Questions and Answers
Question: Which environmental condition usually controls building material sizes?
Answer: Typically wind is the primary (governing) load condition for pole buildings. Snow loading begins to have an important effect at approximately 50 psf and higher.
Question: What is a shear wall?
Answer: A shear wall is a wall that is installed to resist lateral loading such as wind and seismic loads through diaphragm or plate action.
Question: What is the difference between a cantilevered and diaphragmed building analysis?
Answer: A cantilevered building resists lateral loading through post bending only. A diaphragmed building takes advantage of the shear capacity of the wall sheathing to resist lateral loading.
Question: What is the most common gauge of metal sheathing used in pole buildings?
Answer: The most common gauge of metal sheathing used in northwest pole buildings is 29 gauge.
Question: What code covers the design and construction of pole buildings?
Answer: The International Building Code.
Question: How does higher than minimum snow loading impact the structural components of a building?
Answer: Increased snow loading primarily affects purlins, trusses and rafters. However, it also affects post size, footing size and truss attachment.
Question: What is the purpose of a footing?
Answer: The purpose of a footing is to resist vertical movement of the posts due to vertical loading. The footing prevents the post from sinking into the ground over time.
Question: What is the most common size of footing for pole buildings?
Answer: The most common footing size for a pole building in the northwest is 24″ diameter and 6″ thick. Footings can be larger or smaller as required by load conditions.
Question: What effects footing size?
Answer: Footing size is effected by vertical loading and soil capacity.
Question: What commonly happens to the building material requirements when one or more walls have large or many openings (doors and windows) in them?
Answer: Large doors or openings will increase loading in the diaphragm. The walls may require plywood sheathing. When openings are designed into a building, its capacity to resist lateral (primary wind) loading is reduced.
Question: What are some of the things that effect post hole depth?
Answer: Post hole depth is primarily affected by lateral and vertical loading and soil conditions. The post is embedded deeper to resist bending and to gain soil capacity. Soil capacity (lateral and vertical) increases with depth.
Question: When a customer wants a building with three sheathed walls and one open wall, what are the common changes in building materials required?
Answer: A three-sided building will require additional post strength along the open wall, uplift details for the trusses and the purlins and uplift provision at the post hole.
Question: What is truss bracing?
Answer: Truss bracing is used by the truss engineer to stabilize the truss. Truss bracing is required by the truss engineer, not the building engineer.
Question: Who supplies truss bracing requirements and who supplies the truss bracing materials?
Answer: The truss engineer specifies the location of truss bracing; the builder supplies the bracing material. The building engineer has little or no part in the specification of truss bracing.
Question: What inspections by the Building Department are typically required for a pole building?
Answer: The Building Department will usually require a hole depth and diameter inspection, a footing inspection, a framing inspection and a final inspection.
Question: What are the definitions for “boundary” and “field” for wood sheathing nailing?
Answer: Boundary nailing is the nailing around the edges of each panel of wood sheathing. Field nailing is the nailing in the interior of the sheets – typically 12″ o.c.
Question: When are interior posts required?
Answer: Interior posts are usually required only when the post and rafter system is used.
Question: What are stitch screws and when are they required?
Answer: Stitch screws are screws that join sheets of wall and roof metal at their seams. They are required when a higher shear capacity is needed. The use of stitch screws can be an economical alternative to the addition of OSB and plywood sheathing.
Question: What is the structural definition of failure?
Answer: Failure occurs when the analyzed, applied stresses exceed code allowable. A building component “fails” when the calculated stresses are greater than that allowed by the governing code or the manufacturer. Structural failure occurs well before something breaks or falls down. An example of failure is cracked ceiling plaster due to unacceptable deflection. Note that the ceiling does not have to wind up on the floor to constitute failure.
Question: What type of nails shall always be used when attaching to pressure treated wood?
Answer: Hot dipped galvanized – per the International Building Code.
Question: How is eave height defined?
Answer: Eave height is measured from the ground to the intersection of the roof line with the wall line. Eave height is not the clear height under the bottom chord of the truss.
Question: What are four common eave heights of pole buildings?
Answer: 10′-0″, 12′-0″, 14′-0″ and 16′-0″.
Question: What three environmental conditions place loads on a building?
Answer: Seismic, wind and snow place loads on buildings. The two most severe load conditions are usually wind and snow.
Question: What are two of the most common changes in building material requirements when changing from a “B” to a “C” exposure?
Answer: Wind exposure will affect post size and sheathing requirements. Exposure will also effect post embedment depth, girt and even purlin and truss attachment requirements. When exposure increases from “B” to “C” the lateral and vertical pressures exerted on the building increase.
Question: What is the difference between wind exposures “B” and “C” with regards to the surroundings around the pole building location?
