A problem good Structural Engineering could solve…


(In one of the many discussions following the rash of building collapses experienced throughout the Midwest this winter, I received a copy of the email below from Dr. David Bohnhoff, PhD, P.E., Emeritus Professor at the University of Wisconsin – Madison. I reprint it here with the other names removed and with Dr. Bohnhoff’s permission in hopes that his message will reach a wider audience)

“I’m responding to your email and copying a few others on it as I feel the need to get some talking points out in the general public.

For starters the State of WI Uniform Dwelling Code (SPS Chapters 320-325) has absolutely nothing to do with agricultural buildings.  It is a PRESCRIPTIVE code that is only applicable to small buildings.  This would be buildings, for example, whose clearspans seldom exceed 20 or 30 feet.

Larger buildings are structurally engineered in accordance with the governing commercial building code.  In the State of WI, this is a slightly modified version of the International Building Code (IBC) and is referred to as the WI Commercial Building Code (SPS Chapters 361-366).  From a structural design perspective, the IBC is a PERFORMANCE code and it contains verbiage specific to agricultural buildings.  For what could be argued as antiquated (historic) reasoning (more on this later), the State of WI exempts (via SPS 361.02(3)(e))) farm buildings from all provisions of the WI Commercial Building Code.

For reasons (sometimes shear ignorance) there are a number of builders that believe you can build large buildings in accordance with a PRESCRIPTIVE code for small buildings.  Prescriptive codes are codes that PRESCRIBE exactly what size/grade/shape components to use at various locations and how to connect them.  Prescriptive codes are very limited in their overall applicability.  Prescriptive codes “get by with” using simple, uniformly-distributed loads (e.g., a balanced snow load) to determine component size.  Structural engineers are seldom required when prescriptive codes are in play (and that’s one of the main reasons they exist).

When buildings get large, structural engineering gets more complex.  Most loads are far from being nice and uniform.  Wind and snow patterns are highly variant and quite complex.  When you add in parapets, cupolas, ridge vents, asymmetric roofs, steep roofs, intersecting roofs and associated valleys, overshot ridges, and sudden changes in roof height, AND you combine these with snow that can be blown in any direction, THEN (simply put) you have pages and pages of calculations you better perform if you want both an efficient and safe building.  Calculation of loads and load combinations is the first step in the structural design of a building, and not only are these loads dependent on the size and shape of the building you are designing, but they are also dependent on adjacent structures and terrain.  In many areas of the county, seismic loads are a big part of the equation, and obviously add significantly to the work involved in structural design.

Once the engineer has his loads, he/she begins the process of sizing components AND CONNECTIONS to resist these loads.  To design an efficient structure (in order to keep cost down), the engineer is constantly figuring out (1) ways in which secondary structural components and cladding can best be used to reduce the size of primarily structural components, and (2) ways that components can be connected to reduce component and connection stresses.  This takes both knowledge and experience.  A couple hallmarks of buildings that lack structural engineering are primary framing components that have little or no resistance to buckling, and mechanical connections that have little or no strength because fasteners have been inappropriately sized, spaced and/or located (with respect wood connections, fasteners often induce high wood stresses because they are too close together, too close to the end of a component, too close to the edge of a member, or otherwise used in a manner that induces high tension stress perpendicular-to-grain).

If you understand the above, then you know that when someone tries to sell a farmer a building “designed to withstand a BALANCED snowload of XXX psf” that farmer should slam the door in the salesperson’s face.  Obviously, that salesperson and the company he/she represents are not selling structurally engineered buildings.  More often than not, they are selling a building that includes a truss that has been sized using methodology only appropriate for a small, residential building, and it is quite likely that not a single load calculation has been performed, and thus not a single component or connection has been properly sized/detailed for the loads to which it will likely be subjected.  When you sell a structurally engineered building, you talk about the performance codes and standards that were used in its design.  You talk about the code-specified GROUND snow loads in the area that were used IN PART to determine the complex load combinations required to properly engineer the building.

The fact that some companies are selling large agricultural buildings that are not fully engineered is shear lunacy and highly unethical if not criminal.  When these same builders blame the failure of their buildings on a rare heavy snowfall, instead of their lack of providing a structurally engineered building, they are being deceitful/fraudulent.  They also must take farmers and the rest of the general public as idiots.  How else can you convince someone that a rare heavy snowfall is the culprit when the percentage of agricultural building failures due to a given snowfall is magnitudes greater than it is for other commercial and residential buildings in the same area.  Along these same lines, how misleading is it to state something like “the snow loads exceeded those we used in design” when you never structurally engineered the building in the first place?

