Post Installation Tools

          Embedded post foundations are largely responsibility for the cost effectiveness of the post-frame building system.  However, like other foundation systems, installation of embedded post foundations has its unique challenges.  To help overcome some of these difficulties, prototypes of three different tools: a posthole installation shield, a posthole-bottom leveler, and a footing transport and placement cart were designed by Bohnhoff and tested   in 2003.  Research in 2004 was concentrated on refinement of the posthole-bottom leveler.  Research in 2005 resulted in a new version of the footing transport and placement cart and modifications to the posthole installation shield.

          The posthole-bottom leveler is a simple, inexpensive device used to level soil at the base of a hole prior to tamping and placement of a precast concrete footing (a.k.a. cookie).  The use of such a device becomes increasingly important as footing diameter increases.  Without such a device it is difficult to ensure that the base of a hole is not tilted or uneven.  A tilted base results in a tilted footing, and consequently, a significantly reduced area of contact between the footing and post.  Uneven terrain (i.e., high and low points) results in more variant footing stresses and increases the likelihood of future foundation settlement. It is recommended that such a tool be present on any jobsite where precast concrete footings are being placed.

          Sloughing of posthole sides is associated with drilling in noncohesive soils (e.g., sands and gravels with low clay contents) that are of low density, poorly compacted, very wet because of a recent rain or because they are poorly drained, or saturated because they are near or below the water table.  Vibrations that occur when hitting rocks and/or tree roots generally increase the likelihood of posthole collapse. When a posthole side sloughs, the diameter at the top of the posthole becomes larger.  This makes in more difficult to throw material away from the posthole by spinning the auger after it has been brought to the surface.  It also becomes more difficult for workers to prepare the base of the posthole for a footing, to place the footing, and to properly position, brace and backfill the post, even when planking is used to bridge the large opening.  The posthole installation shield is used to prevent posthole sides from collapsing during hole drilling and anytime prior to footing and post installation. It is recommended that all major post-frame companies stock one or more post-hole installation shields for use on jobsites where conditions make it difficult to maintain proper posthole geometry.

          Round, precast footings (a. k. a. cookies), especially those with diameters less than 17 inches, are frequently dropped into postholes.  Unfortunately, regardless of its size, the likelihood of a “dropped” footing landing properly in a hole is extremely remote. When one edge of a footing hits first, the result is a localized soil bearing failure – a failure involving the movement and “loosening up” of a good portion of the surrounding soil.  To avoid damage to the footing base, some contractors use special tongs to lower smaller footing.  Others have wrapped steel banding around the footing, and then removed it after the footing was in place.  Neither of these methods works very well with larger/heavier footings. Footings that are too large for one person to lift up are typically handled with a rough terrain forklift, skid steer loader, front-end loader tractor or similar piece of equipment.  This equipment is not only used to move footings around on the jobsite, but they are also frequently used to lower larger footing into postholes.  In addition to the equipment operator, another worker is typically required to attach/detach footings to/from the equipment and to guide the footings so they do not hit the posthole sidewalls when being lowered.  In other words, it requires two workers and a larger piece of equipment to properly install large precast footings. The footing transport and placement cart enables a single person to transport and place precast concrete footings as large as 350 lbm and 34 inches in diameter, without the use of self-propelled equipment.  Any company that stocks and/or routinely uses footings weighing in excess of 100 lbm should not be without a footing transport and placement cart.

          There are no patents protecting the three post installation tools described above.  They are provided for public use and to stimulate development of similar tools.  In return, we simply ask that any individual or company that uses the designs, or in anyway profits from the designs, help support through donation, the post-frame building research effort at the University of Wisconsin-Madison.  University research is a non-profit venture that can only be sustained via constant support from those who it benefits.

Lateral Load Distribution in a Metal-Clad Wood-Frame Building

          This project involved the construction and testing of a full-scale, metal-clad, post-frame building with the goal of gaining a better understanding of the complex distribution of load in this popular agricultural building system.  The building was erected, instrumentation was installed, and initial tests conducted in 2001.  Research in 2002 was dedicated entirely to testing and analysis of data.  Research in 2003 and 2004consisted of data analysis and computer modeling.

          The test building is 40- by 200-ft with trusses on 10 foot centers.  Trusses are pin-connected to posts, which in turn are pin-connected to concrete piers.  Centered under each interior truss is a hydraulic frame loader (HFL) that is attached by rods to each end of the truss.  An HFL can be set to operate in one of four modes: (1) north load, (2) south load, (3) lock, or (4) float.  Although there are no HFLs under the endwall trusses, the endwalls trusses can be either locked-in-place or allowed to float during a test.