Answer: Exposure “B” is a more reduced exposure than exposure “C”. Exposure “B” is used when the building is surrounded by other buildings, trees and surface irregularities. Exposure “C” is used in generally open areas with little or no protection from the wind.
Question: If a customer does not want a concrete floor in his building, what type of backfill will be used in the posthole?
Answer: The most likely backfill with no floor slab is concrete – full depth. Concrete backfill prevents flexure below the soil surface, allows for better lateral support and prevents uplifting of the post
Question: If a customer wants a concrete floor is his building, what type of backfill will most likely be used?
Answer: The most likely backfill with a concrete floor slab is either granular or sand. The international Building Code actually requires either concrete backfill or sand backfill, however, it has been found through experience that compacted, crushed 3/4 (-) rock is an excellent backfill. The presence of a floor slab also provides lateral support for the post.
Question: What are three options for installing purlins?
Answer: Purlins can be installed stacked and lapped, stacked and butted or face hung. A stacked purlin stacks on top of the truss top chord. It is nailed to a purlin block that is nailed on each side to the top chord of both trusses. When the purlins are stacked and lapped one purlin laps over the other, usually about a foot in length. When the purlins are stacked and butted the two purlin boards butt together right in the middle of the purlin block. Purlins can also be installed flush with the top of the top chord of the trusses
using joist hangers.
Question: When are commercial girts typically required?
Answer: Commercial girts are typically required to resist high wind loads.
Question: What are two options for installing a commercial girt?
Answer: A commercial girt can be installed with blocks nailed to the post or with rotated joist hangers. The commercial girt (which is rotated 90 degrees to a flat girt) is nailed on top of a block and then toe nailed into the post. A commercial girt can also be installed with a joist hanger rotated 90 degrees and nailed directly to the post.
Question: How is a flat girt installed?
Answer: A flat girt is installed by nailing a board (usually a 2 x 6) directly to a post with hot dipped galvanized nails.
Question: What are three grades of posts?
Answer: The most common grades of posts are #2, #1 and Select Structural. The most readily available grade is #2.
Question: What are four common sizes of posts used in pole buildings?
Answer: Four common post sizes are 6×6, 6×8, 6×10 and 6×12. Pole buildings in the Northwest commonly use Pressure Treated (PT) HEM-FIR wood for posts. These posts are referred to as “solid sawn”. Nail laminated posts made for 2×6 and 2×8 boards are commonly used elsewhere.
Question: What are the two most common bay spacings in a pole building?
Answer: The most common bay spacings in pole buildings built in the Northwest are 12′-0″ and 10′-0″. Other parts of the U.S. commonly use 4′-0″ and 8′-0″ bay spacings.
Question: What are the two most common roof pitches of pole buildings?
Answer: The most common roof pitches are 3/12 and 4/12. Roof pitch describes the angle of the roof. It is measured by laying out 12″ horizontal and the appropriate number of inches vertical. It is generally accepted that the 4/12 pitch is “prettier” than a 3/12.
Question: What affects the size of a corbel block?
Answer: Corbel block size (width) is affected by truss bearing area. The length of the corbel is affected by snow loading. Corbels are usually 2x boards that are the same width as the post. The area required to support the truss heel will be specified by the truss engineer on the truss drawings. Determine the bearing area of a corbel block by multiplying the width of the corbel by its thickness (Bearing area of a 2 x 6 = 1.5″ x 5.5″ = 8.25 square inches).
Question: What is the purpose of a corbel block?
Answer: The purpose of the corbel block is to support the load carried by the truss. Corbel blocks attach directly to the posts usually with a combination of bolts and hot dipped galvanized nails.
Question: What effects purlin size, grade and spacing?
Answer: Purlin size, grade and spacing are affected most by bay spacing and snow load.
Question: What is the most common purlin size, grade and spacing?
Answer: The most common purlin size in the Northwest is #2 DOUG-FIR or HEM-FIR 2×6 spaced at 24″ o.c.
Question: What effects girt size, grade and spacing?
Answer: Girt size, grade and spacing are affected most by bay spacing and wind loading due to wind speed and wind exposure.
Question: What is the most common girt size, grade and spacing?
Answer: The most common girt size in the Northwest is #2 DOUG-FIR or HEM-FIR 2×6 spaced 24″ o.c.
Question: What is a pole building?
Answer: A pole building is constructed using the Post Frame style of construction. Post Frame construction uses posts (columns) to directly support trusses. Girts (secondary wall framing) and purlins (secondary roof framing) span between the posts and trusses. The wall and roof sheathing attaches directly to the girts and purlins. Posts may be either buried in the ground or attached at the ground line to a floor slab.