If you want to start a lively discussion among new engineers or contractors, ask them about the differences between shoring, reshoring, and backshoring. They all sound similar, but in reality, perform different jobs at different stages of the construction process. More importantly, the number of shores, reshores, and backshores are project-dependent. They also vary wildly according to climate and speed of construction. So, what are each of these things?
Due to the complexity and property line constraints of modern construction, earth shoring requires a solution that must conform to both engineering and safety guidelines during all stages of construction.
Here are the 5 key concepts to remember for an earth shoring design:
Everyone knows about dams. But have you heard about a cofferdam?
Cofferdams have been around for a long time. People have used these when excavating very large plots of land or building foundations of water-based structures such as bridges or piers. The cofferdam keeps water from flowing into these sites, ensuring a dry foundation.
Shoring existing structures can be a tricky business and the older the building, the trickier it can become. Many older structures do not have drawings of the existing construction and if they do, they are not always reliable. Many buildings go through generations of remodel with additions, renovations and improvisations that are not always documented properly. Without proper documentation, it is sometimes difficult to determine the load bearing members in an existing building and this makes it difficult to shore. If you can’t figure out where the loads are concentrated, you can’t figure out how to safely and economically support anything.
A safety factor is like an insurance policy – you hope you never use it. Basically, a safety factor applies to the strength of a product; that is, the strength of a product is more than what is absolutely necessary for the product to work. Safety factors are expressed in terms of a ratio. […]
When shoring is mentioned, new buildings typically come to mind. However, shoring is also frequently used to support existing buildings, either because the building is failing or building modifications are being performed. While supporting existing buildings utilizing standard shoring equipment performs the same function as new construction—the approach to the design and installation oftentimes is entirely dissimilar.
Shoring existing structures can be a tricky business and the older the building, the trickier it can become. Many older structures have unreliable drawings or none at all of the existing construction. Additionally, many buildings go through generations of remodel including additions, renovations, and improvisations that are not always properly documented. In fact, without proper documentation it becomes difficult to determine the location of the load bearing members in an existing building and consequently complicates the designer’s shoring design and analysis. Simply stated, if the location and magnitude of the loads are unknown, it is impossible to safely and economically design a shoring system.
Qualifications and Requirements
As with scaffolding, all shoring must be designed by a qualified person. Additionally, the services of a qualified engineer will most likely be required, particularly if columns and footings are removed. Load transfer from columns can easily reverse the stresses in the adjacent beams; if the shoring isn’t placed correctly, and any excessive stresses mitigated, structural components and the shoring equipment may be compromised to the point of failure.
Items the engineer will require before initiating a shoring design and analysis include the type of work being performed, the boundaries of work, distance to any excavation, dimensions of the building, and location of load-bearing members. Other pertinent information includes the dead load of the supported area and any anticipated live loads— for example, will an office building remain occupied or will the parking garage remain operational during construction?
Depending on the scope and duration of the project, snow and wind loads may also need to be taken into account. Be certain to consider any special circumstances like required access openings within the shoring equipment and the sequencing of the work which could affect the shoring towers. While drawings, schematics, and photographs can be provided to convey most of this information, in some cases it is easier and more cost effective for the engineer designing the shoring plan to visit the site.
Improper Techniques Can Prove Catastrophic
The consequences of an improperly shored existing structure can be, and usually are, catastrophic. Reports of partially or completely collapsed structures are common and are typically the result of either a poor or non-existent design produced by an unqualified person.
Simply sticking a few shoring posts under the joists and beams in a random fashion, based “on experience,” does not a good shoring plan make! Worse yet is the construction crew that installs various posts in an ill-defined manner, only to be surprised by creaks and groans of a structure that is ready to fail. While the structure is talking to them, they misread the message, only to find that total collapse is imminent.
To avoid such an experience, providing accurate and complete information to your shoring designer will eliminate the risk of collapse and ensure an accurate, safe and economical project. Don’t take chances, if in doubt get a qualified professional engineer involved and maximize your chances of shoring success!
Regulatory Differences Between Shoring and Scaffolding
It is not all that uncommon for a scaffold equipment supplier to also be a shoring equipment supplier. After all, both are used on a temporary basis, have similar components and are designed to support loads. While shoring equipment normally supports heavier loads, scaffold frames can as easily be used as shoring as shoring can be used as scaffolding. (The shoring referred to here is the equipment that is used to either support fresh concrete while it is curing (getting hard) or to support existing structures. It is not the equipment used to restrain soil in excavations and trenches.)
