The Rigid Frame Metal Building

by Steve Carter

The building system consists of primary framing members, secondary framing members, roof system, wall system, and accessories.

Rigid Frame

The prime objective of the Metal Building System is to provide a quality structure. Our buildings are available in a range of configurations - from the small, standard structures to maximum performance structures with creative architectural refinements to satisfy the spectrum of the owner's requirements. The variety of building configurations and sizes offer many solutions to fulfill needs of the commercial, community, and industrial markets.

Standard versus Non-Standard

You will hear the word standard used many times in our business. It is misunderstood more than any other word. Certainly any manufacturer who designs and produces parts that must fit together to provide a completed product has a definite direction or "standard", which is the base of normal application of the product. Consequently, standard items are considered to be those that are commonly manufactured on the production line and those that are purchased by customers.

However, if a situation arises involving something that is "nonstandard", it is still possible and practical to meet that need in many cases. Our engineers believe nothing is impossible but variation from a standard often means extra work, expense, and time. Sometimes this is negligible, but at other times it might be quite involved.

Primary Framing System

Primary framing furnishes the main support of a building. A bearing frame (post and beam) and a main frame (rigid frame) are examples of primary framing. In this text, we will not only be talking about the main frame as a primary framing system, but also about secondary framing members, and bracing that join with the main frames to make up a complete structural system.

Roof Slope

Roof-Slope is defined as the tangent of the angle that a roof surface makes with the horizontal, usually expressed in units of vertical rise to 12 units of horizontal run.

The roof slope of a building is expressed as 1/2 : 12, 1:12, 4:12, etc. A 1:12 roof-slope rises 1 inch in every 12 inches measured horizontally from the side of the building across its width to the peak of the building. A 1 : 12 is what is provided with the Value Building System.

The Main Frame

The main frame (rigid frame) is the primary structural member of the building system. The main frame consists of columns and rafters. Columns are used in a vertical position on a building to transfer loads from main roof beams, trusses, or rafters to the foundations. Rafters are the main beams supporting the roof system.

Strictly speaking, a main frame is structurally stable because of the rigidity of its connections. The main frame members are connected in such a manner as to make the entire frame act as a single unit. Two common types of connections used to connect major parts of a main frame are diagonal and perpendicular.

Knee/Haunch Area of Main Frame

The knee/haunch is that area of the eave where the column connects to the roof rafter. The knee/haunch ties the members together rigidly and converts them into a single unit to carry all loads, vertical or lateral.

Notice that in the area of the knee/haunch, the main frame (rigid frame) is deepest in section, which makes it the strongest area of the frame.

This is required primarily because of the vertical load considerations, but at the same time it enables the frame to offer lateral strength. What does this mean? It means that the strength designed into the frame for vertical loads is also available to carry lateral loads, which might be caused by high winds, earthquake shock, etc.

Because the inside flange of the knee is in compression, a resulting thrust is produced at the inside corner, which is upward and outward. Stiffeners are used to counteract the resistant thrust. Stiffeners are usually extended to the outside flanges and also serve to stiffen the entire web. The haunch connection also serves as a stiffener. Main frames may be considered as arches in their action, in that they produced a horizontal thrust at their base or a tendency to kick outward. Under certain loading conditions, however, an inward thrust might be produced at the base. Main frames belong to a general class called continuous structures because the action and stress travel throughout the entire structure, since all joints are fixed in a structural sense. Because of this, engineers must analyze an entire main frame as a complete unit in itself, and not as an assembly of separate members.

Visualize a big hand grasping the roof rafter of a single main frame at the peak. The hand is alternately pushing down and pulling up on the frame. Since the member is a continuous structure, it is easy to see that the base of the two columns will tend to kick outward or inward, depending on the type of load being exerted.

These thrusts, however, are easily counteracted by a properly designed concrete foundation. We have used the expression "easily counteracted" purposely because a qualified engineer can design an adequate foundation using the reaction charts supplied by the manufacturer. There are many buildings, both over-designed and under-designed, in use today that have improper foundations simply because the person designing the foundation was either unqualified or did not refer to the reactions furnished by the manufacturer.

The building drawings include reaction charts with various loading conditions for standard main frames. Our pricing program produces preliminary mainframe column reactions as well. Make these charts available to your architects and engineers so that foundations will be priced properly and economically.

Main frames are normally connected to the foundation by using the appropriate anchor bolts in a configuration that is described as a pinned condition. This means that the loads transmitted to the foundation are vertical loads and horizontal loads.

