What is false bottom under-framing, you ask. To put it simply, false bottoms are non-structural filler chords attached to the bottom chord of a truss (see below). They are used in a number of different situations including, but not limited to double step ceiling conditions, ceiling height changes, and vaulted ceiling conditions. The benefits of using false bottoms consist of less on-site framing, less expensive truss material cost, and more efficient labor during truss manufacturing. Client preferences, architectural plans, and designer discretion determine when false bottom under-framing is useful.
Briefly, I would like to talk about how a false bottom is created using MiTek. The command can be located under the “Tools” menu in the upper left, by placing it in a toolbar, or by creating a hotkey for it. Once the command is open, the dialog is completed with the information needed for the filler.
In the location box, the option of which side to start from is provided. In this case, the filler started at 5-1/2” in from the left end of the truss. The “End at” field indicates where the false bottom stops. 6’ 8” was used in this example. The starting height of “0” indicates that the underside of the false bottom is the same as the plate height, or bearing height, of the truss. If a false bottom is being applied to create a ceiling height change of 1’, the starting height would be 1’. Either a slope or an end height may be used to create a vaulted false bottom. Extensions left or right will extend the bottom filler chord; however, the vertical filler members are not present over the extended horizontal member. “Stud spacing”, “Min. Stud Length”, “Butt Cut”, and “Stud From” are all adjustable and are used to meet client specifications and building codes.
Alright, now we can move on to the important stuff. The filler chords in a false bottom are non-structural because they don’t contribute to the matrix of the truss itself. However, when analyzing a truss with a false bottom condition there are certain aspects that the designer must consider. Normally drywall is applied directly to the bottom chord of a truss. When there is a false bottom, the drywall is applied to the filler chord instead of the bottom chord in that area of the truss. In other words, the false bottom under-framing must still be analyzed by the software.
In another instance, the filler chords require plates throughout the false bottom and plates to affix the under-framing to the bottom chord of the truss. Sometimes, based on location, plates will overlap in the area where the truss bottom chord and the filler chords join. Overlapping plates are not O.K., they are bad. When plates overlap it causes an insufficient grip between the teeth of the plate and the member of the truss. Improper plating of a truss has a significant structural impact and will cause the truss to fail. In the following example, the plates overlap slightly. The integrity of each plate is intact and so the joint plates and the truss passes.
We want to make sure that the system is setup to provide a proper analysis in the two previous scenarios. Prior to analyzing the truss, verify that the system will plate non-structural members. There are a couple ways to access the plating options in the MiTek engineering program. The edit dropdown at the top left houses the plate options menu. Plating can be viewed by going into truss basics (Ctrl+B). Another option would be to setup a hotkey for the plating options menu.
Once inside the plate options menu, scroll down until you see the following:
Make sure that “Plate separately” is selected from the drop down next to “Non-structural members”. Next, analyze the truss. Analysis of a truss will provide any necessary bracing required. Any bracing displayed by the engineering drawings must be adhered to in the field. As discussed previously, the drywall for the ceiling is applied to the false bottom. Purlin bracing must be used in those areas. If there aren’t any structural issues, the truss will pass and may be saved.
Now we are able to properly create and analyze a false bottom in MiTek. Not only are false bottoms a time saver, but also they are more economic. In a few short and easy steps, fillers may be added to a truss. The Proper analysis provides accurate truss drawings that must be replicated both by the truss manufacturer and the builder.
Those are just a few tips we have. What tips and tricks do you use?
Admin – Gould Design Inc.