5/1/2009

On the Brink of Collapse

Temporary bracing mitigates risk of structural failure in construction projects.

By E. Brian Webb, P.E., C.M.I

What goes up must come down, right? While the laws of physics are themselves naturally noble, when they’re brought to bear on a building that’s supposed to stay up, they can be truly noxious. They also pose a special problem for those charged with determining the cause of collapse.
 
If you’ve been working in the insurance or construction industry for even a short time, you’ve likely experienced the devastating impact of structural collapses. Installation of temporary construction bracing is a critical component to each phase of a successful construction project, but sometimes it is forgotten or purposely neglected. Failure to install adequate temporary bracing can have catastrophic consequences to the overall project schedule, customer satisfaction, profit margin, and worker safety.
 
All building types, whether residential, commercial, agricultural or industrial, are designed to withstand forces from wind, snow, gravity, earthquake and people, as well as numerous other sources. Regardless of the type of force, the building is generally designed to transfer that load into the foundation and, ultimately, into the earth upon which the building is based. The path that the force takes to get to the foundation is called the “load path.” Often, loads are applied into a single component of a building, such as a basement wall or a roof truss. These individual components are not able to resist the force of the load on their own, and they must work synergistically with the other components of the building.

 

During various phases of the construction process, the load path is incomplete. Loads placed into certain components of an incomplete structure are not able to properly migrate into the foundation. This can result in overloading and failure of the member—and even progressive failure of additional building components. To prevent this, temporary bracing must be installed to resist any short-term loads until the structure is complete. The bracing must be strong enough to resist the load and safely transfer it to the ground. In addition, it must be placed in the correct location to carry the load and prevent overloading of the permanent components of the building.
 
When completed, every building or structure should have the strength, stiffness and stability to withstand the stresses of anticipated external forces without excessive movement or deflection. During construction, however, the structural elements that increase stiffness and stability may not be in place yet, leaving the primary structural members vulnerable to otherwise non-damaging forces. The purpose of temporary bracing is to provide the interim stability and stiffness required until all structural elements of the building are assembled.
 
Temporary bracing also serves to dampen the energy accumulated in a structure as it deflects or “sways.” This effect is most noticeable during gusting winds that, despite not being damaging alone, can increasingly push a structure with more force than the previous gust due to latent energy. Properly installed temporary bracing will prevent this cumulative energy from developing and will transfer these intermittent forces to the ground.
 
Basement Wall Failure
Basement walls are designed and constructed to resist the lateral pressure of the soil against the foundation of the house. Once construction is completed, the basement walls are restrained laterally by the footing or basement floor slab at the bottom, and by the floor structure at the top. Until the first-floor structure is in place, the walls are incapable of resisting lateral soil pressure without adequate temporary bracing to prevent collapse; otherwise, they would simply topple over.
 
In the instance shown here, the basement walls of a four-family condominium building had been erected of concrete masonry units (CMU blocks). All of the walls were properly constructed per the applicable building code; however, temporary construction bracing had been installed on only about half of the walls. Photograph 1 shows a wall that collapsed, and Photograph 2 shows a wall that has pushed inward but has not yet collapsed. Neither of these walls had been braced prior to their failures, and yielded under the normal force of lateral soil pressure.

As can also happen, Photograph 3 shows temporary bracing that had been installed but that had subsequently bowed under load. This was because the bracing member was not strong enough to hold the normal lateral soil load placed onto it by the wall. Photograph 4 shows a similar condition where plenty of bracing was installed, but the wall on the right bowed slightly inward because one of the braces had buckled. In this case, temporary bracing was provided, but it was undersized to handle the load.

 

Unfortunately, temporary construction bracing is often left up to the discretion of the contractor or subcontractor. A knowledgeable contractor (or one who has previously been the victim of a bracing-related construction failure) will likely never omit temporary bracing. However, contractors who are working with a tight deadline, or those looking to rush to the next job, may skimp on temporary construction bracing or omit it completely. This increases the risk of partial or complete failure of the structure or its components during construction. How many times have you heard a contractor say, “I’ve been doing it this way for over 30 years”? Any contractor who fails to install temporary bracing because he’s “done it that way for 30 years” is long overdue for a failure.
 
Even properly braced structures can collapse if they are subjected to sufficient forces. However, it is reasonable to expect a properly braced building under construction to withstand forces that approach those prescribed by local building codes for completed structures. Ultimately, the builder is responsible for the stability of a structure under construction. Understanding the forces that act on a structure is key to planning for the temporary measures necessary to protect it from potential collapse.
 
Truss Failure in Structures Under Construction
Wooden trusses (or any truss for that matter) are designed to span between at least two bearing points while carrying a load. Trusses are designed to be economical and are very flimsy until they are installed and permanently braced by roof sheathing or other building components. It is not uncommon to have a 2-inch wide truss that spans more than 60 feet. Failure to provide temporary supports to brace the unstable trusses during erection can have catastrophic consequences.
 
Photograph 5 shows newly set roof trusses that failed (similar to dominoes) due to inadequate temporary bracing support. Not only will the trusses need to be scrapped, but the exterior walls will also need to be repaired or reconstructed because they were pulled inward by the failing trusses, as shown in Photograph 6.
 
When construction failures lead to project delays, damaged materials, and reduced profits, contractors often look to their insurance companies to recover their losses. In the absence of severe weather or other obvious causes, insurance claims professionals should give immediate consideration to the likelihood of inadequate temporary bracing as a root cause of, or at least a contributing factor to, the loss.
 
The Order of Construction
As illustrated above, inclusion of temporary bracing during construction is paramount, but of near equal importance is the order in which a building is constructed. Take, for example, the house under construction shown in Photograph 7. For reasons unknown, the builder decided to apply the exterior wall sheathing to the second floor first. This greatly increased the “sail area” on which wind could act, resulting in significantly increased temporary lateral loads. To compound matters, the builder installed bracing to resist forces in the east-west plane only, leaving the north-south plane unbraced.
 

As Murphy’s Law would have it, winds of approximately 40 mph originating from the south resulted in the complete collapse of the structure, as shown in Photograph 8. Had the second floor been unsheathed (reduced load), or had there been bracing to resist forces in the north-south plane (increased strength), the collapse could have been prevented.
 
Collapses that occur during construction can be difficult to investigate since the resulting debris is never neat and orderly. Failures often result in piles of broken lumber, masonry, bricks, mud, and other items that make determination of the exact cause difficult. However, knowledge of how a building is integrally constructed can help uncover the presence of any temporary bracing that might have been in place at the time of the collapse. A copy of the construction drawings should be consulted, and the assistance of a professional engineer could be warranted to determine the cause of the collapse.
 
E. Brian Webb, P.E., is an employee of Donan Engineering Co., Inc. Donan Engineering has successfully investigated thousands of construction-related failures throughout the Midwestern United States.

 



E. Brian Webb, P.E., is an employee of Donan Engineering Co., Inc. Donan Engineering has successfully investigated thousands of construction-related failures throughout the Midwestern United States.

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