Endwall Frames

Assume a building is 100' long, consisting of four 25' bays as shown above.

The main frames indicated by MF in the drawing above support a roof area of two half bays. The endwall frames indicated by EW, however, only support one half-bay of roof load.

From this you can readily see that the endwall frames need not be as strong as the main frames. It is for this reason that in addition to expandable main frame endwalls, we offer lighter non-expandable mainframe endwalls, or even lighter bearing frame endwalls, depending on your customer's requirements.

The expandable main frame endwall is designed to support two half bays of roof load and can support an additional half bay in the future. The non-expandable main frame is designed to support one half bay of roof load and cannot support an additional half bay in the future. Main frame endwalls do not require any bracing and clear the endwall bays for large framed openings or open wall conditions.

Secondary Framing Members

Secondary framing members are those members that join the primary framing members together to form building bays and provide the means of supporting and attaching the walls and roof. Secondary framing members are:

  • Eave Struts
  • Purlins
  • Girts
  • Bracing

Eave Struts

The eave strut is a roughly C-shaped cold-formed member and is located as illustrated below. Cold-forming is the process of using press brakes or rolling mills to shape steel into desired cross sections at room temperature.

The eave strut provides an attachment and bearing points for the end of the roof sheets and wall sheets. Eave struts are available in nominal depths of 8", 10", or 12" to match the purlin depth. Eave struts are pre-punched at the factory for bolting to the main frames.


A purlin is a secondary framing member that serves to support roof panels and transfer the roof loads to the rafters.

The purlin is Z shaped as shown below. Purlins are available in 8", 10", or 12", depth, and are available in different gauges of steel 16, 14, 13, or 12 to meet various loading conditions.

The continuous purlin is a Z shaped cold-formed member 8", 10", or 12", depth with a 50 degree outer lip to facilitate nesting. The purlins are lapped at each interior frame with the lap varying from 8" to 60" depending upon the conditions. Continuous purlins take into consideration the design advantage of continuous beams. The economy is based on using them on multiple bays where the overlapped splice of the purlin, continuous over the rafter, assists in supporting the load of the adjacent bay.


Girts are secondary framing members that run horizontally between main frame columns and between endwall columns. They are Z shaped members like purlins, also available in depths of 8", 10", or 12", and gauges of 16, 14, 13, or 12.

Standard girt spacing is the first girt at 7' 4" above finish floor and a maximum of 6' there after. This standard spacing fits doors, etc., utilizing optimal design. Other spacing is available to satisfy design criteria. A low girt option is available on request at 3' 6", which stiffens the wall section, and is standard in high wind conditions. For applications where a drilled pier foundation or isolated pad foundation is to be used, a base girt is available at ground level for fastening the bottom of the panel. Girts and purlins are pre-painted at the factory. The manufacturer welds all girt attaching clips to the frames for easier and quicker erection.

Bypass girts attach to the outside flange of the columns, creating a more efficient design. The girt is lapped at each frame and at the first interior frame from the endwall. Bypass girts are used to take into consideration the design advantages of continuous beams spanning from bay to bay.

Flush girts attach to the web of the columns, with the girt face in the same plane as the column face. This provides greater interior clearance.

In addition to playing an important roll in the structural stability of the complete building system, girts also serve the important means of providing the framing for the attachment of wall covering.


In addition to main frames, endwall frames, eave struts, girts, and purlins, the building system must have adequate bracing to make the system stable in a lengthwise direction. Bracing systems transfer wind loads from endwalls and sidewalls to the foundation. Wind bracing systems must include two types:

  1. Longitudinal bracing, for wind on the endwall.
  2. Transverse bracing, for wind on the building sidewall.

Requirements for bracing systems described on these pages are based on the specifications of applicable codes.

A variety of methods are available for providing bracing for wind on the building endwall. Bracing systems of this type serve a secondary purpose of squaring the building. In addition to the standard method - diaphragm action, alternatives include X-bracing (cable or rod), fixed base columns, portal frames, and wind bents attached to column. When bracing must occur in bays where doors or other accessories are required, fixed based columns or portal frames should be used.

Bracing Methods:

Diaphragm Action

Diaphragm action utilizes the diaphragm resistance of the wall panels to transmit lateral wind or seismic forces to the foundation. Diaphragm action utilizes undisturbed sheeting, floor to roofline, and assumes all wall panels are installed correctly.


