PRACTICAL. RESPONSIVE. EXPERT ENGINEERING CONSULTANTS SINCE 1985

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Existing Structure Shoring

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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.

When undertaking the task of existing structure shoring you should consider consulting an engineer – and I don’t just say that because I happen to be an engineer!  The peace of mind that you get from entrusting this work to an engineer far outweighs the risk of liability if something goes wrong during the shoring operation. 

Things that your engineer will need to know before starting a shoring plan 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 is your customer trying to keep the parking garage operational during construction?  Depending on the scope of the job, snow and wind loads may also need to be taken into account.  Be certain to consider any special circumstances like required access openings in the shoring plan and work sequencing that would affect the standing shores.  Drawings, schematics and photographs can be provided to convey most of this information but, in some cases, it is easier and most cost effective for the person designing the shoring plan to visit the site.

If an existing structure is improperly shored, there is danger of damaging the building or of a collapse.  Providing as much accurate information as possible to your shoring designer will help to minimize risk and ensure the most accurate and economical design.  Don’t take chances, if in doubt get a professional engineer involved and maximize your chances of shoring success!

Fall Protection – The Full Package

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It has been said that the best solution for fall protection is to not fall, but as falls account for several deaths on construction sites, it turns out this plan doesn’t work out and will make OSHA very grumpy. This topic may be stale news to the salty veterans who have been around the block a time or two but I would be willing to bet that there are very few who consider all aspects of a fall protection every time they don their harness.

Whether you are the engineer designing the plan or the contractor whose life relies on the plan, there are several aspects of fall protection that need to be considered. The most familiar components of fall protection are the personal fall arrest system and the anchor which the system is attached to. Most anyone who has needed to utilize fall protection in their line of work knows that OSHA requires you to use a personal fall arrest system and be connected to a suitable anchor which is capable of supporting 5,000 pounds or be designed by a qualified person. In addition a fall protection user must consider the anchor location in relation to the work area, the fall distance and a rescue plan which are just as important and easier to overlook.

After determining the personal fall arrest system and a suitable anchor, next, consider the work area in relation to the fall protection anchor: It is always a good practice to keep the fall protection system as close to 90 degrees to the edge of the fall hazard as possible. This will limit the amount of swing in the event of a fall reducing the risk of the worker swinging into an object below.

Next, consider the fall distance to prevent a worker from hitting a lower level or an obstruction below as they fall. This aspect of fall protection has the highest variability and can change with each setup. The fall distance can be as little as a few feet if using a self-retracting lifeline attached to a rigid anchor to upwards of 20 feet with some horizontal lifeline applications.

Finally, any fall protection plan is pointless without a way to rescue the poor soul hanging from the system. The fact of the matter is that the fall is not the only way to cause injury and/or death. The sustained mobility of being suspended and the potential for the harness to restrict blood flow can cause serious issues if the worker is not rescued within a reasonable amount of time.

A well designed and implemented fall protection plan must consider all of these aspects. Fall protection may or may not be your bread and butter, however when you need it, considering only some of the aspects could turn into a very bad day. All good ideas start with a plan but without the follow through you’re just a guy hanging there hoping on a dream.

FAQ’s ABOUT FALLS

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A lot has been said about falls and fall protection. The U.S. Federal Occupational Safety & Health Administration, OSHA, has emphasized fall hazard awareness and increased enforcement of the fall protection regulations for years in the hope that deaths and major injuries due to falls in the workplace can be reduced. Manufacturers and suppliers are complementing the OSHA emphasis by offering a plethora of products that can be used to keep employees from falling. Or, more accurately, to keep employees from falling from heights to levels below in such a manner that they get injured or killed.

Due to the complexity of fall protection, it is not a simple procedure to provide personal fall protection equipment in such a way that it will protect an employee in all situations all the time. Confusing the matter is the inaccurate information, conflicting codes and interests, and a whole bunch of misconceptions about fall protection. Here are a few of the more frequently asked questions (FAQS)

What’s a personal fall arrest system? A personal fall arrest system (PFAS) consists of a full body harness, a shock absorbing lanyard or self-retracting lanyard, a vertical lifeline or horizontal lifeline, and an anchor. Alternatively, the lanyard can be attached directly to an anchor, eliminating the lifeline.