Of particular interest are the applicable regulations for scaffolding and shoring. Is shoring the same as scaffolding? Do the regulations that apply to scaffold erectors also apply to shoring erectors? After all, frame scaffold is just like frame shoring—or is it? These are good questions that deserve accurate answers. The quick, or fundamental answer, is that scaffolding is not shoring and shoring is not scaffolding. Since the work activity, and not the equipment, describes the equipment’s function, it just doesn’t matter if scaffold frames are used to shore up concrete any more than if shoring frames are used to support a work platform.
Applying Federal OSHA standards
Here is how the federal OSHA standards get applied. It is necessary to determine what the elevated platform/deck is being used for. Is this elevated platform used to support employees or materials or both so that the workers can reach the work area? For example, are the employees masons who are laying up brick and block? If so, then the elevated platform, and the supporting structure, is a scaffold. On the other hand, is the primary purpose of the elevated platform to support fresh concrete? If so, then the deck is a “horizontal walking/working surface.” The significance here is that the purpose of the deck must be known so that the correct standards are utilized. What happens when the work surface is used to support employees who are using it to reach a work area and the work surface is also being used to support concrete? What about the walkway around the perimeter of the deck that is used to support concrete? Well, here is how it works: Say that you have an electrician using the deck/platform to reach a location to install conduit and electrical boxes. Since the electrician is using the work surface as a scaffold platform, the platform must be able to support itself and four times the intended load. In addition, the guardrails must comply with the scaffold standards, there must be access in compliance with the applicable scaffold standards and any exposed employees must be protected from falling objects.
On the other hand, if the work surface is being used to support concrete, then it must comply with the applicable fall protection, access and formwork standards that address that work activity, including the concrete and formwork standards, 29 CFR 1926, Subpart Q, the fall protection standards at 29 CFR 1926, Subpart M, and the access requirements found at 29 CFR 1926, Subpart X. Typically, if these standards are met, then in the example where the electrician is using the platform to access her work, she will also be in compliance with the scaffold requirements due to the fact that the scaffold regulations are generally not as restrictive as the fall protection and access regulations for working surfaces.
Where are the Differences?
Where are the differences? For the guardrails, in scaffolding the guardrail height is 38 inches to 45 inches while for an open-sided work surface the range is 39 to 45 inches. Access in scaffold requires that the first step is not more than 24 inches while it is 19 inches for a floor or other work surface. Falling object protection standards are similar for both scaffold platforms and other work surfaces although the safety factor for a scaffold platform is 4 while the safety factor for a deck supporting concrete is whatever the employer feels is correct. (ANSI and the SSFI recommend a 2 to 3 safety factor.)
What should be done with erectors? While the temptation is to consider shoring equipment erectors to be the same as scaffold erectors, and therefore the scaffold standards should apply, the reality is that shoring erectors are not scaffold erectors. Consequently, the scaffold regulations do not apply. This presents a quandary when it comes to the application of the appropriate standards. Is climbing a shoring frame access or a working surface? When do shoring erectors use personal fall protection? Or do they?
Well, here is how federal OSHA views it. A shoring frame is a vertical walking/working surface. Consequently, personal fall protection is required when climbing a frame. Standing on a plank at the top of a shoring tower is considered to be an open-sided platform; fall protection is required. Walking/climbing between shoring towers is not normal access so therefore the access standards would not apply. But the fall protection standards would apply.
As for the walkway around a formwork deck supporting concrete, it is considered an open-sided horizontal work surface. Therefore a guardrail system is expected and it must comply with the regulations in Subpart M where the toprail must be at least 39 inches above the deck and no more than 45 inches.
Focus on Purpose and Safety
Confused? The bottom line is that you need to look at the purpose of the work surface. This will determine if it is a scaffold platform rather than a horizontal work surface. And as for the employees installing the rebar and inserts in preparation for placing the concrete, although they appear to be using that deck to access their work and consequently it appears it would then be a scaffold platform, these employees are doing work to the deck which then makes that deck an horizontal work surface and not a scaffold.
Still confused? Focus on safety—not the regulations.
With many variables to consider, there simply does not exist a “one size fits all” approach to reshoring.
To discuss reshore, we must first start with an understanding of shoring, which, according to ANSI A10.9, is “The vertical supporting members in a formwork system.” The important points here are ‘vertical’ and ‘formwork‘, which signify that we are supporting a wet, horizontal concrete slab. Easy enough? Good, then let us move on to reshore.
Reshore, also according to ANSI A10.9, is “The vertical supporting members that are used to support partially cured concrete after the removal of the formwork.” It is the system of vertical load bearing supports that are assembled AFTER the shoring is stripped or released that are intended to carry the partially cured deck or decks.
Well, for starters, you do not need reshoring if you only have one slab and your shoring system is supported by the ground or other sufficiently rigid substrate. If your substrate is not sufficiently rigid, you might need reshoring to transfer the shoring loads to the rigid substrate or ground.
