Stephen Hudak | Executive Vice-President | Z Squared, LLC
Last time we introduced the phrase “deeper is cheaper,” discussed reducing the span of a rafter by adding interior columns, and depth restrictions versus tapered members. Today we continue our learning with more Primary Framing concepts (that can also be applied elsewhere in PEMB design). First remember the Key Takeaways from last post:
Let’s look at symmetry in geometry and design; pinned vs. fixed base columns; and supported vs. unsupported framing members. The more symmetrical a PEMB’s geometry is usually translates to a less expensive building (or course there are many variables that can impact this). The below chart shows the weight differences (and weight translates to dollars – as steel prices continue to rise).
Symmetry in Geometry
Table 1: Symmetry in Geometry
In the above table the weight differences are slight, but as the building’s geometry increases (width x length x eave height) the cost percentages will be greater. As your snow load increases, the unbalanced snow placement model will also impact cost.
Unbalanced snow is considered for gabled roof pitches >= ½: 12 (see Image 1).
Image 1: Unbalanced Snow Load Model
Symmetry in Interior Column Spacing
Last time we discussed how adding interior columns (up to a certain point) decreases the span of the rafter (or bay spacing) decreases the cost. A rule of thumb I use (where my Ground Snow load rarely exceeds 25 psf) is if the building is 80’-0” or greater in width I consider adding interior columns. I’ve found over time that around 40-50 feet is economical for my loading needs. Remember to consider the additional foundation and labor costs associated with each interior column.
The below table shows four scenarios applicable to symmetry in interior column spacing. The clear span rigid frame is to show how adding interior columns can lessen cost. The other three options show the more symmetrical the interior column spacing that the frame cost us usually less.
Table 2: Symmetry in Interior Column Spacing
Symmetry in Bay Spacing
Symmetry can also translate to the bay spacing of a project. Many times, you have a set of drawings that show where the frame lines must be, but, especially in design-build situations you can make VE recommendations to have symmetrical bay spacing. Particularly in longer buildings where you have more purlins and girts, the erection costs can possibly be lowered if the number of unique parts is decreased.
Table 2: Symmetry in Bay Spacing
In the above example, the weights did not vary significantly, but the different number of purlins has. This can be a benefit to your erector.
Pinned vs. Fixed Column Base
Nearly all metal building manufacturers design their column base connections as pinned. The moment is taken at the deeper haunch and rafter sections (insert phrase “deeper is cheaper” here).
A fixed base connection may be suggested or required at high seismic area, heavy cranes, or where there are no exterior columns (as in a T-Shaped canopy). Think of a “flagpole” where it attaches to the ground and then extends skyward. As the wind whips that flag and the pole wants to topple, the foundation is required to prevent the twisting.
However, your PEMB company should NEVER surprise you with fixed base columns as it may decrease the steel costs but will certainly increase your foundation costs. The below chart compares a frame designed with the standard pinned base versus the same frame redesigned with a fixed base condition.
Table 3: Pinned vs. Fixed Column Base and Anchor Bolt diameter, quantities, and base plate thickness
- Symmetrical building geometry (ridge location) is usually more economical over unsymmetrical.
- Symmetry in interior column spacing and lower the frame cost.
- Symmetrical bay spacing will usually lessen the different number of part marks which will be benefit your erector. Time is money.
Next time we will discuss Supported vs. Unsupported Primary Framing and consider “deeper is cheaper” where it applies to secondary members.
Do not forget to check out our website at www.zsquaredsteel.com and contact us for your PEMB furnish-erect needs!