Metal Buildings 101
Most metal buildings have four outside walls. Two of these walls are called sidewalls. This occurs where the roof meets the walls and aligns parallel to the flush floor without any increase in the height of the walls. The other two walls, called endwalls, show a rising line where the walls meet the roof and the height of the walls change.
The point where the sidewalls meet the roof is called the eave. It often has special trim to dress it up or a gutter to catch the rain flow from the roof. The distance from the bottom of the base plate to the point where the roof and sidewall intersect is where the eave height is determined.
If you visualize a straight line across the endwall by extending the eave line on one sidewall to meet the eave line on the other sidewall, you would be creating a triangular area referred to as the gable. The point where the two rising halves of the roof meet at the endwall is called the peak.
The line where the sidewall meets the endwall is usually finished out with a special piece of trim known as corner trim. The peak receives a plastic or metal cap known as the peak box. The peak box usually receives an identifying mark, or the Kirby logo.
The roof pitch or roof slope is usually shown as a ratio to 12 (i.e., 1:12, ½:12, 4:12, etc.). When inches are used as a basic unit, a 4:12 roof pitch means that the roof rises 4 inches in every 12 inches measured horizontally across the width of the building from the side to the peak of the building.
Kirby metal buildings have three basic dimensions: span (width), length, and eave height. The span is the distance from the outside of the sidewall girt on one side to the outside of the sidewall girt on the other side. The length is the distance from the outside of the endwall girt on one endwall to the outside of the endwall girt on the other endwall. Eave height is the distance from the bottom of the base plate to the top of the eave strut.
SYSTEMS, PARTS & PIECES
Once they are erected, metal buildings can look quite different. There are, however, a number of standard parts and configurations that are common to all. This section concerns itself with standard types of materials, parts, and systems to show how they function to make up the entire building.
All metal buildings are built on top of a foundation. The foundation is generally a concrete slab with concrete footings. The footing is concrete, usually rectangular shaped, poured and formed under a column. A footing distributes the load created by the metal building support member into the supporting soil. Anchor bolts are set in the footing to “anchor” the column or structural members.
A metal base plate is a pre-punched metal plate which comes already welded to the base of the column or structural member. The pre-punched plate fits over the anchor bolts.
If you were to take off the outside covering on the roof and walls of a metal building, you would see something similar to the illustration shown here. This is known as the structural support system, and is divided into the primary support system and the secondary support system.
PRIMARY STRUCTURAL SUPPORT SYSTEMS
Primary support systems furnish the main support of the building. The primary structural support system is more often called the main framing system and can be divided in two basic types – clear span and beam and column. Clear span frames have no interior obstructions between the exterior columns. Beam and column or BC frames have one or more interior columns between the exterior columns, supporting some of the vertical load carried by the frame.
The rigid frame (RF) is probably the most commonly used clear span frame. The rigid frame has tapered or straight vertical columns and tapered or straight rafter sections. A cross section through either a column or rafter section would show an H shape. You will note that typically a rigid frame is deepest in the knee or haunch area where the column is connected to the rafter beam.
SECONDARY STRUCTURAL SUPPORT SYSTEMS
If we go back to a metal building with the outside covering taken off the roof and walls, you will notice in the roof area above the primary support system, a series of z-shaped members spanning between the frames. These are the principal members of the roof secondary support system and are known as purlins. The basic structural shape used by Kirby is an 8”, 10”, or 12” zee section.
Kirby purlins come in a variety of gauges (thicknesses) and the space between them can vary, although 5’ purlin spacing is the most common. There are two basic types of purlins, a simple purlin and a continuous purlin.
The basic purlin provided by Kirby Building Systems is the continuous purlin, however, the definition of both purlins are as follows: simple purlins provide a 4-1/4 “ lap connection for alignment; continuous purlin laps vary from 1’2” to greater according to various conditions. The continuous purlin design often allows the use of a lighter gauge and more economical system, saving the customer money while giving the same or better structural integrity.
Looking back at our metal building with the outside covering taken off, you will notice in the wall areas attached to the columns are 8’ zee shaped members similar to the purlins on a roof. These are the principal members of the wall secondary support framing systems and are called girts. Girts, like the purlins, take the loads imposed on the covering system and transfers them to the frames which, in turn, transfer them to the foundation.
The first girt in a Kirby building is usually at 7’4” and additional girts are spaced at 5’0” intervals up to the eave line. The spacing of girts, however, like the gauge of girts, varies with the loads imposed on them. Kirby offers two basic types of girt systems – flush and by-pass. Kirby’s standard is by-pass girts for both sidewalls and endwalls.
By-pass girts are outset from the columns. Flush girts have the main frame columns inset into their line. The advantage to flush is an increase of usable space inside the building at the column location.
It should be remembered that by definition the building width is measured from outside of sidewall girt to outside of sidewall girt. Therefore, when using the flush girt condition, the building frames are moving out into the girt line. The building does not get wider.
Wind exerts a force on metal buildings and one of the primary design features used to resist this force is bracing. Bracing consists of sets of cables in the roof, between the rafters, and in the walls between the columns.
Specific design criteria that govern a particular metal building will determine the quantity and location of cables. Bracing cables are often called “cable bracing”, “wall bracing”, “wind bracing” or “X-bracing”.
When framing system members or frames are laid out in a predetermined pattern to provide the primary support for the building, the distance between the frames are called bay spacing. The areas between the two frames are called bays.
On interior bays the bay spacing is the distance from the centerline of one interior frame to the centerline of the next interior frame. Bay spacing for end bays is measured from the centerline of the first interior frame to the outside line of the first interior frame on the endwall.
The most common endwall is called a bearing frame endwall, and is Kirby’s standard. It is a modular frame in the sense that it has interior columns. The interior columns are called endposts or wind columns. The two side columns are called corner columns. The bearing frame is designed to carry only one-half of a bay.