Engineering Requirements For Farm Structures

Agdex#:	714
Publication Date:	03/04
Order#:	04-013
Last Reviewed:	03/04
History:	Replaces OMAFRA Factsheet Engineering Requirements for Farm Structures, Order No. 93-009
Written by:	John Johnson - Engineer, Civil Systems/OMAFRA

Table of Contents

1. Introduction
2. Is the New Building Being Built by an Experienced Farm Building Contractor?
3. Are Standard Construction Methods or Plans Being Used?
4. Is the Soil Bearing Strength Typical of Other Building Sites in the Vicinity?
5. Is the Ground Floor Area of the New Building Greater than 600 m²?
6. Is the Side Wall Height of the New Building Greater than 3.6 m?
7. Is the New Building Attached to (or very close to) Another Building Differing in Size or Height?
8. Are There Side Wall Doors or Openings Wider than 3.6 m?
9. Are the Roof Trusses Engineered and Manufactured Offsite, or Built On-site by a Farm Building Contractor?
10. Does the Structure Store or Support Hay, Straw, Bulk Materials such as Grain, Silage, Haylage, Vegetables, Liquid Manure or Other Liquids?
11. What are the Environmental Consequences of a Structural Failure?

Introduction

The Ontario Building Code (OBC) makes it necessary to obtain a building permit for all farm buildings in Ontario with a floor area larger than 10 m² (108 ft.²). The chief building official may require drawings that show more structural detail. A question many producers commonly ask is, "Do I need to contact an engineer to obtain a detailed structural design?" This Factsheet will help to outline situations where further design may be necessary. Some of these conditions are illustrated in Figure 1.

Figure 1. Attached buildings make the design of roof structures complex. Proper snow load calculations are important. (Courtesy: Alberta Agriculture.)

An engineer’s task is to protect humans, animals, crops, property and the environment with structurally sound, functional, but cost effective, designs. He/she must consider many types of loading and must still make certain that the building serves its function properly.

Farm buildings commonly require engineering design for specific structural components (plates, post/stud sizes, bracing, footing sizes, etc.). The Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) has developed standards for sizing common farm building components. These standards are available in OMAFRA Publication 809, Farm Building Standards For Use in Sizing Common Building Components and can be ordered through the OMAFRA Home Page.

As well, the Ontario Ministry of Municipal Affairs and Housing, jointly with OMAFRA, has prepared a Construction Guide for Farm Buildings. This can be ordered from Orderline.com: Information Central for Building Industry Professionals.

Simple parameters such as the building floor area or cost are often used to show how complicated a building is, but for many farm buildings, this does not tell the whole story. This Factsheet lists several questions to ask about your structure. Your building contractor or building official may be able to help you answer these concerns. It serves as a guide to show how complicated the design is and why building officials may request more design details.

Is the New Building Being Built by an Experienced Farm Building Contractor?

The chief building official will be most comfortable in dealing with a builder or contractor who has completed many similar projects. Shortcuts and deviations from standard building practice are more common where inexperienced builders have constructed a building. An experienced builder follows a proper design, is familiar with current building techniques, makes proper connections and knows best how the building must function. The complexities of constructing, temporarily bracing and finishing a large structure are best handled by a person with experience. The largest percentage of building failures occur during construction.

Are Standard Construction Methods or Plans Being Used?

If your building official is not familiar with a proposed construction method, or if the building uses unconventional components, more details may be needed. Standard plans such as those provided by the Canada Plan Service, available for download at the Canada Plan Service website are often acceptable. If your design is not typical, engineered plans may be needed, unless the building’s components can be designed using recognized design tables or information. Engineered plans are required if the building floor area is larger than 600 m² (6,459 ft²). Manure storage tanks and other structures that use reinforced concrete also need to be engineered.

Is the Soil Bearing Strength Typical of Other Building Sites in the Vicinity?

The Ontario Building Code allows footings and foundation systems that have performed well in the past and in soil conditions commonly found in the vicinity. Floating slab foundation systems can be used successfully on well-drained, compacted sand. The building official usually requires an engineering design for shallow foundation systems. Clay soils require deeper, below-frost foundation systems. For unusual soil types (soft or swelling clays, loose sand, etc.) or situations where standard design tables do not cover the situation, contact a geotechnical engineer. He/she determines the bearing strength of the soil and provides recommendations for the type of foundation system to use. This information is used by the project engineer to design a proper foundation system. Before constructing a silo or other heavily loaded structure, it is important to obtain a report from a geotechnical engineering company.

Is the Ground Floor Area of the New Building Greater than 600 m² (6,459 ft.²)?

Large buildings are more complicated. Wide buildings carry very large roof snow loads. This can be magnified if rainfall occurs while the snow is still on the roof. Large trusses are more difficult to install, even in very light wind conditions. It is important for qualified persons to carry out this procedure. Proper truss and wall bracing techniques (temporary and permanent) must be used for these larger buildings.

