in the formwork cross-beam (Figure 3). Calculations using the limit state method were carried out based on the characteristic phases of the process of stopping the fall of a 100 kg mass, taking into account the fall factor (FI) appropriate for individual simulated training ground tests in three load states:

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Phase 1—beginning of the fall arrest process, where a force of 0.4 kN appears in the rope system, causing the post to begin to plasticize;

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Phase 2, where the joint rod is permanently deformed and the force is generated by the plastic joint and is 0.7 kN. In this state, the position of the upper part of the post shifts from vertical to horizontal;

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Phase 3, where the fall is completely stopped and the force reaches its maximum value, reaching 4 kN in the ropes resulting from the mutual angular position of the posts and the rope.

The analysis of the stress state was performed using ANSYS v.16 (2015) software with a uniform distribution of the nodal points on all planes of the shelf. In the simulation, a solid steel rope was used as an anchor point in the set of two posts, and in the remaining cases there was a fixed anchor point. The personal belay system was omitted because the maximum force measured during the experiment was applied to the anchor point. The post in the simulation has a flexible joint, which deforms when the force is applied. The angular deformation values of the post in the actual test were compared in the simulation of the entire system and the results were consistent. In the simulation’s formwork frame, poles and other elements were calculated as low strength structural steel with yield strength of 235 MPa. Individual safety elements between the anchor point and dummy were not calculated because it was during experimental work that only measured the total force in the anchor point.

3. Results

3.1. Test 1—A System of Two Posts and Rope

In the first test, the arrangement of the posts with a plastic joint was uneven. As a result of the fall, both posts were deformed in the elastic joint (Figure 4), indicating that part of the fall energy was absorbed into the plastic deformation of the steel. Due to the fall factor FI = 2.0, the impact force of the weight was 6.42 kN and the free flight height was 2.2 m. As a result of the force, the textile shock absorber was torn along a length of 290 mm and both posts were bent, absorbing the energy of the falling mass. The bending took place on a flexible joint at an angle of 15° and 75°. The dummy stopped above the ground.

The formwork did not move during the test, so it behaved as a rigid structure, and the total energy was transferred from the posts directly to their fastening elements and to the formwork elements. The stress distributions during the fall are shown in Figure 5. Two maxima were observed during the fall. Their presence is due to the mechanism of bending the posts and absorbing the energy of the fall. Based on the obtained measurement results, a matrix of maximum stresses and the maximum stress state in the plane stress state was determined, which result from the structure of the formwork elements. The maximum reduced stress was approximately 91 MPa, which means that there is no negative impact on the structural elements of the formwork system. In the place where the formwork was attached, no deformation of either the opening or the surface of the sections was observed. After the fall, the deformation of the joints was asymmetric, which causes a different distribution of stresses in the system and an extension of the braking time of the falling mass to approximately 0.5 s. Based on the results obtained, the force acting on a single post was determined to be only 1.3 kN at its mounting point. This indicates that more than 5 kN was absorbed by the bending of the posts and the tearing of the textile shock absorber.

$$\left[\begin{array}{cc}{\sigma }_{11}& {\tau }_{12}\\ {\tau }_{21}& {\sigma }_{22}\end{array}\right]=\left[\begin{array}{cc}56& 25\\ 25& 106\end{array}\right] \mathrm{M}\mathrm{P}\mathrm{a}$$

(1)

$$\left[\begin{array}{cc}{\sigma }_1& {\tau }_{12}\\ {\tau }_{21}& {\sigma }_2\end{array}\right]=\left[\begin{array}{cc}45& 0\\ 0& 116\end{array}\right] \mathrm{M}\mathrm{P}\mathrm{a}$$

(2)

The FEM analysis showed that under the conditions of stresses, the degree of effort of the formwork structure in state 1 is negligible and amounts to 1–3%, in state 2 it increases to 7%, and only to 32% in state 3, reaching an effort of 32% (Figure 6). The values determined in the analysis are presented in the table in Figure 6b. Low effort values, even in the maximum impact period of the falling weight, mean that the effect of absorbing the fall energy and decelerating the mass is not transferred to the formwork system. This indicates that carrying out assembly and inspection works on the formwork and falling do not pose a threat to the structure but ensure the safety of the climber. The FEM calculation of stresses in the cross-beam indicate the stresses above the yield strength of steel (over 300 MPa) what could indicate the plastic deformation of element or the hole in the beam (Figure 7). Observations after the experiment reveal the lack of deformation of the formwork frame structure. Such differences in simulation and real measurements are explained by the effect of dynamic force expansion on element and stress distribution in the structure. Short periods of high stresses (below 0.1 s) allow the avoidance of stress concentration and steel plastic deformation. Real values of stress, measured using tensimeters, indicate lower stress values, only reaching up to 116 MPa (Figure 5).

