COMMITTED TO SAFETY AND VALUE ENGINEERING - SINCE 1985
Monthly Archives

October 2012

The Rise And Fall of Elevators

By | Hoists, Resources, Uncategorized | No Comments

Moving vertically on a construction project can be a challenge.  While stairs are normally available, it doesn’t make a whole lot of sense to use those to climb 50 stories unless you are interested in a lot of exercise!  Consequently hoists, elevators and lifts are frequently used to transport workers and materials between the various floors of buildings under construction.  In fact, rare is the project that doesn’t have some type of hoist for moving people and material vertically.

In the interest of making jobsites safe, the American National Standards Institute, ANSI, has developed numerous standards for use on jobsites, including standards for personnel hoists (elevators).  One of the standards which address the machines that move people vertically at construction sites is ANSI/ASSE A10.4-2007: Personnel Hoists and Employee Elevators on Construction and Demolition Sites.  Another standard is ANSI/SIA A92.10-2009: Transport Platforms.  Both of these standards address similar vertical transportation issues but in different ways.

“Personnel hoists and employee elevators” are utilized as the general carrier for workers on jobsites who want to travel between floors while “transport platforms” are more specific in their use.  Although each can be used to transport workers, a hoist is more adaptable to fast movement between floors where the interest is to get from one floor to another while a transport platform is more adaptable to a specific work environment.  In fact, the scope of each standard explains the difference.  A92.10 applies to “Transport Platforms that are primarily used as a tool of the trade to vertically transport authorized persons, along with materials and necessary tools, to various access levels on a building or structure for construction, renovation, maintenance or other types of work.”   [A92.10-2009: 1.1]  On the other hand, A10.4 “applies to the design, construction, installation, operation, inspection, testing, maintenance, alterations and repair of hoists and elevators that (1) are not an integral part of building, (2) are installed inside or outside building or structures during construction, alteration, demolition or operations and (3) are used to raise and lower workers and other personnel connected with or related to the structure.”  [A10.4-2007: 1.1.1]

Although these scopes may appear similar, there are subtle differences.  In the simplest explanation, a transport platform is more portable than a personnel hoist while a personnel hoist is more like a permanent elevator in a building.  The personnel hoist has doors, gates, cages, enclosures, counterweights and other devices not unlike a normal building passenger elevator while the transport platform is a stripped down model.  That is not to say that a transport platform is more hazardous; it’s just not as robust as a personnel hoist.

Each standard has specific requirements regarding the design and fabrication of the machine.  For example, wind loads, live loads, dynamic loads, overturning forces and inertia forces must be considered by the manufacturer.  Of particular concern for the erector is the connection of the machine to the building since improper and/or inadequate connections will lead to catastrophic failure of the entire system.  In other words, the erector better know what he/she is doing!

Keeping in mind that transport platforms are more portable than personnel hoists, the inspection criteria is different for each.  Transport platforms are treated more like scaffolding, that is, a temporary structure.  It is expected that the transport platform operator will visually inspect the machine each day before use.  The user shall “check the operating devices, brakes, emergency stops, the condition of all trailing cables, travel limit switches, guardrails, structural connecting mast ties, cables, guide rollers, and information plates, etc.” [A92.10-2009: 5.1.2.9]  Load test frequency for transport platforms is determined by applicable regulations (local and national) and the manufacturer.  For personnel hoists, “periodic inspections and tests of hoists shall be made at intervals not to exceed three months.” [A10.4-2007: 26.4.3]  Since personnel hoists are more complex and sophisticated in their operation than transport platforms, these inspections are more thorough than the transport platform inspections normally are.

As with all machinery, operators are expected to be trained.  For transport platforms, operators must be trained in all facets of the machine, including inspection, safety requirements, stability, allowable loads, the purpose of placards and decals, to name a few.  The obligations of the operator is not to be taken lightly as he is the individual who will ensure the safe use of the platform. Simply stated, this means that if you are not the operator, you are not allowed to operate the platform.  As a rider, you get to ride, not to operate.

For personnel hoists, they shall be “operated by a competent and authorized operator.” [A10.4-2007: 30.1]  Furthermore, the operators shall be “knowledgeable and capable of performing the duties outlined in the maintenance, operating and inspection manuals and are capable or recording such activity in their log.” [A10.4-2007: 30.3]   Normally, the personnel hoist operator is a designated driver who has no other duties while the transport platform operator may be doing other work once the transport platform arrives at the work location.

In summary, a personnel hoist is a sophisticated machine, similar in design and operation to a building elevator while a transport platform is a device that takes several workers to a work location with their tools and material.  The personnel hoist is truly an elevator, albeit a temporary one that will be removed when the project is completed, while the transport platform is much more temporary by being easier to erect, dismantle, operate and use; in other words it is more like a scaffold than the personnel hoist.

