Structural Aspects of Renovation
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When preparing for a building or barn renovation project, you need to consider that the changes being made could influence the structural integrity of one or several buildings.
When deciding to repair or renovate a farm building, you could inadvertently change the original design of the structure. As well, you may influence the design of other farmstead buildings. When adding a new building to an existing group of farm buildings, you may affect the patterns of snow accumulation. If a higher building or tree windbreak blocks the wind from sweeping across a roof, those affected roof areas will need to be designed for a higher snow load. Often these changes are considered for the new roof but ignored for existing roofs that are affected. In extreme cases, collapse of existing roofs could result.
Renovation or construction of one building may necessitate structural changes in other farmstead buildings to ensure continued integrity as a result of additional loading. The loading changed by adding a new silo, a tree windbreak or a new larger overshadowing building must be considered.
When renovation takes place, a change in occupancy sometimes results. If you renovate a farm building to accommodate, for example, a retail area for selling fruit and vegetables to the public, the building may no longer be considered a "low human occupancy" structure. The Ontario Building Code requires that a building be designed to withstand higher loading in a "high human occupancy" situation. Since roof structures must be constructed according to the Ontario Building Code, an engineer should be approached for a renovation plan.
When adding a lean-to or an addition to an older building, consider supporting the new structure so that it does not rely on the older building for bracing or support. Assuming that the older building can carry large additional loading is common. This is not always the case.
Where possible, instead of hanging a lean-to on the older building, place a row of posts and footings as close to the existing buildings as possible. Include sufficient wind bracing in the new structure to support itself. The attachment to the older building should be non-structural. Figure 1 shows a new structure that provides its own bracing and support system. This is the preferred method of construction. The older lean-to that it replaces relies on the main structure for support.
Figure 1. In this renovation, a small lean-to has been replaced by a structure that does not rely on the older building for support.
Large doorways are commonly added to existing buildings to allow for large equipment access. Doubling the width of a door in a load bearing wall will require that the supporting lintel have at least four times the strength of the beam that it replaces. Using a steel beam or other specialized materials will often be necessary. If the wall was originally braced to posts that have been eliminated, additional wall bracing will be necessary to offset that loss. Often the post size on each side of a large doorway will need to be increased. As well, footings on each side of the door may need to be larger than the original post footings.
Renovating an older building to change stall location, spacing, etc. is very common. Care must be taken when moving or eliminating support posts to re-establish the integrity of the support system. This may mean eliminating older timber beams and replacing them with steel beams. Ordinarily, you will need to pour concrete footings at the new post locations. If you were to place support posts on an existing concrete floor, they may punch a hole through the concrete and will not provide the necessary support. Another concern that can be overlooked when relocating posts and beams in a barn floor is that roof support posts may be resting on the existing beams. Care should be taken to ensure that the roof support structure is not compromised by the renovation.
Sometimes, farmers decide to remove the barn floor in a two-storey building to allow more headroom for a workshop, machinery storage, seed cleaning operation, etc. The barn floor provides stability for the sidewalls of the building. As well, the floor assists in transferring wind loads to the corners of the building. When removed, the stiffening effect of the floor must be replaced. Since each circumstance will be different, engineering assistance is necessary to ensure that the integrity of the building is maintained.
Another renovation often occurs where livestock production on a second storey of a two-storey barn is desired. Pouring concrete onto an existing floor is quite possible. If the floor originally could carry a mow full of hay or straw, it is likely that it can support a slab of concrete, 100 150 mm (4 6 inches) in thickness. A 150 mm (6 inch) thickness of concrete weighs approximately 3.6 kPa (75 pounds per square foot). By contrast, 20 feet of hay or straw can weigh 7.7 kPa (160 pounds per square foot) or more. The concern occurs if the floor is to be used for hay or straw storage at some point in the future. The additional 3.6 kPa (75 pounds per square foot) that the floor weighs will reduce the overall carrying capacity of the mow floor.
When replacing a section of wall or a beam in a two-storey barn, jacks must be used to temporarily carry the weight of part of the structure. To do this, temporary beams must be located as close as practical to the beam that will be moved. Temporary beams are commonly needed on both sides of the existing beam. Jacks can be used either at the top or base of temporary posts. Care should be taken to place the posts or jacks on several layers of planking. This will distribute the weight carried by the posts so that they will not punch through the concrete floor or sink into a soil base when lifting begins. Temporary bracing should be used when lifting the structure. If instability occurs at a time when the permanent support system has been removed, structural collapse could result.
Increasingly, builders are using steel roofing and wall sheathing to supply wind bracing for the entire building. This is known as diaphragm bracing. Several special fastening details are necessary for this to take place. In the future, if you replace this steel with new steel without consideration for these details, the structure could have little or no bracing remaining.
