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The Rigid Frame
Metal Building System
In a previous blog we discussed the different Metal Building Systems in the Norsteel Product line. We touched on the basic concepts and terminologies that are important for customers to understand, as they work with their Building Consultant to design the steel building that will ultimately meet all of their requirements.
As mentioned, Norsteel’s Rigid Frame Steel Building Systems, are by far our most versatile product line. Rigid Frame Metal Buildings can be customized to accommodate unlimited applications and project needs.
This is precisely because of the way they are engineered.
In this blog, we go over the structural support system of Rigid Frame Steel Building Systems. A general understanding of the structural support components will help customers to gain an appreciation of how all the components work together to provide integrity and structural support for their steel buildings. This knowledge is what allows us at Norsteel, to provide our customers with innovative, cost-saving solutions. Because once you understand how the pieces go together to produce strength and support, you also know how to provide unique design solutions without sacrificing that integrity. This in turn, allows us to be creative in our approach to every steel structure.
This diagram provides an inclusive look at the main components of a rigid frame metal building system. Not all steel structures are pre-engineered with all of these components. There are options and necessities which are dependent on the steel building application, the size and, the specific building codes and loadings for your particular building on your particular site. Several components must be added when there is, for example, a mezzanine or a crane in the building; or when there is snow shadow or significant point loads. But for the most part, this diagram is a good place to begin your introduction to the basic elements that make up a simple metal building system with a Rigid Frame.
The Structural Support System of a Pre-engineered steel building is divided into 3 main parts:
These 3 structural systems are engineered to absorb the loads and forces that are acting on the building and they transfer those loads into the foundation system.
When we speak about the Primary Structural Support of a Pre-Engineered Steel Building, essentially, we are addressing the framing of the building. The Primary structural support system is the main support of a pre-engineered steel building. For this reason, we often refer to the primary structural support systems as the main framing system of the building. It follows that when a frame is at an Endwall, it is called an Endwall Frame, and when a frame is within the building, it is called an Interior Frame.
Rigid Frames and Endwall Frames are laid out in a carefully engineered pattern to provide the primary support for the building. The distances between the centerline of each of the frames is called a Bay. Bay Spacing is an important factor in determining the design of your structure. The required width of each bay is something that customers should consider because it will have an effect on where you are able to place windows, service doors, and larger garage, overhead and bi-fold doors along the sidewalls of your steel building.
The rigid frame is the most commonly used frame in pre-engineered Steel Building Systems, and so it stands to reason that it is responsible for why we identify this as a Rigid Frame Pre-Engineered Steel Building. It is the Rigid Framing System on a pre-engineered steel building which offers the structural integrity to this construction. Without the Rigid Frame, there is no pre-engineered solution.
Generally speaking, a Rigid Frame is designed to bear the load for half the bay of the roof on either side of its location.
Although the majority of Rigid Frames are typically clear-span in design, the Rigid Frame is also available in a modular version.
Rigid Frames are either Clear Span (no interior columns) or Modular (with columns). Modular frames have 1 or more interior columns between the exterior columns, that are responsible for supporting some of the vertical load that is carried by the frame.
Tip: A Rigid Frame can be used in the interior framing as well as at an Endwall of a steel building
By far the most frequently used primary framing in a Pre-Engineered Steel building is the Clear-Span Rigid Frame. We call these frames “Clear-Span” simply because they have no interior columns or support between the exterior columns. An amazing thing for all customers looking to achieve 100% usable interior space! In fact, it is the Clear-Span Rigid Frame that revolutionized construction and made metal buildings such an incredibly versatile and innovative investment.
The typical Rigid Frame on a steel building has tapered vertical columns and a tapered rafter. These are the I Beams of the steel building - meaning that if you took a cross-section through either a column or rafter section, you would have an I-shape.
Typically, the rigid frame is thickest in the area where the column is connected to the rafter beam. We call this area, the Haunch. The haunch is the thickest part of a Metal Building system simply because it is a load bearing spot in the building – it must support great force, and so, we must reinforce that area with steel.
Tip: Often, we have a customer who is trying desperately to achieve every square inch of usable space. In such cases, we employ a straight column rigid frame instead of our standard tapered column. This also comes in handy when we are trying to achieve a certain clearance under the haunch that we would otherwise not be able to accomplish. Rigid, Clear-span Frames with straight columns are usually less cost effective because they require the building be fortified in other (less ergonomic) areas.
