Shouldering the Load - MC Magazine Spring 2003 - Concrete Publications

Shouldering the Load

A Look at Lifting Mechanisms.

By M. Chris Osment
Chris Osment is a freelance writer based in Russellville, Ark.

Proprietary design or “homegrown”? Threaded coils or stamped sheets? Steel or plastic? One of the biggest challenges confronting precast concrete manufacturers in day-to-day operations is in the moving and handling of their finished products.

It’s not enough to have a flawless mixing and casting production line. Unless the precast concrete products your operation produces are relatively small, you’ll have to commit to one or more of the various forms and designs of lifting inserts and mechanisms. This decision, which isn’t an easy one, gets even stickier if you have a variety of clients who each use or insist on a different design.

Apart from the practical and market-driven considerations, precast manufacturers must consider a number of laws and standards when choosing what sort of lifting devices to incorporate in their products. For U.S. manufacturers, foremost among these are the U.S. OSHA regulations set forth by the federal government. 29 CFR 1926.704 sets the specific load requirements for lifting devices and mechanisms. It requires that inserts used in tilt-up products be capable of supporting at least two times the maximum intended load, while the inserts used in all other precast products must be capable of supporting four times the maximum intended load. In addition, the lifting hardware (chains, spreader bar, etc.) itself is required to support a minimum of five times the maximum intended load.

The apparent disparity in load capacities defined by the OSHA standard actually makes sense when examined in the context of component usage. Tilt-up products are typically cast, picked up and moved, and set into place – and never (or very rarely) moved again. Precast products may be cast, subjected to further architectural treatments such as sandblasting, then held in a storage area until needed or sold – with one or more lifting, transporting and lowering operations between each of these steps. Finally, the lifting hardware is given the most stringent requirements because it serves as the focus of the lifting and transporting operations, is subject to the greatest stresses through use and reuse, and is subject to the most potential for slight installation variations.

OSHA isn’t alone in its stipulated minimums; these same requirements are set forth by the American National Standards Institute (ANSI A10.9, 1983), the American Society for Testing and Materials (ASTM C857-95, C890-91, C913-98) and the National Precast Concrete Association (Quality Control Manual §2.3.1).

Unlike the United States, Canada maintains no uniform body of standards applicable to workplaces in the private sector. Each of the 10 provinces, three territories and the federal government has its own occupational safety and health legislation. For more information and territory-specific data, visit http://www.canoshweb.org/en/legislation.html or contact the Canadian Centre for Occupational Health and Safety (CCOHS) at 800-668-4284.

In addition, various other OSHA standards indirectly deal with the lifting and moving of precast products. While these regulations do not deal with precast products specifically, they all concern devices and mechanisms often used in lifting or moving precast products, and precast manufacturers should become familiar with the sections that pertain to their operations. These standards are summarized in the sidebar “Standards of Lifting.”

One final safety note concerning load limitations: Be certain your employees are familiar with the effects of additional stressors on the load. For instance, if you’re lifting a precast product that is 90 percent of the rated maximum, you’re clearly within the operational guidelines. However, if you were moving that product at a high rate of speed, lifting it from a “set” position/configuration or hauling it over rough terrain (resulting in jouncing and jostling of the load), the additional stress caused by such factors could result in the hardware’s failure. In addition, such circumstances or even “normal duty” over a prolonged period of time could result in a weakened or deteriorating insert mechanism. For this reason, lifting hardware should undergo a regular inspection routine.

Given the fairly straightforward requirements for lifting inserts and devices, a novice might conclude that there would be a correspondingly limited variety of equipment. As any industry insider knows, however, that novice would be uninformed. In fact, the myriad options and configurations available can be problematic, to say the least, and are often a source of frustration and confusion – especially when product passes from a party committed to a certain kind of device to a second party utilizing a different method entirely.

Precast concrete professionals divide lifting devices into different categories depending largely on personal and company preferences. While no hard-and-fast classification exists, the various methods share a common purpose and some common characteristics.

In nearly every case, an insert (commonly made of steel, though there are some exceptions where specialized forms of plastic are used) is cast into the precast product with a portion of the insert extending out of the product; this portion serves as the “handle” for lifting and moving operations. This “handle” portion is often recessed into the surface of the product; this recessed portion can be filled in for aesthetic purposes after the product is set into place. The portion of the insert embedded in the concrete will typically have flanges, coils or “limbs” extending from it to lend strength and resistance to the insert.

The preceding paragraph sums up the commonalities between the different types of lifting inserts and devices. Listed below are descriptions of the most widely used techniques. While some pros and cons of the various means will be discussed, no effort is made to identify the “right” or “best” solution – though we will spotlight a commonly employed wrong answer.

