Wind and earthquakes impose lateral loads on a structure that must be resisted. We call these loads “drag loads” in the MiTek Engineering software. While the application of wind and earthquake loads is very different when it comes to truss designs, they both are designed as if they are horizontally applied to the structure when considering the overall resisting system.
Wind loads develop pressure on the wall and roof elements. The magnitude of this pressure is proportional to the square of the wind speed. Earthquake loads create ground movements and the structure within the specific earthquake zone must be able to withstand the lateral accelerations caused by these movements. Both wind and earthquakes create powerful torsional forces within the structure what can shear a building apart. Wall, floor and roof systems must be designed to resist these lateral forces in addition to supporting vertical loads. In accordance with ASCE 7, a Drag Strut is a structural element (could be a truss) installed parallel to an applied load that collects and transfers diaphragm shear forces to the vertical-force-resisting element or distributes forces within the diaphragm or shear wall. Properly designed drag strut trusses, shear walls or roof diaphragms and their connections will transfer lateral loads to the foundation and then safely into the ground.
Using the MiTek Engineering software, the process of designing the drag strut truss is easy. The truss technician or the truss designer is not responsible for calculating drag loads in the structure. Location, magnitude and direction of these loads must be provided by the building designer (required to be registered professional / structural engineer) as a total load in pounds (lbs.) or as uniform load in pound per lineal foot (plf). Once loading information is provided the MiTek Engineering software has the ability to handle drag loading automatically using the appropriate dialog window.
Let’s run an example to see how the tool functions.
Let’s assume the Framing Plan indicates drag force (DF) = 5.0K (5,000 pounds). First the Truss Manufacturer must place a truss at the location of the DF designation in line with the direction of the DF. Second, the Truss Manufacturer needs to design this Drag truss to be able to collect the lateral load on the top chord, where this load is applied and transfer it to a full or partial shear wall usually in-line with the truss either vertically (bearing on the bottom chord of the truss) or horizontally (truss span - an opening between two linear walls). For this example we will transfer the lateral loads vertically to a shear wall below. Let’s assume the shear wall plan specifies locations of the shear walls as follows –a 5’ long shear wall in line with the drag strut truss at the left end and a 7’ long shear wall in line with the drag strut truss at the right end of the drag strut truss. Since the framing plan does not specify whether the drag load is due to lateral loads generated by the wind or seismic forces the conservative approach is to apply drag load in all load cases.
Step 1 – choose the Loading Dialog Window and check on Drag loads.
Step 2 - click on the Drag Load tab at the top of the Loading Dialog Window.
Next specify a Total Drag Load of 5000 lbs as given in this example and Horizontal Drag Load Resistant Locations along Bottom Chord.
Distance 1 – the starting point for the drag load resistant location along bottom chord. Since a 5’ wall is directly in line on the left end, select from left, and distances will be from 0 to 5’
Distance 2 – ending point for the drag load resistant location along bottom chord. Since a 7’ wall is directly in line on the right end, select from right, and distances will be from 0 to 7’
Edit DOL’s default for Dry lumber and Plate grip is set to 1.33. You can change the duration of load to 1.60 but be aware some building jurisdictions may require 1.33.
After selecting OK you will be returned to the Truss Basics Window. The Drag Load cases will be created automatically, applying the specified drag load in each load case. Existing load cases will be duplicated twice, once with a drag load direction to the left and once with drag load direction to the right.
Below is the picture of the Regular Drag Load Case #1 Left. As you can see, the Drag Load is applied as a plf across the entire top chord (5000 lbs : 24’ = 208.33 plf) and resisted by an opposite force over the user specified locations on the bottom chord (5’ + 7’ = 12’ – total length of locations specified on plan; 5000 lbs : 12’ = 416.67 plf).
After the truss is analyzed a note can be found in the notes section of the Engineering Drawing stating “This truss has been designed for a total drag load 5000 lb. Connect truss to resist drag loads along bottom chord from 0-0-0 to 5-0-0, 17-0-0 to 24-0-0 for 416.7 plf”. Uplift/overturning reactions can be found in the reaction section of the Engineering Drawing.
If you have Engineering version 7.2.0 or later you can take advantage of using the Advanced option on the Drag Load Dialog window. Let’s review a few combinations from Advanced option which can help you make the design of drag strut trusses more accurate.
The Advanced Drag Load Dialog Window defaults to “Wind” + “Seismic” and “Include all live loads”. This is the equivalent of the original Drag Load Dialog Window that was described in example above.
In most common roof truss designs, drag loads are mostly from the wind forces. If drag loads are due to wind, the most critical load cases to apply these loads are wind load cases.
When “Wind” and “Include all live loads” are checked, the drag load will automatically be applied in each wind load case. Existing load cases will be duplicated twice, once with a drag load direction to the left, and once with drag load direction to the right.
Selecting “Wind” and “Include only dead loads” will apply drag load in wind load cases that do not contain live loads. The dead loads that resist the uplift from the wind are restricted to being 0.6 times the dead loads used in general load tab, except on certain types of agricultural use trusses.
“Seismic” in Advanced option is used in regions where seismic activity is the critical drag load factor. If drag loads are due to seismic forces the most critical load cases to apply this load are non-wind load cases. There will be no drag load applied to wind load cases.
According to section 1605.3.1 of IBC 2006 - “flat roof snow loads 30 psf or less need not be combined with seismic loads”. That is why if you are in non-snow area you do not have to have live load in drag load cases, unless otherwise specified by building designer. Selecting “Seismic” and “Include long term loads” will automatically remove short term live loads and will leave floor loading, storage loading and dead loads in the basic non-wind load cases to which drag loads are added.
If you are in an area where flat roof snow exceeds 30 psf, select “Seismic” and “Include all live loads”. The Drag Load cases will be created automatically and specified drag loads will be applied in each non-wind load case. Existing load cases will be duplicated twice, once with a drag load direction to the left and once with drag load direction to the right.
The Advanced option will cover most of the requirements a building designer may have with drag loads. Selecting “Wind” will create drag load cases for wind load cases only. Selecting “Seismic” will create drag load cases for non-wind load cases only. If you select “Include long term loads” program will remove short term live loads from the load cases but leave floor, storage and dead loads in load cases to which drag loads are added. Selecting “Include only dead loads” will remove the base load cases of all but dead loads. These items are specified for each loading condition, so it is possible to have seismic only drag and wind only drag loading conditions for the same truss.
The program will not automatically print drag load cases on the truss design drawing. Unless otherwise specified, it will only print the note(s) in the note section of the Engineering Drawing, similar to the one that was shown in example above. The note will state whether truss was designed for wind or seismic drag loads, and what load combinations were chosen.
It is the responsibility of the truss technician or the truss designer to review and understand the building designer’s drag loads detail and incorporate these requirements into the design of the roof truss system.
Please note that when designing drag trusses, it is recommended that the truss be fully triangulated so as to avoid uplift reactions that cannot realistically be connected to the supporting structure.
If you have any questions regarding above information please feel free to contact the MiTek Engineering Team.