Reshoring is required when you are pouring a concrete slab and your shoring system must be supported by lower floors that may not have achieved full strength. It may be necessary to spread the shoring load to more than one of the partially cured slabs or floors below. This is done with reshoring.
You have heard the old saying ‘There’s more than one way to skin a cat’. Well, there is also more than one way to design reshore. Many times, it is based on the shoring designer’s preference, the slab thickness of the supported structure or the load limitations of the equipment to be used.
One approach, the Simplified Analysis, is spelled out in ACI 347.2R. There are several factors to consider but it all boils down to how much load your newly placed slabs can handle and what the load carrying capacity of your shores and reshores are.
The Simplified Analysis starts with several assumptions. First, all slabs are identical. Second, shores and reshores are aligned one-to-one from floor to floor. Third, ground level or other base support is rigid and shores are spaced closely enough to treat the shore reactions as a distributed load. Finally, shores and reshores are infinitely stiff relative to the slab.
The Simplified Analysis assumes that all slabs are identical but we know that in real life this is not always true. If you invest the time and money up front when designing the shoring layout, it will pay off in the end. That means that you must look at all of the levels or floors prior to beginning the shoring design so that you can economize the layout and reuse what you can in the reshore design. It is easiest and cheapest to maintain the same shoring/reshoring footprint from floor to floor. This pre-planning saves design time and labor hours.
The question then asked is ‘Why do you have to loosen then re-tighten shoring to re-use it as reshoring?’ In many cases, you need to loosen or drop the shoring system to allow the slab to deflect to its natural shape so that it can carry its own weight. By allowing the slab to carry itself, we then transfer only the shoring loads from above through the slab and into the reshore system preventing the accumulation of loads. Thus, the reshoring carries the load from only the shored slab.
So when do you not have to loosen and re-tighten shoring to re-use it as reshoring? When the shoring has been designed to carry the accumulated loads from the slab that it currently holds plus the supported slab or slabs above it, the current slab does not have to carry its own weight so there is no need for it to deflect. If the slab does not need to deflect, the shoring does not have to be loosened.
‘Can I always drop the deck and re-tighten the shoring to act as the reshore?’ The simple answer is no, you cannot. You must have the same leg-for-leg pattern for the shoring and reshoring and you must ensure that the partially cured slab can handle the loads imposed from the shoring system. Sometimes, the shored floor above is heavier than the current floor and the current floor slab cannot handle the loads imposed by the upper floor(s). If you do not have a leg-for-leg reshore layout or it is impractical to reshore in a leg-for-leg pattern, an engineer may be able to provide you with a reshore layout based on a structural analysis of the partially cured slab or slabs.
‘Can I always employ the “one level of shoring and two levels of reshoring” rule-of-thumb?’ No, you cannot always employ the same shoring and reshoring patterns. The number of levels of reshoring is based on the strength of the partially cured slab, the design of the structure and the capacity and frequency of the shores. Some structures are designed to allow more loads on a floor slab than others and these things need to be considered when designing a reshore pattern. The thickness of the slab plays a crucial part in the reshore design because the thicker the slab, the heavier the shoring and reshoring loads become which could require more levels of reshoring to handle these loads.
Multi-level reshore requires preplanning to maximize and economize both the shoring and reshoring layouts. Not all projects are the same and all reshore patterns are not the same. There are several factors to consider when determining how many levels of reshore are required for a given project. No matter what design or method of analysis you choose, always utilize a qualified shoring and reshoring designer to save time and money to keep your project safe and on schedule.
Applying the correct regulations and working safely while constructing shoring & formwork
When it comes to providing fall protection and access for shoring and formwork erectors, OSHA scaffold standards apply to scaffolding and nothing else; so what does a shoring or formwork erector do? The most efficient answer is to not fall while erecting the equipment.
Shoring Tower or Elevated Work Platform?
In order to apply the correct regulations, the first step in providing fall protection and access for shoring and formwork erectors is to determine if they are actually erecting shoring towers or an elevated work platform so they can reach the work area.
While both scaffold construction and shoring construction involve the assembly of towers, the use of those towers is completely dissimilar. Shoring towers are used to support a deck that is used to support concrete. Scaffold towers are used to support an elevated platform that is used to support workers or materials or both. But aren’t shoring decks supporting workers who are installing the rebar and inbeds? Yes, but those workers are doing work to the deck, not workingfrom the deck. And that’s the big difference.
Once it has been determined that the towers being constructed are shoring towers, and the deck on top is the formwork for the concrete, then appropriate fall protection and access standards can be applied.
