Is it Real Protection?

By David H. Glabe, P.E. / January 15, 2007

Do you think you are really protected from falling objects when passing by a construction site?  Sure, you stand under a scaffold platform that’s fully decked and assume nothing will harm you.  But how do you know that platform is strong enough to protect you from falling objects?

Falling object protection requirements are addressed in a variety of standards, including the Federal Occupational Safety and Health Administration (OSHA) standards, American National Standards Institute (ANSI) standards, and the International Building Code (IBC) standards.  OSHA’s standards address falling object protection for scaffold users in Subpart L of the Construction Standards and for other workers passing under a scaffold in Subpart M of the Construction Industry Standards.  ANSI addresses falling object protection for scaffolds in ANSI A10.8-2001 and the IBC concerns itself with pedestrian safety during construction.

There are several options that can be used to protect people from the hazard of falling objects.  One of the more common options is the use of toeboards and this can be an effective choice for scaffold platforms in many instances.  Canopies can be used to provide protection for employees working on scaffolds and they can also be used to protect employees passing under the scaffold.  Canopies are especially effective for protecting the perimeter of a jobsite, if they are designed and constructed properly.  In fact, most urban jobsites are required to provide a canopy/covered walkway since sidewalk access is normally expected, even during construction.

To safeguard pedestrians, the IBC specifies the strength and dimensional parameters for sidewalk canopies.  While many cities adopt these standards, others have there own specific requirements.  If you are the provider of a sidewalk canopy, you are expected to know what these requirements are.  Basically, covered walkways must withstand the force of a falling object.  The IBC requires that all walkways “shall be designed to support all imposed loads and in no case shall the design live load be less than 150 pounds per square foot (psf).”  Installing a tubular welded frame scaffold with a few wood plank on top and plywood will not suffice for these loads.  Besides, the force of falling debris can easily exceed the 150 psf load requirement and will most certainly exceed the capacity of plywood and wood plank.  For example, if an errant worker were to drop a 20 pound object from 3 stories above the canopy, the force could easily be above 1000 pounds.  In fact, from ten stories the results would not be well for anybody under this type of canopy!

Besides the strength requirement, the IBC specifies the minimum clear width and height of a covered walkway.   The minimum allowable width is 4 feet and the minimum height is 8 feet.  Of course, both of these dimensions may need to be greater to accommodate the expected pedestrian traffic.  (Again, some cities will have stricter regulations.)  Lighting, splash shields, and barriers may also be required to protect pedestrians from construction activity and adjacent traffic.  In certain instances concrete traffic barriers may also be required.  Wind loads must also be considered, particularly if the walkway is enclosed.  Based on these requirements, how well do scaffold frames perform under these circumstances?  The quick answer is: It all depends.

On average, a one tier tall scaffold frame can support about 3000 pounds per leg.  Since the ledger capacity is dependent on the width of the frame, that capacity will vary, decreasing as the frame width increases.  Let’s say that you decide to install a covered walkway utilizing 5 foot wide frames and 10 foot crossbraces.  The IBC requires that each bay of the scaffold must support at least 7500 pounds.  Since you want to be competitive, you cleverly use three wood plank and an old piece of plywood for the cover.  You nail it down so it doesn’t go anywhere and decide it looks pretty good.  That platform may safely hold 1800 pounds, if you play with the safety factor.  The last time I checked, 1800 pounds is less than 7500 pounds by a substantial margin.  Well, let’s just fully plank the thing and go have coffee.  Now you are up to maybe 3100 pounds, still a bit less than the required 7500 pounds!  It looks to me that maybe plank and plywood might not be the best choice for a proper cover.   Also remember that dropping a concrete block from ten stories will also have an impact, literally, on the cover.  Even if the cover can hold the load, don’t forget the ledger of the frame; it probably isn’t strong enough either.

What’s the solution?  The simplest solution is to keep people out from under the work above.  Since that isn’t always the practical or even permissible answer, a properly designed cover is required.  The qualified designer must ensure that it will work.  This means that it has to be designed to stop whatever missile will hit it.  One solution is to construct a catch platform directly under the falling object so that it doesn’t fall all the way down to the covered walkway, gaining speed and force as it drops.  As in fall protection, not even letting an object begin to fall is the best solution.  Since there are many variables, there are many solutions.  A covered walkway/canopy that looks good may not be any good.  Chances are, scaffold frames with plank and plywood won’t meet the minimum strength or dimensional requirements.  Keep that in mind the next time you walk through a covered walkway at a construction site.

Tags: construction site hazards falling object protection OSHA Standards & Regulations Resources Safety Hazards

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