UPDATE: 7/1: Article corrected to state cantilever system was used, not an outrigger system as was originally stated.
UDATE 6/26: Authorities to launch criminal probe into Berkeley balcony collapse. See full story in San Jose Mercury News.
UPDATE 6/26: Collapsed Berkeley balcony had significant dry rot, says City of Berkeley in report. See full story in Berkeleyside.com
Author’s note: On behalf of the structural engineering community, I offer my condolences to the families who have lost their loved ones due to this tragedy. Amongst all the questions and concerns, it is clear to the general public that this should not have happened. The engineering community is therefore motivated to learn from this experience, and to prevent this unfortunate incident from reoccurring.
With due respect for the ongoing investigation by the City of Berkeley, I wish to highlight a few general observations with regard to comments on the tragic balcony collapse in Berkeley, California, that were posted on ENG-TIPS.com and to suggest an appropriate response. Being that I do not have first-hand access to pertinent information, such as the set of construction documents approved by the City of Berkeley for the construction of Library Gardens, the apartment complex where the collapse occurred, my observations are limited to information made available at the time this article was written. Specifically, I am relying upon photographs posted on ENG-TIPS.com .
First, allow me to address the issue of quality control. Ideally, quality control should be evident during the process of detailing, construction and maintenance of any structure. While quality control may well be present during the detailing process, there generally tends to be less quality control on the job site, with periodic on-site inspections, at best, and virtually no quality control in terms of maintenance. By and large, the structural elements are neglected during their serviceable life. As the saying goes, “out of sight, out of mind.”
That being said, the building code does give credence to general construction practices and encourages such practices to continue. This is particularly evident with regard to wood construction. General construction practice has proved the test of time. It is generally understood that structural elements within a structure do not need to be routinely inspected for their performance during their serviceable life if there is no cause for concern. This understanding, however, presents a catch-22: How do we know that there is no cause for concern unless routine inspections are performed? We are suddenly jolted into awareness of a problem once a tragedy occurs.
The Need for Inspections
I would therefore like to propose that a change be implemented to the building code and enforced by the local jurisdiction. A simple routine inspection — perhaps on an annual basis — should be implemented for the balconies on any structure, and particularly for those balconies that are cantilevered. Of greater concern of course would be those structures that are more accessible to the general public as opposed to private residences.
Typically, when such inspections are specified, they are conducted as a spot check involving a random selection of elements for inspection. Inspectors assigned to this task can be trained to the specific task of spotting problems. These inspections may address a variety of related concerns. However, they should be focused upon the general structural integrity of the balconies. These are primarily visual observations that rely upon what is visibly evident without penetration of the building envelope. At times, non-destructive means are employed to penetrate the surfaces — x-ray, radar, etc. — that provide a much better and detailed observation of the present condition of the members in question.
Water Damage a Strong Possibility
The photograph above reveals a few things an inspector should look for. Without implying the exact cause of the damage, we can confidently state that the cause for this failure does involve water infiltration. The ongoing investigation being conducted by the City of Berkeley, involving an in-depth forensic analysis, will be required to determine just how this infiltration occurred. Therefore, I will not offer opinion in that regard. My observations, rather, are general in nature.
The potential for water infiltration is — and has been — a constant concern for those of us in the construction industry. As we know, water often presents itself in three forms: ice, liquid and vapor. In all its forms, water presents a challenge to wood construction. Generally speaking, and for a variety of reasons, wood members in the presence of moisture tend to deteriorate. This deterioration is a relatively slow process that typically takes several years — several wet cycles — to have a significant impact upon the structural integrity of the wood member. A great deal has been written about this subject and there is no need to go into detail about it here. Suffice it to say that the building code is well versed on this subject.
The common practice is to provide a moisture barrier to prevent water infiltration. Product manufacturers have developed and marketed many viable options to address this problem. The key to developing the building envelope is employing such manufactured products in such a manner that they work in concert to create an impenetrable barrier to moisture. The inspector should therefore be looking for evidence that the building envelope had been compromised.
The inspector should be familiar with various wood preservative treatments that are commonly employed. These treatments are most often employed when the structural wood members are exposed to the elements: infestation, sun, wind and rain. Wood preservative treatments are generally employed to prevent, or slow, the eventual progress of deterioration. There are also various types of wood often employed with exposed wood construction, such as cedar, redwood and a type of wood commonly referred to as ironwood. These are wood species that are naturally resistant to deterioration due to infestation.
Outrigger versus Cantilever
The inspector should also be aware of the type of structural design employed to construct the balconies. Extending the length of a beam by attaching another beam along each side is a common method of providing balcony support. Another method is to extend the balcony joists themselves so that each joist carries its respective load directly back to the main structure. The first method thus described is often referred to as the “outrigger” method because it resembles an outrigger canoe. This method provides support in a manner that is less redundant than the other method described. In terms of life safety — which is the lifeblood of the building code — redundancy is a critical issue. Generally speaking, a less redundant structural system poses greater risk.
An inspection of the drawings of the building on ENG-TIPS.com show that this balcony used a cantilever system.
Wood Not Always Inappropriate
That being said, there should be no reason to consider an outrigger system to be inappropriate — or even the general use of wood in balcony construction to be inappropriate — if properly detailed and constructed. Furthermore, I do not wish to imply that faulty detailing or construction in this case caused damage. I merely wish to point out that the failure of one outrigger beam would cause collapse of the entire balcony whereas failure of one cantilevered joist would not necessarily cause collapse of the entire balcony.
Armed with this knowledge, the inspector should pay particular attention to the present condition of the outrigger beams that support the balconies. If there is any doubt as to the possibility of water infiltration to these outrigger beams, further investigation, by non-destructive means (as discussed earlier), should then be conducted. The inspector must either be completely satisfied that the member has not been compromised, or he or she must require replacement of the member.
Indeed, the structural engineering community should have had the foresight to address this issue before lives had been lost. However, with this issue laid before us, we cannot ignore its importance. My hope is that appropriate action is taken in response to this tragedy.
The Author
Richard Flower, P.E., is a LEED Green Associate with 27 years of experience as a civil engineer focused on structural engineering. He is currently a senior structural engineer with Complere Engineering. He is well versed in the structural engineering of wood, concrete, steel and masonry structures. Flower was an independent structural consultant for residential construction in Los Angeles and Ventura counties, and was involved with the investigation of structural damage as a result of the 1994 Northridge earthquake in Los Angeles. He has a BS in architectural engineering from California Polytechnic State University.