The Ontario Building Code and the National Farm Building Code require engineering designs for all roof systems in farm buildings.

Is the Side Wall Height of the New Building Greater than 3.6 m (12 ft.)?

High sidewalls and two-storey farm buildings are more difficult to brace (temporary and permanent) against wind. This is one of the most common shortcomings in new farm buildings. Where standard bracing techniques are not being used, make sure an engineer designs the bracing system (Figure 2).

Figure 2. Typical wall-framing detail for a post-frame building.

Is the New Building Attached to (or very close to) Another Building of Differing Size or Height?

When attached buildings, or those built closely together, differ in height, snow can slide or drift between the roofs.

For example, a one-storey tie-stall dairy barn is added to the end of an older two-storey barn. Snow can sometimes slide or drift from a higher roof to a lower one, adding large loads to the lower roof. A new lean-to (Figure 1, item 7) may add loads to an attached existing building that it was not designed to carry. The new lean-to may not be very large in floor area, but can be quite complicated structurally. Sometimes the roof is sheltered from the wind by a row of trees, a taller building or a projection such as a chimney. That roof will carry a heavier snow load since the wind cannot blow the snow away. It will need to be stronger than an unsheltered roof.

In situations where there is unusual loading, contact an engineer to determine the loading and to recommend a design for the structural components affected by those loads.

Are There Side Wall Doors or Openings Wider than 3.6 m (12 ft.)?

Roof trusses usually rest on the side walls of buildings. They transfer roof loads to the foundation and footing system through the wall frame. With typical widths of modern farm buildings of 12 m (40 ft.) or more, there are large loads to transfer, which increase as the door width is enlarged.

For example, with a total roof load of 2.0 kPa (42 lb/ft.²) and trusses spaced at 1,200 mm (48 in.), the load on one end of a 12 m (40 ft.) truss is about 1,500 kg (3,300 lbs.).

Plates and lintels spanning large wall openings must be designed accordingly. As a rule of thumb, if the opening width is doubled, the strength requirement increases by 4 times. Often it becomes necessary for steel or built-up wood beams to be used when the spans become large.

Are the Roof Trusses Engineered and Manufactured Offsite, or Built On-site by a Farm Building Contractor?

Many companies pre-engineer and manufacture roof trusses. Truss designs, bracing and anchoring techniques are complicated, especially in wide buildings. The wood connection points are critical to the design. If the roof system has multiple spans with interior posts and beams, then the design for the posts, plates and for the bracing is critical. The structural components of roof systems must be engineered unless standard plans or specifications can be followed.

Does the Structure Store or Support Hay, Straw, Bulk Materials such as Grain, Silage, Haylage, Vegetables, Liquid Manure or Other Liquids?

The design of silos, bins, potato storage structures or liquid manure storage structures is critical. The consequences of a structural failure can be severe, both environmentally and from a human and animal safety standpoint. The Ontario Silo Association maintains reinforcing schedules and standards for the construction of silos in Ontario. The Canada Plan Service also supplies designs for horizontal silos, fruit and vegetable storage structures, manure storage structures, etc. These designs usually require verification by an engineer. The Nutrient Management Act (NMA) requires that all new liquid nutrient storage structures be engineered. To provide a level of security for surface and groundwater, a geotechnical evaluation is also required. The NMA also requires the engineer to inspect the project during construction and verify that the structure meets all required standards at completion. For any non-standard loading circumstances, consult an engineer for a design.

What are the Environmental Consequences of a Structural Failure?

The environmental consequences of the failure of a liquid manure storage tank holding several hundred thousand litres of manure could be substantial. The design of these structures requires careful attention and they must be engineered. Standard plans can be obtained from the Canada Plan Service web pages at the Canada Plan Service website. These plans require verification by an engineer.

Structural failure does not always mean that the building falls down. It could mean that the insulation becomes wet from a poorly designed vapour barrier, or that it does not perform to its potential. Economic losses could be substantially greater if animal health or stored crop losses result. Proper functional design is often as important as structural design.

Building codes are primarily designed to ensure that public safety concerns are addressed. "Occupancy" indicates the density of people inside a structure and how efficiently they can be evacuated. A low human occupancy farm building means that there is not more than 1 person/40 m² during normal use. For farm buildings of low human occupancy, the risk of human injury in a structural failure is lower than for a building that contains a high human occupancy. Ontario’s building codes and standards are in place to try to reduce the possibility of injury or death due to structural failure.

Table 1 is a check-list of items to consider when designing a farm building. It will hopefully help you to understand why the chief building official may ask for more design information than you think is necessary.

Every farm building is different, and each one has its own special structural and functional design needs. Does your proposed farm building require special design considerations?