Often, a customer does not take issue with having a clear-span design. In such cases, we have the option of using a Modular Frame. Modular frames are simply Rigid Frames with intermediary columns, and the spacing between the intermediary columns does not have to be equal. What is important here is the understanding that these extra columns provide strength and integrity to the rigid frame, redistributing the load of the force exerted on the haunch. And because the columns provide strength and integrity to the overall structure, Modular Frames can be used to bring the overall cost of a structure down. When used where they can be hidden within the architecture (e.g., in aisles or bleachers) the use of a Modular Frame does not impact the overall design of the interior of the steel structure.
The clever use of both types of framing is one way that your experienced Norsteel Building consultant will work with you to achieve the best solution for your specific building. Depending on the application for your steel building, we may incorporate both rigid frames as well as modular frames within the same design.
Tip: The dimensions between interior columns are measured from the outside of the sidewall girt to the center-line of the first interior column. On all other interior columns, the column spacing dimension is measured from the center-line of one column to the center-line of the next column, until the last interior column is reached. The column spacing on the last interior columns is again measured from the center-line of the column to the outside of the sidewall girt.
The most common Endwall frames are called Post and Beam Endwalls. As the name implies, Post and Beam frames are comprised of Corner Posts, End Posts and Rake Beams. They are designed to support the load for half a bay of the roof and to support the addition of framed openings for windows, service doors and, garage or roll-up doors.
In some cases, a Post and Beam Frame may be used as an Interior frame. We call this a Recessed Endwall. In such cases, a rigid frame is used to provide a huge overhang porch and the post and beam is used in the second bay as the entrance way. Many farm-house designs are laid out in this way in order to achieve the look of a large covered front veranda. Such designs are also used for drive-through windows of retail outlets, and for some car ports.
In still other cases, the Endwall Frame must be designed as a Rigid Frame, so that the structure is already prepared for future expansion and length may be easily added to the structure at a later time. In this case, we call it an Expandable Rigid Frame Endwall. In some cases, an Endwall may be designed as a rigid frame so that it can be capable of holding the weight of a large opening or hangar door.
Tip: Metal buildings are relatively easy to expand by lengthening, which involves disassembling bolted connections in the Endwall, removing the wall, and installing an additional clear-spanning frame in its place. The removed Endwall framing can often be reused in the new location. Matching roof and wall panels are then added to complete the expanded building envelope.
When we are speaking about the Secondary support system of a pre-engineered metal building system, we are referring to the Purlins and Girts. These structural members are the steel components that run horizontally across the roof and walls, spanning the primary framing.
Girts and Purlins are essentially the same with the only difference being their location on the building, and they have 2 main functions: To transfer the loads to the primary framing, and to provide a surface onto which the roof and wall panels are screwed.
We refer to the secondary system that provides structural support to the roof as the Purlins.
Purlins run horizontally between the frames of the roof. They are all-steel, z-shaped members. The purlins on a rigid frame steel building are typically 8 and 10 inches deep, but are available in 12 and 14 inches when loading conditions require thicker reinforcement. The depth of the purlin as well as the spacing, is determined by the engineering itself and is dictated by both the design of the structure as well as its location.
The job of the purlins is to transfer roof loads to the primary structural support system, which in turn transfers the loads to the foundation.
We refer to the secondary system that provides structural support to the walls as the Girts.
Girts run horizontally between the frames of the walls and are attached to the columns. They are Z-shaped, similar to the purlins on the roof, and also come in C-shape which can be used around framed openings. The Girts take the loads imposed on the covering system and transfer them to the frames, which in turn, transfer them to the foundation.
The spacing of the Girts, varies with the load imposed on them. The depth of the Girts used in a structure is also determined by the particular loading requirements for that structure. Like purlins, girts are typically 8 or 10 inches in depth. Occasionally, 12 and 14 inch secondary members are necessary for specific applications and site locations.
Endwall Girts: The Girts on the Endwalls are Inset within the post and beam framing. This means that the girt is flush with the primary framing and does not take away any clear space from the interior of the structure.