Are You Stable?

By | Resources, Scaffolding | No Comments

Tarps and other enclosure materials, such as plastic sheeting, are typical materials used to create a desirable work atmosphere.  Many scaffolds are enclosed in screening and debris netting—I recall one resort project in Aruba where the scaffold was wrapped in a mesh to ensure, so I was told, that construction debris would not blow into the adjacent swimming pool.  In reality it was there so the guests below couldn’t see the less than productive construction workers staring at them!  And, of course, now that outdoor temperatures in North America are slowly falling, thoughts of a cozy work environment on a supported scaffold become more frequent, resulting in more scaffolds being wrapped in some type of enclosure so that work can continue.  It is interesting that wrapped scaffolding has been frequently discussed and written about and yet each year scaffolds fall over because somebody wrapped the scaffold without giving much thought to the effects that the enclosure would have on the stability of the scaffold.  Of course, one of the keys to a successfully constructed scaffold is making sure that the scaffold doesn’t fall over; this is especially important for the individuals who happen to be using the scaffold!

The concept of stability is straightforward:  The forces that want to knock the scaffold over have to be resisted.  How can this be done?  While there may be a number of methods that can be used, there are three that are most commonly used by scaffolding designers and erectors:: tying the scaffold to another strong structure that can resist the forces; guying the scaffold tower to a suitable anchor that can resist the forces, and; making the scaffold large enough so the size and weight of the scaffold are adequate to keep the scaffold from falling over.  Since the stability of asupported scaffold is desirable, standards and regulations have been written to address the issue.  The U.S. Federal Occupational Safety & Health Administration, OSHA, requires that “Supported scaffolds with a height to base width ratio of more than four to one (4:1) shall be restrained from tipping by guying, tying, bracing, or equivalent means….” [29 CFR 1926.451(c)(1)]  The standard goes on to require that when the scaffold is tied to an existing structure, it has to be tied at a frequency of no more than 30 feet horizontally and 26 feet vertically for scaffolds wider than 3 feet, and 20 feet vertically for scaffolds 3 feet and narrower.  (In California the requirements are more restrictive.)

Unfortunately, this regulation can be very misleading for the simple reason that it doesn’t address varying field conditions.  Keeping in mind that the OSHA scaffolding standards are minimum requirements and not directions or instructions, the qualified person who designs the scaffold shall determine the proper means and methods for ensuring the stability of a scaffold.  Also keep in mind that a qualified person will not guess at what is required to ensure scaffold stability.  Unfortunately, the reality is that too many scaffold erectors and users think that experience is a great method for determining what it will take to keep the scaffold from falling over.  While the OSHA mandated requirements may work for a scaffold not wrapped in plastic, the same tying requirements will be woefully inadequate for a scaffold wrapped in a tarp and subjected to a violent winter storm.  (Lucky for many wrappers, the enclosure material rips into pieces and blows off before the scaffold is yanked from its’ moorings!)  When a scaffold is wrapped in a quality enclosure, that is a netting or enclosure that is resistant to tearing, the scaffold instead will rip, bend and ultimately fail.

Interestingly, #9 wire is often used to secure a scaffold to a structure.  While this can work with an open scaffold design, it very rarely is adequate for a wrapped scaffold, even if the ties are “doubled up.”  Remember, guessing never has worked well as a substitution for a properly designed and erected scaffold.

So, what is the worker to do?  The answer is easy, logical, and in compliance with the applicable standards and good scaffolding engineering practice.  Have a Qualified Person design the scaffold.  In the case of a wrapped/enclosed scaffold, it will probably take the skills and expertise of a Qualified Professional Engineer who can design the scaffold for the anticipated forces at the specific scaffold location and for the specific time of year that the scaffold will be exposed to external forces from the wind and other environmental conditions.

If you think that you are qualified to design an enclosed scaffold answer yes or no to these statements.  (If you answer no to any of them, you are not qualified to design an enclosed scaffold):

I know where to find the information that tells me what the design wind loads are for my scaffold location;

I am familiar with the American Society of Civil Engineers (ASCE) Standard, Minimum Design Loads for Buildings and Other Structures wind loading criteria;

I know the strength of #9 wire and why it shouldn’t be used for wrapped scaffolds;

I can calculate the forces that are a result of a 100 mph breeze;

I know how to calculate overturning moments and forces due to pressures;

I know what the effects of a partially wrapped scaffold are;

I know what happens if the windows are open;

I know what effects a building corner or roof has on a wrapped scaffold;

I know my limitations.