          Using the versatility of the HFLs, 22 different loading were applied to each of 10 different building configurations.  Different building configurations were obtained by adding and removing: the ridge, chord reinforcing hardware, roof-to-sidewall fasteners, roof panel stitch screws, sidewall steel, and eave trim.  This experimental design was replicated twice for a total of 440 building tests in 2002.  During each test signals from 225 different transducers were recorded every 4.7 seconds.  With a test time of at least 3 minutes each loading usually generated at least 10000 data points.  This was obviously an unwieldy amount of data to analyze without significant data reduction.

          Throughout early 2003, research effort was primarily dedicated to data reduction.  This was accomplished by first calculating an average horizontal frame force for each 4.7 second scan.  The second step in data reduction was to linearly regress the output from each transducer on the average horizontal frame load values.  After these regression analyses, the data file for each load case was reduced to 204 values.  

          During the latter part 2003 and throughout 2004, research effort was dedicated to the modeling of full-scale building behavior.  In the end, a model with three displacement degrees of freedom (DOF) per building frame was selected.  These include a displacement parallel to the frame, and two displacements perpendicular to the frame – one at each side wall.  The 3-DOF/frame model contains four different simple spring elements (a.k.a. truss elements): a frame element, roof cladding element, chord element and wall cladding element. 

          The primary task during the modeling phase of the study was to determine axial stiffness properties for each of the four elements, or more specifically, to find element stiffness values that were a function of building configuration, and thus could be used to accurately predict full-scale building behavior regardless of building configuration or distribution of applied loads.  This turned out to be a formidable task, requiring thousands of computer simulations because of the interdependency of wall cladding, roof cladding and chord force element properties.  Nevertheless, a set of element properties were selected that do a very good job of predicting building displacements as well as the in-plane bending moment and shear forces between building bays.  The results of this modeling were presented at the 2004 ASAE/CSAE International Meeting in Ottawa, Ontario. 

          The 3-DOF/frame model is embodied in computer program DAFI3 (Diaphragm And Frame Interaction 3 dof/frame).  Once a pre- and post-processor are added to the program, it will be made available to the general public. 

          As part of this ongoing study, a closer assessment of chord forces will be undertaken.   This should ultimately lead to improvements in current standards for diaphragm design.

Accuracy of Corrugated Metal Panel and Trim Installation

          In January of 1999, the National Frame Builders Association (NFBA) published Accepted Practices for Post-Frame Building Construction: Framing Tolerances.  In preparing this document, UW-Madison researchers conducted an extensive field investigation to determine just how accurately post-frame building frames are constructed. 

          In March of 1999, the NFBA Technical and Research (T&R) Committee proposed that NFBA pursue the development of a second construction tolerances document; one that covered metal trim and corrugated panel installation.  In October of 2002, the committee identified items for inclusion in the document. At this same meeting, it was agreed that University of Wisconsin-Madison researchers would conduct the field research required for document development.

          Actual data collection commenced during the summer of 2003 and was completed in early June, 2004.  A total of 52 buildings were surveyed.  Items investigated included:  (1) panel plumbness; (2) roof-to-wall panel rib alignment; (3) corner trim squareness; (4) corner trim connection to wall panel; (5) wainscot panel alignment; (6) roof panel offsets at eaves; (7) variations in roof panel overhang; (8) misalignment of wall panel ends (e.g., saw-tooth effect); (9) fit at openings; (10) dings; (11) scratches and scrapes; (12) crimps/kinks; (13) horizontal fastener alignment; (14) fastener drive depth; (15) fastener driving angle; (16) fasteners missing framing; and (17) irregular fastener patterns.

          Data from field investigations were tabulated and analyzed, and then summarized in a technical paper presented at the 2004 ASAE/CSAE International Meeting in Ottawa, Ontario.  This information was subsequently used by Bohnhoff to draft the first version of a document entitled Accepted Practices for Post-Frame Building Construction: Metal Panel and Trim Installation Tolerances.  This document went through five revisions based on feedback from the NFBA T&R Committee.  The final version was assembled by Bohnhoff in August of 2005, at which time it became an official NFBA document.

Concrete Pier-to-Wood Post Connector Design

          A greater number of post-frame buildings are being constructed using precast or cast-in-place concrete piers.  This increased interest in concrete piers can be attributed to following seven, largely-interrelated reasons.