3.2. Test 2—Single-Post System

Only a single post was used in the test. This means that the angle between the safety rope at the beginning of the fall will depend primarily on the attachment point and the position of the carabiner relative to the post. During the fall, the smallest possible angle was simulated, causing the falling force of the mass to be parallel to the axis of the post and the plastic hinge. As a result of the force, the post was slightly bent and twisted (Figure 8), indicating that despite the unfavorable force system and the initially small bending moment, the joint was not deformed. The simulated fall factor was FI = 2.0, the impact force of the weight was 5.6 kN, and the free flight height was 1.6 m. As a result of the force, the textile shock absorber was torn along a length of 395 mm. The dummy stopped above the ground.

The formwork did not move during the test, so it behaved like a rigid structure, and the total energy was transferred from the post directly to its mounting elements and to the formwork elements. A slight deformation of the post resulted in a shortening of the free time to only 0.2 s, causing an impact force on the formwork of approximately 3.8 kN.

The distribution of stresses as a function of time when a weight of 100 kg is dropped on a single post is shown in Figure 9. Two maxima were observed in a very close time interval. This is due to the high stiffness of the system and the failure of the elastic joint to operate. This caused the falling mass to stop, causing an elastic effect on the formwork system. The maximum reduced stresses are relatively high at 202 MPa, and the obtained values do not cause the deformation of the structural elements of the formwork. The maximum impact force on the formwork was approximately 3.8 kN, and in the conditions of the post bending, it should be reduced due to the plastic deformation of the joint.

Conditions favorable to joint deformation are the bending moment when the falling mass will not move in a direction close to the axial direction relative to the joint axis. The matrix of maximum stress and maximum stress state in the plane stress state based on the measurement data looks as follows:

$$\left[\begin{array}{cc}{\sigma }_{11}& {\tau }_{12}\\ {\tau }_{21}& {\sigma }_{22}\end{array}\right]=\left[\begin{array}{cc}214& 13\\ 13& 188\end{array}\right] \mathrm{M}\mathrm{P}\mathrm{a}$$

(3)

$$\left[\begin{array}{cc}{\sigma }_1& {\tau }_{12}\\ {\tau }_{21}& {\sigma }_2\end{array}\right]=\left[\begin{array}{cc}219& 0\\ 0& 182\end{array}\right] \mathrm{M}\mathrm{P}\mathrm{a}$$

(4)

The system with the lowest plasticity was analyzed, i.e., the system in which the force from the falling mass is parallel to the axis of the post. Then, the bending moment that initiates the bending is the smallest and is the result of moving the axis of the anchor hole away from the axis of the post. In the simulation (Figure 10), as well as in the field conditions, the pole bends during falling, but the initial fall force is the highest already in phase 1 and close to the value for phase 3. This indicates that, during the beginning of flow, the system behaves as a rigid system and all energy is transferred to the structural elements of the formwork at the point of installation of the post. In phase 2, when plastic deformation of the column occurs, the degree of strain decreases, indicating that the fall energy is absorbed by the plastic flow of the joint.

The FEM calculation of stresses in the cross-beam indicate the stresses above the tensile strength of steel (over 1000 MPa), which could indicate not only the plastic deformation of element or the hole in the beam, but could also fracture it (Figure 11). Observations after the experiment reveal a lack of deformation and cracks in the formwork frame. Similar to the double-post system differences in the simulation, and the real measurement values are explained by the effect of dynamic force expansion on element and stress distribution in the structure. The short period of high stresses (below 0.1 s) allows to avoid the stress concentration and the steel plastic deformation. The real values of stress reach 188 MPa.

3.3. Test 3 and 4—Arrester Hook

During the fall using the formwork hook, a fall factor (FI) of 1.0 and 1.3 was used, which corresponds to the typical factor when moving or working with a formwork hook. Hook use may occur when the formwork system is not yet fully assembled, which means that the forces may cause the formwork elements to detach together while the person is working at height. The simulation results showed that the stresses were very short and exceeded the elastic range (Figure 12). Thus, when a structural element falls, it may undergo temporary elastic deformation but without permanent deformation. In the case of the repeated dynamic impact, the presence of high momentary elastic deformations may reduce the plasticity of the structure openings and initiate fatigue cracks in future operation. The design conditions of the formwork system make such a case unlikely. The reduced stresses that occur in the system are relatively high, approaching the yield point of the material (for S235 steel) and up to 235 MPa for FI = 1.3 and 128 MPa for FI = 1.0, respectively. These stresses correspond to the force acting on the formwork of 5.1 kN for a free fall of 1.2–1.4 m and 4.43 kN for a free fall of 1.0 m. Despite the fall, the hook was not deformed (destroyed) at FI = 1.3 and 1.0.