Removal of "nuisance" bracing in farm buildings is common (by accident or not). A brace could be accidentally broken off with a loader or deliberately removed if the bracing happens to damage the cab of a combine. In hay storage buildings, machinery sheds, etc., if the knee bracing is in the way, it often disappears! In one instance, a loaded corn crib was starting to lean drastically to one side. Upon further examination, it was noted that the owner had removed a large portion of the original X bracing to make unloading of the corn easier. Wind and sway bracing is an important part of a building. If one type of bracing is removed, it is important that the bracing be replaced by another means.
Truss members should never be cut or eliminated unless specified by an engineer. These members often carry very large forces. Adding hanging laying cages to a roof truss is quite common. This should only be done if an engineer has approved the roof design or the modifications required to do so.
If the reconstruction requires new footings and foundations, care must be taken to pour these on undisturbed soil. The possibility of large settlement is always present if a supporting structure is placed on fill.
When adding gutters to a structure, it is common to excavate along the existing foundation wall. Care should be taken not to undermine the foundation. A structural collapse could result. This is especially true where a silo or other heavily loaded structure is located outside and very close to the wall. The potential for instability is often increased where there is a high water table or poorly drained soil conditions.
The Ontario Building Code allows a person to move a structure or to rebuild it without redesign if several conditions are met. Firstly, the structure must be rebuilt in the same form. Secondly, it must not be moved to a location that will differ in snow loads, etc. However, this may be the case if it is moved to a new location on the farmstead. Other buildings may influence the accumulation of snow on its roof. As well, wind forces may be increased if the building is not in a sheltered location. The only consideration that the Ontario Building Code addresses is the new foundation. Often, concrete stave silos are moved to a new location. If the silo is reconstructed in the same configuration, the building official may only be interested in the design of the new footing.
Post-frame buildings will eventually need to have the in-ground part of the post replaced. Commonly, the existing wood below ground level will be removed and a new timber or concrete support will replace it. In post-frame building design, it is assumed that the post will be continuous at ground level. The fixity of that point will help the building to resist wind loads. It is difficult to replace that fixity when either a new piece of timber or a concrete pier is attached to the old post. Additional wind bracing could be necessary to offset this weakness. The completed joint should ensure that the building is well anchored against wind uplift. This type of renovation should be carried out under the supervision of a structural engineer.
When roof trusses are located in corrosive atmospheres, the metal gusset plates or nails may eventually deteriorate. Sometimes, this can cause the roof to collapse or to be significantly reduced in strength. Deterioration of roof truss gusset plates is discussed in OMAFRA Factsheet, Corrosion of Roof Truss Gusset Plates (Order No. 10-071).
Corrosion of reinforcing bars in silos and in concrete floors and slats covering stored manure is an issue that is currently being researched. As the concrete deflects or is stretched under load, tiny cracks can develop. These cracks allow corrosive liquids and gases (including water and oxygen) to start to corrode the steel reinforcing bars.
Farmers commonly drive tractors and skid-steer loaders on reinforced concrete floors and slats. The underside of these slatted and solid floor areas should be inspected periodically. Care should be taken not to enter a space where confined toxic gases could be present without appropriate breathing apparatus and a safety line. If corrosion is taking place, the under side of the floor or slat will be discoloured with rust stains. In those cases, engineering advice should be obtained.
Older barns with pine board siding are commonly re-sided with steel. By doing this, several changes occur. This can add to the wind forces that the older building must resist. Previously, some of these wind forces were dissipated as it was, in part, allowed in through the cracks between the siding boards. With solid steel sheathing eliminating this wind movement, an increased pressure occurs. Wind pressures are very significant on the large wall surfaces of a two-storey barn. An engineer should determine if additional bracing is required. As well, the natural ventilation provided by a board-sided barn is eliminated by the addition of steel siding. Because of this, moisture is not so easily removed from the loft area. To offset this, louvered or screened vents should be added both to the wall areas and to the barn peak in the upper storey of the barn.
Power nailers have found their way into the toolboxes of most construction crews. They make the task of repetitive nailing easier. The nails that these guns use are not as large or as strong as the common or spiral nails that are often required for structural connections. Construction crews are often not aware of this. Power-driven nails should be used in structural connections only when their use is designed and approved by an engineer.
In Ontario, for new construction and for structural renovation, the Ontario Building Code requires that you obtain a building permit. When you discuss the project with the municipal building official, he or she will indicate what critical design information they will require (if any) before issuing a building permit. In some circumstances you will need to contact an engineer to obtain a design for some or all building components. Therefore, applying for a building permit well before the planned start of construction is wise.
A farmstead is a continually evolving group of buildings. In many ways these structures influence each other. In your renovation, if you make changes or add to a building, additional considerations may be necessary to assure the integrity and continued service of all buildings influenced.
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