Is it true that I can use either a guardrail system or personal fall arrest system when working on a supported scaffold such as a frame or systems scaffold? That is true although the guardrail will be much more effective unless you are using the fall arrest system for fall restraint.

What is fall restraint? Fall restraint is using a personal fall arrest system to keep you from going off the edge of an exposed platform edge. It’s like hooking up the employee to a leash.

Why is a guardrail system more effective than a PFAS? A guardrail system keeps you on the platform or floor while a PFAS catches you after you have decided to leave the platform or floor.

I went bungee jumping once and found it to be exhilarating. Does one get the same thrill from falling off a floor while wearing a PFAS? I don’t know—I haven’t done either one although I want to jump off a bridge attached to a rubber band—sounds like fun. Falling from heights utilizing a PFAS, on the other hand is a whole different experience. While it is often perceived that no injury will occur due to a fall, the truth is quite the opposite. While there are those who experience no injury, typical injuries include severe bruising and intestinal damage. Frankly, the only thing worse than falling while wearing a PFAS is falling without a PFAS.

That doesn’t make sense: people use PFAS daily and I don’t hear of any injuries. What gives? The fact of the matter is that employees utilize/wear PFAS but very, very few actually use it. In other words, although employees wear harnesses and are attached to anchors, they rarely actually use the harness because they don’t fall from heights. Consequently, since they don’t fall, they don’t get hurt.

I have been told that my PFAS anchor has to hold 5,000 pounds unless it is designed by a qualified person, that is someone who knows how to design the anchor and system. Is this true?
Yes it is. The OSHA regulations and other codes require that the anchor you use has to be “capable of supporting at least 5,000 pounds per employee attached, or shall be designed, installed and used as part of a complete PFAS which maintains a safety factor of at least two and under the supervision of a qualified person.” [29 CFR 1926.502(d)(15)]

Are you telling me that before I attach my lifeline or lanyard to an anchor I must have someone determine it can hold 5,000 pounds? Yes.

Come on, no one does that. Everyone eyeballs the chosen anchor and estimates its strong enough. You mean I cannot do that? That is correct: OSHA says you cannot do that.

But it works; I mean that is what everyone does so isn’t it okay? It works because you don’t fall and therefore you never actually use the anchor! Just because you hook off to something that you call an anchor does not an anchor make. In other words, just because it looks good doesn’t necessarily mean it’s going to work. While not recommended, you must jump off the floor to see if your anchor will work.

Why does everyone get away with guessing as to the strength of the anchor? That’s easy; the regulation isn’t enforced. Besides, all the safety folks are happy if the guy is “tied off.” Luckily we don’t have too many employees jumping or falling off floors.

Isn’t tying off the same as utilizing PFAS? No way. You can tie off to anything, including yourself. Properly utilizing a PFAS means that you have selected an anchor that will support 5,000 pounds or you have tied off to an anchor designed by a qualified person in compliance with the mandatory OSHA regulations.

And what are those mandatory regulations? Here are a few: Limit the freefall to 6 feet; stop within 3.5 feet, (known as the deceleration distance); limit the force on the body to 1,800 pounds; and the most important, don’t hit the surface below.

That sounds complicated; is it? Yes, it can get very complicated to design a system that provides 100% fall protection and be in compliance with all of the applicable codes and OSHA regulations. Fortunately, the fall protection equipment manufacturers have done an incredible job of consistently developing new products that can be used to assist employers in protecting employees from fall hazards. It is amazing the changes that have occurred since I first got into the business many years ago. Unfortunately, too many employees lack the training to use the equipment properly. Fortunately, very few employees ever get the opportunity to actually use their PFAS!

How do I obtain the training to utilize and maybe use my PFAS correctly? There are numerous seminars that offer fall protection training. However, I suggest first contacting the manufacturer of your equipment since it should know its products. To learn about the applicable regulations, select a seminar that fits your needs, such as user, inspector or competent person. And finally, verify that the instructor is qualified to teach the seminar.

