Stephen Hudak | Executive Vice-President | Z Squared, LLC
30%. That is roughly the amount of Primary Framing steel on the average pre-engineering metal building (PEMB). You can quickly take out chunks of money within this 30% by getting creative with your primary framing layout/bay spacing and the type of frames you use.
First, let’s look at how a framing member is designed. Remember this phrase, “Deeper is Cheaper.” All members in a PEMB have a responsibility. The term “systems construction” is more fitting than “pre-engineered” (as no building is sitting on a shelf waiting to be shipped) and the Primary framing, bracing, purlins, girts, and even wall/roof panels do their part in the total building design.
Each member is responsible for a certain amount of loading and, with primary framing, the most obvious is the tributary loading. To think about it in simple terms, take a frame line…one half of the bay on either side added together is the tributary width multiplied by the frame span is the total tributary area (see figure 1).
The resulting cost is considerably less for the frame with the interior column. A basic principle is the more you reduce the span of a member the more you reduce its cost – up to a certain point. At some point you will be adding more steel to the frame thereby increasing its cost and you must also remember that everywhere you have an interior column you have more foundation and erection expense – these must be considered into the whole building price. Plus, you may be limiting the movement inside the building for your customer with additional interior columns. If you lease space to potential customers wide-open spaces can be a selling point even though you may be paying a bit more. Remember, Value Engineering is not always about low cost, a better building is more desirable.
Let’s look at the results from the above graphic comparing the Clear Span frame with the Modular frame with one Interior column:
Every component in a PEMB has and requires a certain amount of steel to handle its loading responsibility. Primary frames have flanges and webs; Rod Bracing has a diameter; Secondary (purlins and girt) have depth and legs and gauge; even roof and wall panels come in varying gauges to add strength – this can be considered as the member’s cross-sectional area.
Remember the phrase “Deeper is Cheaper”? This where depth comes into play in the economy of design for frames. A typical clear frame has tapered members. This is to allow for steel to be where it’s needed and to be removed where it is not. PEMB framing can gain economy over hot-rolled, conventional members that are continuous in depth through the member. If the framing member’s depth must be restricted to achieve certain clearance requirements or to hide a column in a wall, for example, the depth must be restricted and in order to achieve the required cross-sectional area the member’s flanges must be wider and thicker – which is usually more costly that allowing the cross-sectional area to be achieved with the member depth.
In Figure 2 below, the depth of the Clear Span frame column is greater than that of the Modular column. Remember the tributary area is calculate such that the Modular frame is divided across the frame span.
In Figure 3, the Clear span frame has been designed with no depth restrictions and to be held at a specific depth (12” maximum straight, in this example).
- “Deeper is cheaper”
- The more you reduce the span of a member, the more you reduce the cost – up to a certain point.
- Allow tapered frames when possible. When not possible, be careful of restricting the depth too much.
Next time we will investigate the economy of symmetry, pinned vs. fixed base, and supported vs. unsupported