Over the years I have visited and read about an incredible number of agricultural building failures.  I have seen piles of dead animals, and yet the problem with non engineered buildings has only gotten worse.  Why is this you may ask yourself?  Why does it happen in the first place?  Why hasn’t the government did something about it?  Why hasn’t the industry did something about it?  Why don’t the insurance companies care?  Where are the lawyers in all of this?  These are all great questions with reasonable answers.

First, why has the number of agricultural building failures increased, especially at a time when the number of farming operations has decreased?  Simply stated, larger and larger non-engineered buildings are being constructed.  Unfortunately, there is a double whammy that comes into play here.  As previously explained, larger buildings get hit with more complex loads, and if a building is not engineered to handle these loads, the probability of failure increases.  Second, when you double the size of a building, you double the number of components in the building.  In the case of a non-engineered building, you double the number of undersized components and/or connections.  Thus a building that is twice as large has double the probability of a localized failure.  The problem is that this localized failure can bring down a large portion of the structure if you are not careful.  For this reason it is fundamentally important to incorporate mechanisms into large buildings that limit the extent of a progressive collapse (something that is absolutely not done in a non-engineered structure).  

So as absolutely nutty as it is to put up a large building without structurally engineering it, why is it done?  The answer is simple, there is no law requiring structural engineering due to the exemption provided in SPS 361.02(3)(e) AND builders who engage in the practice of selling and erecting non-engineered buildings can undercut the sales of those who don’t.  The problem is, those who erect non-engineered buildings are putting people and animals in danger (extreme danger in many cases), and are sullying the reputation of the entire industry.  Almost without exception, those erecting non-engineered buildings are small, local builders who (1) do not have a national reputation to withhold, and (2) don’t have deep pockets. 

Perhaps only people that engineer buildings understand and appreciate the true dangers and hence insanity of erecting (and then occupying) a structure of absolutely unknown strength.  To structural engineers involved in agricultural building design, NOT following the structural requirements of the governing commercial building code is crazy given the fact that the code sets MIMIMUM criteria.  If you aren’t going to engineer a building in accordance with loads considered the MININUM for your project, then pray tell, what loads are you going to use????

The IBC, which was adopted (with modification) as the commercial building code in Wisconsin, is a code that is as applicable to agricultural buildings as it is to other commercial buildings.  The WI administration code exemption that allows for the construction of non-structurally engineered farm buildings is outdated.  In many cases, code exemptions for farm buildings are as old as the code themselves.  The first building codes were largely fire codes (much like today’s codes) that were put in place to protect loss of life and property from large conflagrations (e.g. fires that consumed entire villages in some cases).  Since farm buildings were small and located in rural areas where they were isolated from other buildings, there was little concern regarding loss of life and adjacent property when they did burn (which they often did) and hence they were exempted from building codes.  As codes have changed so have farm building exemptions.  While farm buildings are still largely exempt from fire, ventilation and energy codes, they seldom are exempt from electrical and plumbing codes, and some jurisdictions no longer exempt them from structural codes.  The latter recognizes that large farm buildings need to be structurally engineered.  In some jurisdictions (e.g. Arkansas) farm buildings must be designed and constructed in accordance with the governing commercial code, but there is no enforcement (i.e., there is no required plan submittal and no required on-site inspection).

The confusion surrounding the structural design of farm buildings has made it virtually impossible for insurance companies to offer better rates for buildings that are structurally engineered in accordance with a specific code, then for ones that have not been structurally engineered.  To this end, farmers that purchase properly engineered buildings are not getting the break due them, in fact, the more large, non-engineered buildings erected, the higher their rates become.

Builders who sell and erect non-engineered buildings (typically at the expense of reputable companies) have no incentive to change their practice.  Given that insurance companies continue to insure the buildings they erect, why change?  As soon as one of their buildings fails, they are right back in the farmer’s yard telling the farmer not to worry as they will take care of him/her like they always have.  They blame the failure on a rare heavy snowfall (or on the truss manufacturer or some other supplier), and then they put up the exact same non-engineered building.  It’s a double win for these builders (two buildings and two pay days).  So why should they change their practice?  Your answer may be “so they don’t get sued”.  To this I ask, when was the last time a hard-working, independent dairy farmer (not a horse farmer) sued a hard-working local builder?  Given that they could go to the same church, have friends in common, or even be related, you can pretty much guess the answer.

Make absolutely no mistake about it, the rash of agricultural building failures is virtually entirely due to the construction of buildings that are not structurally engineered by builders who in many cases could care less.  They are not among the farmers, the reputable builders, the component supplies (who often get blamed for the failures), or the insurance companies who would all benefit by requiring large farm buildings to be structurally engineered.

David R Bohnhoff, Ph.D., P.E.

Emeritus Professor

Biological Systems Engineering Department

460 Henry Mall, Madison WI 53706″

Thank you, Dr. Bohnhoff!