The ABCs of an Efficient Temporary Wall Bracing Plan

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A common concern for many of our clients is to improve the schedule of a job in order to increase revenue and profit. One of the most common ways for a project to gain time in a schedule is to install temporary wall bracing, typically using tilt-up style metal braces. When trying to design the most efficient temporary wall bracing plan, one might want to consider what I like to call the “ABC’s”:

A. Angle: brace capacities are given as an axial load.  After calculating the required horizontal bracing force, the designer must consider how the angle of the brace is going to transfer that horizontal load into an axial load.  This can drastically affect your brace spacing if your brace angle is 60 degrees versus 45 degrees.

B. Bottom: this is typically the main complication of a bracing plan.  The temporary brace resists the overturning of a wall near the top, but there is still the total horizontal load that needs to be resolved at the bottom.  For example, assume that the average load against a 12’ high wall is 5,000 lbs, and it is applied at 1/3 the height (this scenario is similar to backfilling a wall).  The overturning of that backfill is (5,000 lbs) X (12 ft) x (1/3) = 20,000 ft*lbs.  If the brace is installed at 10’, then the required horizontal capacity is (20,000 ft*lbs) / (10 ft) = 2,000 lbs.  However, if the original load against the wall is 5,000 lbs and the brace is only resisting 2,000 lbs, then the bottom of the wall still needs to resist 3,000 lbs.  Typically this is accomplished by installing the slab on grade.  If the slab on grade is not installed, then the designer must analyze the wall itself to resist the load or specify an additional permanent support.  If the wall itself is not sufficient, then it is typically in the best interest of the contractor to install the slab on grade.

C. Connection: connections will need to support shear loads vertically on the wall, horizontally on the slab, and vertically on the slab.  There may be limitations in the existing structure due to substrate thickness, edge/spacing distances, and ground bearing capacity.

By keeping these guidelines in mind, designers maximize the efficiency of bracing for the contractor and the project.

Seismic Retrofit of Existing Buildings

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The process of evaluating and designing the retrofit of existing buildings differs from the conventional structural design of new buildings. The current state-of-the-art analysis and design approach for the seismic evaluation of existing buildings is founded on a performance-based philosophy. There are two parts to a performance-based analysis and design.

First, there is the establishment of a performance objective. This answers the question for the designer and the owner, “What degree of damage to the building am I willing to tolerate in the event of an earthquake?” It is not economically feasible to design all buildings to a performance objective that limits all damage or allows the building to remain fully operational and allow immediate occupancy following an earthquake. Therefore, performance objectives exist that allow a certain degree of damage to occur while still protecting life safety and preventing building collapse.

Second, there is the establishment of the seismic demand used in the analysis of the building. Statistical analysis is used to determine the probability of the maximum considered earthquake (MCE) occurring at the building site at any given time. The MCE demand level varies based on the time frame considered and the probability that there will be ground motion at the site that exceeds the MCE (i.e. 5% probability of exceedance in 50 years). Together with these two variables the mean return period of an earthquake can be established (i.e. it can be expected that an earthquake of ‘X’ magnitude, or the MCE, will occur approximately at least every 975 years).

There are various performance objectives and seismic demand levels that may be considered. Any given combination of performance objective and seismic demand level will result in a varied stringency of analysis and design. Combining a strict performance objective (i.e. operational post-earthquake) with an earthquake of relatively long return period (2500 years) will likely result in a robust, yet potentially expensive, design.

In conventional structural analysis and design, the seismic demand used for the design of the seismic force resisting system is reduced by a system-wide Response Modification Factor, R. This coefficient is established based on the ductility of the lateral system selected for design. The R-Factor is intended to act as a representation of the ability of the lateral system to dissipate energy as it flexes, bends, and undergoes inelastic deformation under seismic load.

In the evaluation of existing buildings, the concept of reducing the demand to account for ductility in a system is captured by using component specific m-factors. Rather than reducing the seismic demand, m-factors are applied to scale up the strength or capacity of individual structural elements that experience ductile or “Deformation Controlled” failure. These m-factors vary by component and allow the design professional to apply a uniform seismic demand to the system while modifying the strength of each individual element of the system according to its ductility. This philosophy is ideal for seismic retrofits that require the introduction of an entirely new lateral system or the strengthening of only a few discrete components.