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Horizontal Lifelines For Steep Pitch Applications

Jul 12, 2016 7:10:30 AM

When most people hear the phrase “fall protection” their minds immediately gravitate toward construction, loading and unloading railcars and trucks, or inspecting and maintaining crane rails, industrial machinery, and rooftop HVAC equipment.  Each of these scenarios requires work at height, and as such, fall protection is needed.  That said, there are countless additional applications for fall protection systems, and our recent horizontal lifeline installation on the Oculus Transportation Hub in the World Trade Center District is a case in point. 

WTC Hub - Ground Level

The World Trade Center Transportation Hub serves 250,000 daily commuters and draws visitors from around the world. The work of internationally acclaimed architect Santiago Calatrava, the WTC transportation hub rivals Grand Central Station in size and stature. According to Calatrava, the Oculus was designed to resemble a bird being released from a child’s hand. The years of delays and construction cost overruns associated with this project are well documented elsewhere, but this edition of Tech Talk discusses the challenges we faced while designing and installing the fall protection system for the structure.

WTC Hub 2

If you look carefully at the pictures above, you’ll notice an arc of overhead glass panels running the length of the concourse.  Each of these panels forming the skylight open—and they require periodic maintenance from the top of the structure.  At first glance, you’d think the main challenge here is finding a way to install a discrete fall protection system that does not detract from the architectural lines of the building.  We’ve done numerous fall protection systems on historic buildings and newer structures rich with architectural detail, but in this case, concealing the system from ground level was only part of the story.  The steep rooftop inclines meant that we needed to find a way to prevent a horizontal lifeline system from turning into a zip line in the event of a fall.

wtc 3

From an OSHA compliance standpoint, we needed to design a system capable of limiting the maximum arresting force applied to a falling worker’s body to 1,800 pounds (OSHA 1915.159(b)(6)(ii)) and bringing the individual to a complete stop and limiting the maximum deceleration distance to 3.5 feet (OSHA 1915.159(b)(6)(iii)).  Our systems typically exceed OSHA requirements by limiting maximum arresting force to 900 pounds, but eliminating the potential zip line effect caused by the steep roof and bringing a fall victim to a complete stop within 3.5 feet presented significant design challenges.

To address these concerns, our engineers recommended the use of, and incorporated a tab towards the downhill side of each lifeline post to accept a rope grab system (5/8” diameter polyester/polypropylene rope and a mechanical device for locking onto the rope).  The rope grab device locks up during a fall, allowing the user to repel down from the nearest post to the catwalk in a safe manner. In addition to ensuring workers don’t tumble down the entire length of the lifeline system during a fall, the design has the added benefit of eliminating the need to install separate anchor points (one to serve as a single point anchor and the other dedicated to the horizontal lifeline) as shown below:

Dual Pupose HLL Single Point Anchor Post

This application illustrates the importance of an in-house engineering department.  In addition to the actual system design, our PE ran the calculations to verify the existing structural support used to attach the posts (beneath the roof deck) could safely handle the load forces during fall arrest.  The completed horizontal lifeline system consists of 800 feet of 316 stainless steel 7 x 7 cable, 54 intermediate posts, and 4 end anchors.  The pictures below were taken during the installation phase of the project:





With proper design and installation, effective fall protection systems can blend seamlessly into the design of even the most stunning architectural landmarks.  Most rooftop fall protection applications don’t have the same set of requirements outlined in this post, but our mission remains the same from project to project—keeping workers safe at any height.  To learn more about fall protection, or to discuss the specifics of your application, contact the safety professionals at Diversified Fall Protection for further assistance.