1. Durability. End users have more confidence in the long-term durability of a concrete foundation than they do in a preservative-treated wood foundation. This is largely due to the poor performance of many under-treated solid–sawn posts.  It is important to note that to date, they are no documented failures of mechanically-laminated wood posts that have been properly treated for ground contact with CCA (chromated copper arsenate) 

2. Reduced availability and/or higher cost of CCA-treated lumber.  Effective December 31, 2003, no wood treater or manufacturer could treat wood with CCA for most residential uses.  While posts for agricultural and commercial buildings can still be CCA-treated, the partial ban on CCA significantly reduces the amount of wood that is CCA-treated, making it more difficult and expensive to obtain. 

3. Corrosiveness of CCA alternatives. Alternative CCA treatments include ACQ (Alkaline Copper Quat) and ACC (Acid Copper Chromate).  Like CCA, these alternative treatments rely on copper toxicity for effective protection from decay organisms.  Unlike CCA, they are not time-tested and tend to leach more copper into the environment. To combat leaching, more copper is used in initial wood treatment.  This higher copper concentration results in yet more loss of copper, and increased galvanic corrosion when metals less noble than copper (e.g., magnesium, zinc, iron, steel, aluminum) are driven into or brought into direct surface contact with the treated wood.  Excessive corrosion of metal fasteners is of primary concern to engineers concerned about structural integrity and hence safety of building occupants.  

4. Reduced use of preservative treated lumber.  Where possible, engineers try to eliminate preservative-treated lumber because (1) it costs more than non-treated lumber, (2) it generally requires use of more expensive, less-corrosive fasteners, and (3) preservative wood treatments are pesticides which can make eventual disposal of preservative-treated wood problematic. The cost of preservative treatment alone will drive engineers to use posts featuring treated wood spliced to untreated wood in an effort to save money for posts not requiring above ground treatment.  Use of concrete piers in this situation eliminates the treated wood altogether, as well as the additional assembly costs associated with joining treated to untreated dimension lumber.

5. Lumber length.  Lumber becomes increasingly expensive (on a board foot basis) in longer lengths.  Additionally, dimension lumber is not readily available in lengths longer than 20 feet.  When concrete piers are used, the overall length of the wood post is generally shortened by four to seven feet.  This means engineers are using shorter, less expensive lumber to obtain the same building heights, and can also build structures with 20 foot eave heights using unspliced sidewall posts.

6. Ease of building disassembly. Agricultural and commercial buildings have a relatively short functional design life.  It is therefore beneficial to be able to easily disassembly the building components for use in a more functional structure.  This is much easier to accomplish when wood posts are attached to concrete piers.

7. Recycling. If history teaches us anything, it is that reuse of lumber treated with a particular preservative is largely dictated by restrictions placed on use of the preservative after it has been in use for several years.  For example, it is not possible to reuse lumber treated with pentachlorophenol in buildings because of restrictions placed on its use in 1984 (pentachlorophenol is no longer available to the general public, although it is still used industrially as a wood preservative for utility poles, railroad ties, and wharf pilings). Some researchers have suggested that the development of good organic-based preservative wood treatments may result in restricted use of all heavy-metal based preservatives, thus making products treated with CCA, ACQ and ACC of little value several years from now.  If this is the case, anything that can be done to replace preservative-treated wood with untreated wood may increase future value of a building.

Despite  the increased use of concrete piers in post-frame building construction, and the number of post-frame buildings that have been erected on concrete frost walls and grade beam foundations, very little attention has been focused on concrete-to-wood post connections. Common steel brackets used by the post-frame industry to attach wood posts to cast-in-place concrete are treated as pin connections in design because of the lack of bending strength and stiffness of (1) the steel bracket-to-concrete connection, (2) the steel bracket-to-wood post connection, and/or (3) the steel bracket itself .  With concrete-to-wood post connections that lack bending strength and stiffness, the building designer must rely entirely upon diaphragm action and/or on rigid column-to-truss connections to handle horizontal forces applied to the structure.

Development of a concrete-to-wood post connection with significant bending strength and stiffness will help concrete piers with attached wood posts behave like unjointed beams.  This would enable design engineers to either reduce overall post size, or rely less on diaphragm action or rigid frame design for building stability.  This, in turn, would make concrete piers more attractive to builders, which would ultimately decrease dependence on preservative-treated lumber.