Reducing FI from 1.3 to 1.0 reduces the impact force of the hook on the formwork and thus reduces the reduced stresses. However, the momentary stress values are significant and may lead to plastic deformation in the assembly holes and cross-beams when subjected to repeated dynamic effects (

Table 1. Stress matrices for fall factors 1.0 and 1.3.

FI = 1.3	FI = 1.0
Maximum stress matrix:
$\left[\begin{array}{cc}{\sigma }_{11}& {\tau }_{12}\\ {\tau }_{21}& {\sigma }_{22}\end{array}\right]=\left[\begin{array}{cc}212& 12\\ 12& 236\end{array}\right] \mathrm{M}\mathrm{P}\mathrm{a}$	$\left[\begin{array}{cc}{\sigma }_{11}& {\tau }_{12}\\ {\tau }_{21}& {\sigma }_{22}\end{array}\right]=\left[\begin{array}{cc}128& 2\\ 2& 124\end{array}\right] \mathrm{M}\mathrm{P}\mathrm{a}$
Maximum stress matrix in a plane stress state:
$\left[\begin{array}{cc}{\sigma }_1& {\tau }_{12}\\ {\tau }_{21}& {\sigma }_2\end{array}\right]=\left[\begin{array}{cc}207& 0\\ 0& 241\end{array}\right] \mathrm{M}\mathrm{P}\mathrm{a}$	$\left[\begin{array}{cc}{\sigma }_1& {\tau }_{12}\\ {\tau }_{21}& {\sigma }_2\end{array}\right]=\left[\begin{array}{cc}129& 0\\ 0& 123\end{array}\right] \mathrm{M}\mathrm{P}\mathrm{a}$

). The stress distributions are shown in Figure 13 and Figure 14. Stress analysis was carried out in the complete fall arrest system due to the rigid structure of the hook and the absorption of fall energy only by the personal safety system. The results of the calculation indicate that the structural effort reaches up to 87% in the area where the hook is attached and is reduced to 9% in the side elements of the formwork (Figure 15). The FEM stress distribution indicates the stress concentration in the hole edge. The values in this area are above the tensile strength of steel, which could indicate the plastic deformation or fracture in this area (Figure 16), but such cross-beam material defects were not observed.

4. Discussion

Field tests performed using a 100 kg dummy and direct protection elements used in the construction and operation of formwork structures showed that the fall of a climber does not cause any negative effects on the formwork structure. In systems based on horizontal fall arrest systems based on posts with a flexible joint, the deformation of the joint absorbs the fall energy, and thus the forces acting on the formwork are relatively small, and in the case of a fall factor of 2.0 they are only 1.3 kN for the two posts in the system. In the case of a single post in an unfavorable position of the climber, the action of the plastic joint is limited, which means that shock absorbers, e.g., textile shock absorbers, are necessary to absorb the falling energy, but at the same time, the forces acting on the formwork increase (3.8 kN). The forces that occur cause stresses in the area of the mounting holes of 91 MPa for two posts and 202 MPa for a single post. The stresses obtained are below the yield strength of the steel used for the formwork frame (usually with a yield strength of at least 235 MPa), which means that there is no deformation of the assembly holes and therefore no damage to the formwork structure. In the case of hooks used to move around the formwork both during and after assembly, the fall factor is much lower and is most often 1.0, and less often 1.3.

The rigid mounting of the hook to the assembly holes causes the impact forces of the hook elements on the formwork frame to be 4.4 kN and 5.1 kN, respectively, but causes short-term stresses that exceed the yield strength. Despite the high stresses, the structure of the formwork does not deform or deform, which indicates that it behaves only elastically. The results obtained indicate that the use of direct fall arrest systems does not damage the formwork system, and that the forces generated are insignificant compared to the loads that occur during concrete pouring. The total effort of the structure at the hook installation site is up to 87%, ensuring that the structure elements are not damaged.

5. Conclusions

The tests in real conditions and simulations have shown the following:

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The use of a system based on flexible posts increases safety during work at height, and the fall energy is absorbed by the safety system and not transferred to the permanent elements of the structure.

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The use of a single post generates a bending moment and a twisting moment in falling conditions. Depending on the direction of the fall, the plastic hinge may have a limited effect (fall parallel to the post axis).

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The use of arrester hooks to move around the formwork systems provides the possibility of obtaining a constant point of support, but in a fall case, the greatest loads affecting the formwork are generated. The stress value does not cause damage to the formwork elements.

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The highest stresses are observed for the arrester hook (240 MPa) and the lowest for the double-post system (118 MPa); this confirms that during a fall the formwork is not deformed. However, the stress values that were reached indicate that during the concrete filling process, the total stresses in the formwork are very high and need to be considered when designing a formwork system for operation.

Future work is needed to extend the FEM model for the prediction of stress in formwork under extreme conditions, i.e., wind speed, and also to predict stress in regard to formwork length and height and the stiffeners localization base on different formwork models and lifeline systems.