Notes on Performance Specifications and Aspects of Wind Design in Tall Buildings
The June 2020 issue of Structure magazine included an article titled, “Performance Based Wind Design, The Next Frontier” by several authors including Russell A. Larsen, P.E, S.E. Magnusson Klemencic Associates. The article caught my interest on more than one level.
One was the recognition of the continuing move in engineering toward performance-based specifications and away from traditional prescriptive specifications. The article discussed work associated with a “pre-standard” which is a document/tool to consolidate the thinking of what is expected to become the standard (or specification) for peer/professional review and comment.
There are two broad strategies for an engineering design specification: prescriptive specifications OR performance specifications. In a prescriptive code, the framework “says,” “Do it this way” (and resulting performance is implied).
Prescriptive codes, by and large, are better suited to singular and independent, less complex, more common, sub-systems or components. As complexity of an assembly or system increases, a prescriptive approach will tend to require overly conservative, often restrictive, more costly, and poorly optimized solutions.
A performance-based approach is explained by the statement that “first and foremost, the performance approach is the practice of thinking and working in terms of ends rather than means. It is concerned with what a system or product is required to do, and not with prescribing how it is to be constructed.”
Few engineered systems are designed and built from a simple recipe or prescription. While sometimes there may be select prescriptive paths or solutions, more commonly they are designed upon established and accepted performance criteria.
Current design engineering is increasingly moving from prescriptive requirements to performance-based specifications which:
Provide a target and baseline metric for all potential solutions.
Promote flexibility and innovation in contrast to prescriptive specifications.
Support optimized, cost-effective solutions in contrast to prescriptive specifications.
Provide incentives to the clever, innovative designer and builder who can develop the most cost-effective solutions to meet the overall performance requirements. Another point of interest for me was noticing that in Figure 2, reproduced below, the specification makes a distinction between “Occupant Comfort Performance Objectives” (e.g. acceleration) and “Operational Performance Objectives” (e.g. Drift).
This caught my eye since I had previously assumed that building drift (lateral deflection) criteria related in large part to occupant comfort; that the acceleration parameters were ‘baked into’ the drift assessment. Since I have but peripheral experience in high rise design, I figured I’d ask Mr. Larsen about drift checks and this was his response.
Drift checks have historically fallen into two camps. The first is to limit non-structural damage to interior walls (partitions) and exterior envelope (cladding). Both systems tend to have joints that allow some degree of independent horizontal movement between successive floor diaphragms – but too much movement will overcome the allowances and cause distress, cracking, leaking of enclosure seals, etc.
The second camp of drift checks consider the overall movement of the roof level relative to the earth. The origin of these checks is multifaceted. Overall movement has been used as a proxy to limit building acceleration and as a proxy to limit P-Delta force amplification. On 500ft + towers there can be elevator shaft verticality concerns that start to crop up for shafts servicing the full building height.
Lastly, there is also concern of drift over a property line – although with wind the roof movements are small enough that this last concern is rarely of practical concern. Given that most of these roof drift checks can be checked in more accurate ways – many folks do not check roof drift but instead evaluate the specific building movement concerns directly.
It is interesting to note that Mr. Larsen does allude to the fact that drift has been a proxy limit on acceleration and therefore, in fact, related to occupant comfort. Mr. Larsen’s answer is further enlightening as well as to concerns and design aspects for tall building design, in this case with respect to wind loading.