When diaphragm action of the panels is inadequate or not allowed, the first alternative is to provide cable or rod bracing between columns. X-Bracing transfers longitudinal forces to the foundation.

Fixed Base Columns

If the openings in the wall are such that they do not allow for the use of X-Bracing, then fixed base columns may be used. A fixed base column is a column with special base plate condition, which allows wind load to be transferred to the foundation. Therefore, fixed base columns will induce a moment to the foundation, thus requiring a special foundation design.

Portal Frame                                              

If neither X-Bracing nor fixed base columns are acceptable, a portal frame (wind bent) can be used. A portal frame is an I-shaped section of built up material consisting of two columns and a rafter, running parallel to the sidewall, and attached to the web of the sidewall columns. As a standard the portal frame usually does not induce a moment to the foundation.

Brace to Interior Main Frame

A method of bracing used for an open bearing frame endwall is to provide bracing in the roof of the end bay. In this case, the lateral forces on the endwall are transferred to the first interior main frame. The main frame is then designed to resist this additional lateral force.

Flange Braces or Purlin Bracing

Flange braces are structural members that attach purlins, girts, and eave struts to primary structural members (columns or rafters). Purlin bracing is an angle connecting the bottom flange of adjoining purlins to prevent purlin roll.

Flange braces are used to prevent the main frame from twisting or buckling laterally under the load. They are an essential structural part and must be installed properly at all locations. Flange braces can also be very useful as an erection aid to align the purlins and eave struts for easier and lower cost roof installation.

Structural Paint

All primary framing members are factory cleaned to remove loose dirt, grease, mill scale, etc. They are then painted with a red oxide primer. The purpose of this primer is to provide temporary protection of the steel members during transportation and erection. Touch up may be required after erection. Red oxide primer also provides a surface that is chemical and corrosion resistant. Therefore, it is not necessary to put an additional finish coat of paint on the framing members. However, if it is desired, finish paint may be applied over the red oxide in the field. However, consult with the paint supplier for the compatibility and proper preparation of steel before the application of any finish paint. It is also recommended that a test patch of the finish paint should be applied to test for compatibility.

Secondary framing members are pre-painted by a company specializing in coating of metal products with a baked on red primer. Due to the special coating required for roll forming these members, they can be difficult to repaint.

Galvanized Steel

For over 140 years, galvanizing has had a proven history of commercial success as a method of corrosion protection in a myriad of applications. Galvanizing can be found in almost every major application and industry where iron or steel is used. The utilities, chemical process, pulp and paper, automotive, agricultural, and transportation industries, to name just a few, have historically made extensive use of galvanizing for corrosion control.

All of our buildings are also available in galvanized steel as a special option. Two types of galvanized material are used:

  • Hot Dip Galvanizing
  • Pre-Galvanized

Hot dip galvanizing is the process of applying a zinc coating to fabricated iron or steel material by immersing the material in a bath consisting primarily of molten zinc. The manufacturer sends the fabricated material, such as, primary and secondary framing members, to the galvanizers.

Pre-Galvanized material is used for secondary members only. The pre-galvanized material used is of 55 grade and adheres to ASTM A653 specifications. The coil of pre-galvanized material is delivered to the Manufacturer and then the pre-galvanized secondary members are fabricated.

Clearspan Buildings

Clearspan buildings allow for the maximum use of interior space, which is particularly important in manufacturing plants, warehouses, offices, and retail stores where uninterrupted space is required. Size flexibility also pays off outside where optimum land use is an equally important consideration.

Virtually every symmetrical, unsymmetrical, and single slope building size and shape is possible as a standard product. Inside the clearspan building you have almost total flexibility in determining the height, width, and roof slope you want: building widths from 20' - 150'; eave heights from 10' - 30'; and roof slopes from 1:12 to 4:12. Building widths of 80' or less are available with the option of straight columns instead of tapered columns.

Lean-tos are available for future expansion or additional space. A lean-to can be designed to match the eave height and roof slopes of the clearspan building if the building was originally designed to take on the loading of an additional lean-to load. Lean-tos are available in widths from 8' - 60', eave heights from 8' - 30', and roof slopes from 1/2:12 to 4:12.

Modular Buildings

A modular building (with interior columns) is specially designed for large buildings such as manufacturing plants, warehouses, truck terminals, and retail stores. Interior columns are either built up 'H' columns or pipe columns. 'H' columns are mandatory in a building with a top running crane. Modular buildings combine the proven practicality of a rigid frame with almost unlimited size flexibility.

