Temporary outdoor event structures are exposed to wind loading, crowd loading, rigging loads, and other environmental forces that must be resisted by the structure’s lateral stability system. For many common event structure types, including stage roof systems, delay and lighting towers, and large tent structures, that lateral stability system consists of guy lines and anchoring hardware. Guy line systems are among the most frequently under-engineered elements of temporary event structures, and failures in guy line design or installation have contributed to structural collapses at events. Understanding the engineering requirements for these systems, and the specific failure modes they introduce, is essential for anyone responsible for the procurement, installation, or monitoring of temporary event structures.

Guy Line System Design Requirements

When temporary structures depend on guy line and ballast systems to resist gravity and lateral loads from self-weight, wind, and seismic forces, their design is critical to the integrity of the structure (Event Safety Alliance, 2013). Guy line systems must be used in a manner consistent with the engineering calculations; specifically, the location of ballast and the angle of guy lines must not deviate from the site plan drawings without the approval of a qualified engineer. A guy line at an angle or in a location different from the design configuration imposes different forces on both the structure and the anchor than the engineer calculated; even small deviations can substantially reduce the system’s ability to resist the design loads.

Site conditions vary for each application and each venue, and must be independently engineered for each event. The engineering specification for the guying system must be included in the overall engineering documentation for the specific event. The specification must consider a worst-case condition, which must be clearly identified by the qualified engineer (Event Safety Alliance, 2013). For outdoor events, the worst-case wind loading condition is typically the most critical design scenario for guy lines; the engineer must establish the design wind speed for the specific site based on applicable standards such as ASCE/SEI-7.

Guy Line Hardware Specifications

All fittings, cables, and attachments must be specified by a qualified person and must have sufficient safe working load for the forces they will experience. Guy line assemblies should be designed and rated based on their weakest component; the safe working load of the entire assembly is limited by the lowest-rated element in the load path, from the structure connection through the cable and all hardware to the anchor (Event Safety Alliance, 2013).

Pretensioning and Slack Prevention

Guy lines must not be slack and must be pretensioned as specified by the qualified engineer. Any slack in a guy line system will be amplified by wind loading and other lateral forces, and can result in structural instability (Event Safety Alliance, 2013). The mechanism of failure is specific and well understood: wind loading will deflect a structure with slack guy lines until the slack is taken up and the guy becomes taut. This sudden transition from slack to taut creates an impulse load on both the structure and the anchor that greatly exceeds the static design load. This condition compromises both the strength and the stability of the structure and can lead to unpredictable behavior and catastrophic collapse. On site, this translates to a practical requirement that guy lines be verified to be at the specified pretension before and during event operations, and that any slack observed during monitoring be addressed immediately.

Guy Line Force Effects on the Structure

Tension on guy lines increases the loads imposed on the structure in two distinct ways: by pulling horizontally on the trusses and by pulling down on the towers. These loads include both the pretension applied during installation and the additional tension induced by wind and other lateral forces during event operation. The cumulative effect of these tension loads is to reduce the total load capacity available to support production loads (audio, lighting, scenic elements). These tension forces must be considered in the overall structural design; an engineer who designs the structure for production loads without accounting for the additional compression loads introduced by guy line tension will underestimate the total loading on the columns and towers (Event Safety Alliance, 2013).

Earth Anchors Versus Movable Ballast

The expresses a clear preference: fixed point anchors such as earth anchors or permanently installed venue anchor points are preferable to movable ballast such as “Jersey” barriers (Event Safety Alliance, 2013). If fixed point anchors are used, they must be tested for the anticipated maximum loading before structure erection. Earth anchors of known capacity that have been properly installed and tested provide a reliable, defined resistance to the loads imposed by the guy line system.

Movable ballast systems must be designed to resist uplift, sliding, twisting, and rolling resulting from the guy line tension applied to the ballast, as well as environmental forces on the ballast itself, such as wind and earthquake effects. The resistance provided by movable ballast is not equal to the weight or mass of that ballast, and is typically far less (Event Safety Alliance, 2013). Where a guy line load or capacity is provided on erection drawings without a specific quantity or size of ballast, a qualified engineer must be consulted to determine the equivalent amount of ballast mass required to resist the load generated by the structure. This calculation is more complex than it may appear: the resistance of ballast to sliding depends on the frictional force between the ballast and the ground surface, which varies with the bearing surface condition, the weight of the ballast, and the angle and point of application of the guy line force.

