How to avoid failure of a tower crane’s foundation? A systematic design approach

Applauding or expressing enthusiasm may represent assertive and uncooperative behavior among observers during a tower crane failure event. These observers may be aware that the contractor is fully insured, and there are no associated injuries (refer to the video for instances of operational failures).

However, empirical evidence indicates that the repercussions of tower crane failures extend beyond mere economic losses. These incidents can tragically result in fatalities or physical injuries. Moreover, insurance coverage does not fully address the costs associated with project delays or the restoration of tarnished reputations for contractors and designers. Recognizing that engineering design is a collaborative effort, harmonized cooperation among all project stakeholders and a systematic approach are essential for minimizing failure probabilities in construction. While a complex design process for tower cranes may yield only a fragment of the overall project, experienced engineers understand that reusing drawings and specifications for new projects is an incorrect approach when certifying geo-structural endeavors. This perspective aligns with my previous articles and recommendations for designing geographically distributed infrastructures. (https://www.dhirubhai.net/pulse/robust-design-certification-methodology-installing-moradabadi-phd?trk=portfolio_article-card_title. and https://www.dhirubhai.net/pulse/framework-uncertainty-management-design-certifying-moradabadi-phd?trk=portfolio_article-card_title).

The following procedure outlines a systematic design approach for tower crane foundations, aimed at minimizing failure probabilities and preventing foundation-related incidents during construction. This approach was successfully implemented in designing a pile-supported foundation for a tower crane in Ennis, commissioned by Coolsivna Company. By adhering to this method, project teams can enhance safety and reliability in tower crane installations

The design procedure

In large-scale projects, the deployment of numerous tower cranes across expansive geographical areas is common. However, gathering comprehensive design input data and predicting geotechnical behavior at the project’s outset can be challenging. To address this, an ‘observational method’—also known as the ‘learn-as-you-go method’ in geotechnics—is often recommended for such projects.

Figure 1 illustrates the design process, which begins with the reception of project data and relevant documents, including conceptual designs, drawings, contract documents, site investigation reports, and the tower crane’s engineering documentation. To mitigate uncertainties and enhance design confidence, effective communication among all project stakeholders is crucial. If basic information suffices, a desk study is conducted. This study typically explores various aspects, including:

1. Buried services and existing structures
2. Previous geotechnical investigations
3. Drainage and hydrogeological conditions
4. Local climate factors
5. Characteristic wind pressures
6. Topographic maps
7. Previous experiences in the area
8. Nearby investigations

Subsequently, a preliminary site investigation is planned, followed by a pre-design process that involves:

1. Specifying design rules
2. Reviewing design concepts and past practices
3. Evaluating plant specifications and loading values
4. Conducting geostatistical analyses to define ground parameters
5. Determining specific design parameters
6. Identifying validation tests
7. Planning new site investigations

These processes should adhere to accepted standards and guidelines, such as Eurocodes, EN 14430 E25, CIRIA C761, ICE specifications for piling and embedded retaining walls, and TII CS-SPW-0 1600.

To comply with design rules and standards, a conceptual design phase may be necessary. During this phase, load combinations for the crane (e.g., in-service and out-of-service scenarios) are developed, along with alternative foundation geometries and reinforcement requirements (e.g., strip foundations, rafts, or ground beams). The conceptual design calculates foundation dimensions, reinforcement needs, and the minimum ground bearing capacity required to support the structure based on load combinations and relevant structural analysis.

Tower cranes experience dynamic loads, but static and pseudo-static methods are generally sufficient for most situations. Dynamic analyses can be disregarded if the following criteria are met during design and operation:

1. The crane operates for no more than ten hours per day.
2. Planned lifts do not exceed six per day.
3. There is no risk of shock loading.
4. Usage remains below 70 percent of capacity.
5. The mast height exceeds 20 meters.
6. The jib length exceeds 30 meters.
7. The tower crane is not permanent.

By comparing preliminary site investigation data on ground conditions—including bearing capacity—with the maximum pressure exerted by the structure on the ground, the designer can determine whether a shallow foundation is suitable. If a shallow foundation is appropriate, the proposed foundation(s) from the conceptual design are assessed for bearing capacity, maximum settlement, differential settlement, sliding, and surface and groundwater effects (if applicable).

The designer must address the surface and groundwater table, either by proposing a new drainage design or by assessing whether the design ratios remain acceptable given the reduced bearing capacity of the ground due to water presence, as appropriate.

When there is a need to enhance ground bearing capacity, a feasibility study becomes essential to explore various viable alternatives. Geotechnical engineers, considering local technologies and experience, may recommend options such as cast-in-place reinforced concrete piles, driven piles (steel, concrete, or timber), or other soil improvement methods (such as secant piles, micro piles, soil mixing, jet grouting, auger cast, or drilled shafts). From both technological and economic perspectives, the feasibility study should outline the constraints and considerations associated with each proposed alternative. This study may yield drawings, diagrams, reports, and schedules, enhancing decision-makers’ confidence.

For the selected alternatives, detailed geotechnical and structural checks are necessary. Geotechnically, both driven and cast-in-place piles require assessment for lateral displacement, settlements, head bearing capacity (if applicable), frictional resistance (if applicable), water table effects, and behavior under cyclic loads.

However, geotechnical checks alone do not ensure local and global pile stability. Structural assessments are crucial, including checks for slenderness, bearing capacity, axial forces, shear, and relevant interaction design ratios. Slenderness analysis may dictate buckling or shell analysis based on pile type. Additionally, fatigue and creep analyses may be necessary for steel and concrete piles, considering probable cyclic loads and environmental conditions affecting service life. Corrosion considerations may impact pile sizes or necessitate protective measures in humid or corrosive environments, depending on the structure’s expected lifespan.

When a deep foundation solution is recommended for structural support, a compatibility check ensures alignment with surface ground beams, slabs, or shallow foundations. Depending on stress regimes and geometry, further considerations arise, including interaction joints between piles and surface elements, contractual eccentricity allowances, pile cap requirements, and verification of dowel bar lengths, numbers, and sizes, or pile embedment lengths for proper connection between buried and surface structures.

If dynamic analysis can be disregarded based on safety criteria, the design is finalized and subjected to risk assessment. Once safety is assured, the design proceeds to the construction phase.

Following the observational method, designers may defer certain site investigations to the construction period or order post-validation tests (especially for piles). If these investigations confirm the sufficiency of the design, platform or foundation construction commences. Coordination between the project supervisor for the design process (PSDP) and the project supervisor for the construction stage (PSCS) may be necessary.

Figure 2 depicts drawings of one of the proposed alternatives, along with selected pages from a comprehensive analysis report for a tower crane’s foundation in Ennis, executed under the directorship of David Lally. The successful implementation aligns with the summarized methodology outlined earlier.

To conclude, in large-scale construction projects, tower crane foundation design plays a crucial role in ensuring safety and stability. Beyond economic losses, tower crane failures can result in fatalities and injuries. A systematic approach involves data collection, feasibility studies, conceptual design, structural checks, and dynamic analysis. Successful implementation requires collaboration among stakeholders and adherence to standards