One design gaining widespread popularity in the industry is the use of threaded inserts. The threaded insert resembles a large coiled spring, and a number of these are cast in the piece being worked. As discussed above, the insert will generally have a number of legs, loops, etc., welded to its outside surface to increase anchorage and holding power. The complement to the threaded insert resembles a screw that is threaded into the insert’s “track” (which is greased to facilitate threading) providing a strong and secure connection.

Threaded inserts are characterized by the size of the insert’s threads. Generally speaking, the threads in coil-threaded inserts are large and coarse; they have the advantage over their finer-threaded cousins in that they are less liable to become fouled by foreign matter getting into the threads. While such debris can cause binding and jamming in the smaller-threaded coils, the coarsely threaded varieties are much less susceptible to such problems.

The next two classes of inserts are so closely related that we’ll examine them together. These are the forged and stamped inserts; collectively, they’re also known as “quick-connect/disconnect” inserts, as the process is substantially quicker than with coiled inserts. Forged inserts start with a round steel bar. One end is forged into a “head” while the end that is cast into the concrete is forged into a “foot” which is formed so that it “holds” the surrounding concrete. Stamped inserts, on the other hand, begin as a flat piece of steel into which several holes are punched. A clasp device similar to that used in jewelry is used to hook up to the holes in the portions of the sheet extruding from the product. Once again, the embedded portion of the steel sheet is typically fitted with additional limbs and anchorage devices.

These quick-connect devices have the stated advantage of taking less time to hook up and release; this is countered by the claims of some industry professionals that this economy comes at the expense of stability that a threaded insert offers.

Quick-connect inserts come with another major caveat, one that is shared with coiled inserts: They are proprietary in nature. Given that there is no set standard or definition for these types of inserts, every manufacturer could conceivably be using a design or specification that is unique to their organization. This poses problems when the product changes hands; manufacturers must either provide the hookups for their inserts at no charge (resulting in lower profits when the hookups inevitably fail to find their way back) or require the customer to either purchase, lease, rent or place a deposit on the hookup devices. This can be irritating to the customer and foster some ill will down the road.

Such considerations have given rise to the widespread usage of another type of insert known as utility anchors. Depending on whom you ask, the name is either a reference to these inserts’ versatility and ease of use or because they first became popular with precast manufacturers providing products to utility companies. Utility anchors often resemble a cutaway view of a light bulb: A bent steel bar protrudes from the concrete forming essentially a giant eye-hook, while the lower portion of the “bulb” is threaded or “legged” in the body of the product. Utility anchors have the great advantage of not requiring any sort of proprietary hardware or hookups, as standard crane hooks can easily interface with the exposed portion.

Another form of lifting insert isn’t an insert at all; in certain specialized areas, lifting grooves provide an excellent alternative to hardware inserts. Generally speaking, lifting grooves are primarily used in precast products that are rectangular in shape. A horizontal groove is cast into the sides of the product, and a sling apparatus uses these grooves for lifting and moving the piece.

Some manufacturers use prestressed steel strands in a configuration almost identical to the utility anchors method. Prestressed strand is similar to a very high-strength rubber band, a property that illustrates one of its primary weaknesses as an insert. As a rubber band is stretched, its length increases – with a corresponding decrease in the cross-sectional area. Prestressed strands behave in much the same way. This expansion and contraction of the strand can introduce weaknesses in the insert as the concrete around the strand loses its grip on it. For this and other reasons, you won’t find any strand manufacturers that will recommend using their product to precasters as a lifting component.

While it may sound as if the prestressed strand design is the wrong answer, it’s saved from that designation by the black sheep of the “homemade” insert family: the use of reinforced steel bars, or rebar. While enterprising souls have used rebar as a lifting insert in a variety of different ways and designs, it most commonly takes a form resembling utility anchors. Whatever the configuration, however, rebar should never be used as a lifting insert. Rebar’s strength can be unpredictable in this application, and it’s more susceptible to wear and fatigue than the approved insert designs. In addition, should you have an accident involving a product that uses rebar as an integral part of its lifting mechanism, you’ll likely face an OSHA inspector wanting to see your testing certification showing that the failed rebar met OSHA standards.

This limitation is further outlined in the National Precast Concrete Association’s Quality Control Manual, which states in the commentary for section 2.3.1 (Lifting Devices and Lifting Apparatus): “Because of their brittle nature, reinforcing bars should not be used as lifting devices.” Clearly, when it comes to lifting mechanisms, “rebar” is a four-letter word.

Now that you’ve met all the players, you’re probably asking, “Which lifting insert is best?” The answer is “it depends.” As in most situations, you’ll have to decide which lifting mechanisms will provide the best fit for your purposes. In this context, “best” means the highest combination of safety, efficiency and cost effectiveness – in that order.