Poultry House Structural Failure Analysis
Poultry houses are often considered to be simple, even low-tech structures comprised of a truss-roof system, a simple stick-frame wood wall with studs or posts and a masonry block wall or reinforced concrete foundation. While not complex to construct it is no less complex in how the structure carries loads (dead, wind, snow, or ice) from one part of the structure to the ground. The purpose of any building is to protect people or animals from the elements. A properly constructed structure must be able to carry the applied loads while protecting its occupants. Even though engineers have a significant amount of knowledge about structural design, sometimes light weight structures like poultry houses are ignored because of the low-tech concepts used in their construction. Engineers probably know as much or more about more complex structures than wood-frame buildings such as those used in poultry houses because of the emphasis that design codes place on loss of human life, rather than on contents.
Structures can be considered to be highly complex systems in which all parts of the system work together to accomplish a task. For a structure, this task is to transmit loads from one part of the structure to the other until they are transmitted to the ground. While a structure is naturally thought to be like a chain in which failure occurs through its weakest link, in a structure the weakest link may not have be the cause of the failure. For example, a poorly built foundation can create stresses in other parts of the building which can eventually cause failure in other parts of the structures.
While all structures are required to be built to code, few agricultural structures ever go through the scrutiny of that of other commercial structures. Some people believe that because they are building agricultural structures that they are exempt from rules established by code-writers.
Even if agricultural structures are not built to satisfy codes they should be built using good construction practices. Most often these construction practices were learned through years of doing. Poultry house designs have changed over the years, like most structures they have become more larger and wide-open with fewer structural supports (columns supporting the roofs) and wider spans to increase the capacity of the structure. While this has increased the overall bird capacity of the structure and made it easier for clean-out the structure has become more complex in the way it supports external loads. While some of these on-the-job tried and true building practices may have worked years ago for older style houses, more modern poultry houses have increased their spans while eliminating any interior supports. By increasing the span and decreasing vertical supports within the structure stresses within the structural members have increased and it has become even more important that the structure be designed and constructed using proper construction practices.
Many builders assume that if they use factory built trusses then they will not be responsible for any possible house failures. While these trusses may have been constructed under controlled conditions, factory built trusses are designed and built only to carry those loads that were assumed in their design. Often these truss builders only design the truss for downward loads and do not take into account all of the loads that are transmitted through the structure. Properly designed trusses must not only be designed to transmit vertical loads (which is what many truss manufacturers design them for) but also horizontal loads which are placed on them by external loads (winds or unbalanced snow/ice loads)
We normally think of most structures as supporting only downward loads, but based on the roof slope and roof height most poultry houses must support large upward (updraft) loads that can exceed the weight of the structure actually lifting it off its foundation. Therefore, proper anchorage of the structure is mandatory.
Because the profit margin in many wood-frame buildings like poultry houses is very small most builders of these type structures tend to cut-corners where ever possible. Much more attention is paid to the equipment housed within the structure than the structure itself. Modern poultry houses are not some chicken coop which houses only a few chickens, but rather a highly complex facility whose purpose is to provide comfort to its occupants. (in this case the chickens). This is a facility which is very expensive to construct, insure and maintain.
Purpose of the Project:
Obviously, no one wants any structure to fail. Not only is this cost prohibitive to replace, but it also disrupts the use of that structure. However, much can be learned from the failure of a structure i.e., what not to do again A primary focus of the study will be studying poultry houses where failures have occurred. During these visits the all aspects of the houses structure will be examined and documented. Engineering reports of poultry houses which have previously failed because of ice, wind or snow will also be studied. Poultry houses which have not failed but which are suspected of having sub-standard construction will also be studied. If needed we will identify and even potentially test items that we think are potential causes of failures within a structure to determine what construction practices should be prevented. Based on our findings a bulletin will be written on common causes of poultry house failures to help poultry producers, builders, poultry and insurance company personnel so that they can hopefully identify potential structural weakness before they become critical.