The fall protection standards are found in Subpart M of the OSHA construction standards, 29 CFR 1926. The access standards are found in Subpart X of the same standards.
Access or Work Surface?
Are the erectors climbing frames to construct a tower using the frames to gain access, or working on an “unprotected side or edge of a work surface?”
Per OSHA, when the erectors are climbing and assembling the frames, it is a “vertical work surface.” So if an erector is climbing/assembling a shoring frame, it is not access—it is a work surface and fall protection is required to be compliance. It’s reasonable to conclude that both a guardrail system and safety net system aren’t applicable, so that leaves the personal fall protection system to get the job done.
Fall protection is required once a worker is six feet above the level below. The anchor to which the lanyard is attached must hold 5,000 lbs. unless it is designed by a Qualified Person and has a safety factor of two. This can be extremely problematic at low heights above the surface below since it is very rare to have an anchor point above the worker. (Shoring is normally used to construct the top floor of a building with the erectors constantly above the top floor. While dismantling, it is equally difficult, near impossible to provide fall protection at the same low heights since lateral movement and limited fall distance are mutually exclusive—you normally can have one or the other but not both.)
When it comes to providing fall protection for the erectors constructing the deck— installing the joists, stringers, and plywood— there is some allowance for “leading edge” erectors. A leading edge is the edge of the formwork that is constantly changing due to the addition of additional decking. It’s considered to be an unprotected side and edge during periods when it is not actively and continuously under construction. Leading edge erectors are to be protected unless the employer can demonstrate that it is infeasible or creates a greater hazard to use fall protection. It is the employer’s obligation to prove infeasibility, not OSHA’s obligation to prove feasibility.
In a Nutshell
Fall protection is required when a worker is more than six feet above the level below whether on frames, vertical wall forms (except the scaffold platform attached to the wall forms where the scaffold standards do apply), or horizontal formwork—easy in principle, difficult, near impossible to execute. A company’s Competent Person is the one responsible to evaluate the situation and determine the applicable regulation.
While fall protection will keep you from falling off an elevated surface, proper access is necessary while getting to that work surface. Access is straightforward; if you are using a portable ladder, you must comply with the manufacturer’s recommendations and the regulations found at 29 CFR 1926.1050 through 1060. Make sure you have a ladder that has the correct capacity, is in good repair, and is installed correctly. This of course would include the correct angle of installation and ladder extension above the deck.
An employer has to have to have its Competent Person evaluate each situation, and when the situation changes – which may be daily—re-evaluate to determine the feasibility of providing fall protection in compliance with the applicable OSHA standards. It is expected that this can be done no matter the circumstances unless proven otherwise; and proving a negative is almost impossible.
How to Ensure Safety While Shoring an Existing Structure
Shoring existing structures can be a tricky business. Typically, the older the building is the more complicated the shoring of it can become. Many older structures do not have drawings of the existing construction and if they do, they are not always reliable. Many buildings go through generations of remodel with additions, renovations and improvisations that are not always documented or constructed properly. Without proper documentation, it is sometimes tough to determine the load bearing members in an existing building which makes it challenging to shore it properly. Difficult access to these documents does not give you a free pass on when and where to shore. If you can’t figure out where the loads are concentrated, you can’t figure out how to safely support anything.
When undertaking the task of shoring an existing structure, you should consider consulting a Professional Engineer – and I don’t just say that because I happen to be one! The peace of mind that you get from entrusting this work to an engineer far outweighs the risk of liability if something were to go wrong during the shoring operation. An experienced professional can help you determine not only how to safely shore the structure, but how to do it in the most economical way possible.
Pertinent information that you will need to know before starting a shoring plan includes but is not limited to: the type of work being performed, the boundaries of work, distance to any excavation, dimensions of the building and location of load bearing members. Other relevant information includes the dead load of the supported area and any anticipated live loads. Examples of live loading conditions that need to be evaluated include determining whether the office building or parking garage will remain operational during construction. You also need to consider if there are any special circumstances such as required access openings in the shoring plan or work sequencing that would affect the standing shores. Do snow and wind loads need to be taken into account? Be certain to consider all of these circumstances when developing the conceptual shoring design. Drawings, schematics and photographs can be provided to convey most of this information. However, in some cases it is easier and more cost-effective for the engineer designing the shoring plan to visit the site.
If you do not follow the correct protocol and improperly shore an existing structure, there is increased likelihood of damaging the building or of a complete collapse during construction. Choosing to retain the services of a Professional Engineer and providing them with as much accurate information as possible will help minimize risks and ensure the most accurate and economical design possible. Don’t take chances; if in doubt regarding the stability of a structure, always err on the side of safety. Get a Professional Engineer involved and maximize your chances of shoring success!