Sidewall Girts: On the Sidewalls of the structure, standard practice is to provide bypass girts. All software design programs default to this type of girt. This means that the girt bypasses the frames and is attached on the outside of the primary frame line. In many cases, you have the option to recess, or inset the Girts within the framing. When customers are trying to maximize interior space, we use flush or inset girts as opposed to bypass girts.
Tip: It is important to take the diminished clearance imposed by the structure itself into consideration, when determining the actual measurements of interior space.
One last thing to mention when we are discussing secondary structural members of a metal building rigid frame system, is that the Eave Strut is located at the intersection of the roof and the exterior wall. It acts as both the first Purlin and the last Girt. This is important because the building eave height is measured to the top of this steel member.
The final structural support system of a Rigid Frame Steel Building is the Bracing system. As its name implies, the bracing system functions to resist the forces of the elements and to transfer these loads to the secondary and primary framing systems. There are several types of Bracing Systems that are used in Pre-engineered Steel Buildings. The most common are Rod Bracing; Portal Frames and Diaphragm Bracing.
Rod bracing or X-Bracing, is by far the most common type of bracing used to brace the building against forces from the elements, on a steel building system. The rod bracing in a steel structure is found in the roof between the rafters, and in the walls between the columns. Where the bracing goes and how much is needed depends on the design of the building and its location. A structure that is situated in a municipality with high wind load for example, will require more bracing than one in a different municipality with mild wind. Likewise, a structure designed with an open wall, will require more bracing than a structure that is completely closed.
Tip: The location of Rod Bracing on a rigid frame steel structure is pre-engineered and cannot be moved on site without seriously affecting the integrity of the metal building system. When you work with your Building Consultant to arrive at a design that you are happy with, one important thing to discuss is the location of service doors and windows. Although both are field located (meaning that their precise and final locations can be determined on the job site), it is important to ensure that the bracing for the building will not interfere with the desired openings.
Where it is not an option to use X-Bracing in a steel building, or when we require more strength in order to provide rigidity and structural stability to the metal building, a portal frame is used. Essentially, a portal frame is a rigid frame that is pre-engineered to reinforce the steel structure. By lining the bay along the sidewall of the metal building, the portal frame reinforces the structure to resist strong longitudinal loads. Typically, if there is a portal frame on one side of the building, there is also a corresponding portal frame directly across from it, on the other side of the structure.
Tip: Again, here it is important to take the diminished clearance imposed by the structure itself into consideration, when determining the actual measurements of interior space. The portal frame is as thick as a rigid frame and will add significant feet in all directions. Make certain to discuss the minimal clearance available in each bay with your Building Consultant so that you are clear what size door openings will fit in the space.
Another type of bracing that is used to establish structural integrity in a pre-engineered steel building is Diaphragm Bracing. Diaphragm Bracing is inherent in most metal building systems, and results from the presence of the cladding – the exterior panels, that cover the primary and secondary framing of the steel building. Cladding provides the building with aesthetically pleasing colour, and it obviously finishes the wall and roof skeleton of the building. In doing so, it provides yet another defensive bracing system against the loadings that impinge on the structure itself.
Tip: There are several different options for Wall and Roof systems. Insulated Metal Panels; stucco finishes, faux wood and brick options - Wall cladding comes in several different configurations and several colours. The possibilities are numerous and will be discussed in more detail in future blogs.
In this blog, we went over the structural support system of the Rigid Frame Steel Building System. We discussed the primary and secondary structural systems and we discussed bracing. We discussed how these 3 structural systems are engineered to work together to absorb the loads and forces that are acting on the steel building and to transfer those loads into the foundation system.
An understanding of these structural components will help customers to gain an appreciation for the engineering behind their steel buildings, and how they are constructed. It will help them to plan for the design of their steel structures and for the location of functional access items like doors and windows. Understanding the structural support systems of Rigid Frame Steel Buildings – how they are engineered to work together and why they are important in the design of their structures, is a great way for customers to begin a dialogue with their Building Consultant. It will also help customers to discuss construction when they are working with general contractors and erecting crews.
Here we have provided a general overview of the structural support system of a Metal Building System that uses a Rigid Frame design. In a future blog, we will discuss other components, things like door and window framing; gutters and downspouts; overhangs, insulation, and other accessories which can be added to all our Pre-engineered Steel building products.
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