With a building that is 100' wide or less, the building can be designed with both clearspan frames and modular frames. This could serve the benefit of having a portion of the building with an unobstructed floor area while maintaining the cost savings of a modular building.

Modular buildings are also possible in any symmetrical, unsymmetrical, and single slope building size and shape as a standard product offering. Inside the modular building there is almost total flexibility in determining the height, width, and roof slope: building widths from 40' - 500'; eave heights from 10' - 30'; roof slopes from .25:12 to 4:12; and interior module spacing from 20' to 100'. Modules are defined as the space between interior columns. THE STANDARD BUILDING is limited to 8 interior modules but more modules are available on request. Building widths of 40' - 80' are available with the option of straight columns instead of tapered columns. Lean-tos are also available for future expansion or additional space if the original main structure had been designed to support the additional load of a lean-to.


The lean-to is ideally suited to give that extra space needed alongside the building. The lean-to ties in at or below the eave of the building and can provide a variety of uses, from just a covered area to a completely enclosed addition to your building. A lean-to structure has only one slope and depends upon another structure for partial support. A lean-to can be located level with or below the eave of the supporting structure.

A lean-to is limited to 60' wide as standard and only has a straight column at the low side and a rafter. The rafter attaches to the supporting structure's column. Therefore, it is imperative that the bay spacing of a lean-to equals the bay spacing of the supporting structure.

Endwall guidelines for Lean-tos:

  1. A lean-to with a bearing frame endwall may be attached to buildings having a bearing frame, an expandable main frame, or a non-expandable main frame endwall.
  2. When the lean-to does not extend the full length of the main building and begins or ends at an interior main frame, the bearing frame endwall is the standard condition but also could be a main frame endwall if necessary.
  3. If an expandable or non-expandable main frame endwall is used on both the lean-to and the main building the endwall may be completely open.

Endwall Types

Endwalls are available in three basic types:

  • Expandable Main Frame
  • Non-Expandable Main Frame
  • Bearing Frame

Expandable Main Frames

The expandable main frame endwall is a combination of the standard main frame with endwall columns. The endwall columns do not support the rafter but serve only as columns for attachment of endwall girts and transmit the wind load into the foundation and structural frame.

The expandable main frame's largest advantage is that it provides for easy expansion. Since it is a main frame it will carry the design load of a full bay, and it can remain in-place if the building is expanded.

Non-Expandable Main Frames

The non-expandable main frame endwall is still a main frame with endwall columns, but cannot be used for future expansion. The non-expandable frame can only carry the design load of one half bay.

Both the Expandable and Non-Expandable main frame endwalls provide more flexibility and ease in locating large framed openings or entrance doors. Locate the openings by simply adjusting the endwall columns spacing. Also, the main frame endwalls do not require any form of bracing, therefore, X-bracing or portal frames will not interfere with large openings.

Bearing Frames

A bearing frame (post and beam endwall) is our standard endwall condition. The endwall columns are generally made of C channel and at times can be back to back C channel. The bearing frame is designed to support only one half bay of roof load, and cannot be used to expand the building in the future.

The endwall columns support the channel rafter and also serve as columns for attachment of the endwall girts and transmit wind load into the foundation and structural system. Bearing Frame Endwalls also require a form of bracing, whether it be X-bracing, portal frames, or diaphragm action.

The use of a bearing frame endwall is a matter of economy. You will usually find the prices of the bearing frame endwalls to be less than one half the cost of the expandable main frame endwalls.

Endwall Cost Considerations

It is important to recognize that the different types of endwalls can be interchanged to offer advantages in specific applications.

The expandable clearspan main frame endwall can provide an entirely open endwall up to 150' wide. This could be the answer to a covered truck dock across the end of the building; or, total flexibility in placement of framed openings.

It is also possible to interchange the interior modular main frames comprised of different modular spacing. For example:

The 120' wide building could have 3 - 40' wide modules or 2 - 60' wide modules. By interchanging some 60' module frames within the structural system we can retain the lower cost of the interior columns yet provide larger unobstructed areas.

Also, using the 3 - 40' modular main frame endwall in place of the 2 - 60' module spacing, you would be able to place an overhead door in the center of the endwall without difficulty.

Many times the ability to interchange frames and endwalls can bring about cost reductions, which will amount to several thousands of dollars. These can be very important savings if you are working against competition or a low budget.