Ballast Attachment Requirements

Guy line attachment to movable ballast must be designed to engage the entire ballast weight. Do not attach guy lines to non-structural embedded elements such as loops, eyes, or bales at the ends of ballast items such as Jersey barriers, concrete blocks, water tanks, or steel plates. Unless these attachment points have been specifically tested for the anticipated loading, they are not reliable (Event Safety Alliance, 2013). The correct approach is to wrap the attachment around the entire ballast element in a manner that cannot slide and that does not bias the loading to one end. Note that the force required to lift one end of a ballast element is less than the total weight of the ballast; attachment to one end can cause the ballast to rotate rather than resist the load, dramatically reducing the effective resistance.

Guy Line Hazard Control

Where guying is used, guy lines and their anchors must not create obstructions or hazards to the public or to event workers (Event Safety Alliance, 2013). Guy line anchor locations within public access areas must be barricaded to maintain the integrity of the guy lines and to prevent injury from vehicle or pedestrian impact. Guy lines that run through public circulation areas present a trip hazard and must be marked and barricaded; where guy lines cannot be redirected to avoid public areas, overhead crossing points with adequate clearance are required.

Fall Protection for Structure Erection

OSHA’s construction standards require fall protection for any employee on a walking or working surface with an unprotected side or edge six feet or more above a lower level. Specifically, 29 CFR 1926.501(b)(1) requires protection through guardrail systems, safety net systems, or personal fall arrest systems. The Bureau of Labor Statistics reported that in 2010, 35 percent of construction worker fatalities resulted from falls, making fall protection the single most consequential safety requirement for structure erection work (Event Safety Alliance, 2013).

Relevant OSHA construction standards for fall protection applicable to temporary structure erection include: 29 CFR 1926.451–1926.454 (scaffolding requirements and training); 1926.501–1926.503 (fall protection duty, systems, and training); 1926.1051–1926.1060 (stairways, ladders, and related training); and 1926.1423 (cranes and derricks fall protection). Employers must provide PPE including hard hats and harnesses where the wearing of such equipment is determined to be an effective means of preventing injury, must train employees in its proper use, and must ensure that both the training and the equipment meet applicable requirements. Employers must also establish rescue plans to ensure safe rescue of any fallen person, considering all potential fall locations and the time required to safely accomplish a rescue (Event Safety Alliance, 2013).

Protection from Falling Objects

During structure erection and disassembly, materials must not be lifted over the heads of people working or passing below. Secure “no-go” areas below working areas must be established to prevent harm to people below. Ground riggers responsible for controlling the ground area beneath overhead work should be clearly identifiable, such as by wearing high-visibility vests. When it is necessary to lift lighting, sound, or other equipment overhead, all items in the rig must be secured or tied off before the lift begins (Event Safety Alliance, 2013).

Lifting Equipment Requirements

Any organization using lifting equipment such as forklifts and scissor lifts has a duty to provide qualified operators and physical evidence demonstrating that the equipment’s last inspection has been completed. Equipment rental arrangements must ensure that the equipment is accompanied by the necessary documentation and is in good repair (Event Safety Alliance, 2013).

After positioning rigging and similar equipment integral to structure erection or operation, a competent person must inspect the lifting equipment before it is put into use. All persons involved in erecting and dismantling temporary structures must be appropriately trained in the safe techniques of high-level rigging. Training programs for safe high-level rigging techniques are commercially available; workers performing high-level rigging during structure erection must have completed appropriate training and assessment before working at height (Event Safety Alliance, 2013).

Conclusion

Guy line and anchoring systems are among the most technically demanding elements of temporary event structure safety, and their failure modes are both specific and well documented. Pretensioning requirements, ballast calculation, attachment geometry, and anchor testing are engineering decisions that must be made by qualified professionals for each specific event and site; they cannot be reliably estimated from visual inspection of the structure on the day of load-in. Fall protection during structure erection is a legal obligation that reflects the reality that structure assembly work carries among the highest fall fatality rates of any construction activity. Together, these requirements define a framework that, when implemented, allows temporary event structures to be erected and operated with acceptable risk.

References

Occupational Safety and Health Administration. (n.d.). 29 CFR 1926.501: Duty to have fall protection. OSHA. https://www.osha.gov

Occupational Safety and Health Administration. (n.d.). 29 CFR 1926.451–1926.454: Scaffolding standards. OSHA. https://www.osha.gov

American Society of Civil Engineers. (2022). ASCE/SEI 7: Minimum design loads and associated criteria for buildings and other structures. ASCE.

Entertainment Services and Technology Association. (2013). ANSI E1.21: Entertainment technology — temporary entertainment structures. ESTA.