The following is a list of some of the most common problems MiTek engineers see when reviewing customer truss submittals. Often the trusses need to be changed in order to be sealed and the objective of this document is to avoid truss designs that are sent to engineers to be sealed and then must be changed.
7 Most Common Errors
1. Wrong building or wind codes.
Be familiar with what building code your state requires and be sure to use a wind code compatible with the building code you are using. Example: Pennsylvania requires IBC/IRC2006 at the time of this writing which in turn requires ASCE 7-05 for wind and snow.
2. Special connection required to distribute bottom chord loads equally between all plies.
Be sure it is possible to transfer loads from one ply to the next on girder trusses.
Each ply of a girder is designed to carry an equal amount of load. When load is attached to one face, a portion will have to be transferred to the other plies. If the loads cannot be transferred, the warning “Special connection required to distribute bottom chord loads equally between all plies” will appear in note #1 on the truss drawing. Try going into design info, nails/screws/bolts, and check the “use verticals for load distribution” box. This will greatly reduce the amount of times you get the special connection note. You can also try changing the type of nail or fastener you are using. In addition, always try to add vertical webs directly above large concentrated loads. If these options do not fix the problem, submit the truss to your design engineer for review.
3. Bearing undersized.
Many times you will come across trusses that give the warning “Required bearing size at joint(s) are greater than input bearing size.” This is due to the bottom chord and/or top plate crushing from the reaction of the truss.
One way to correct this problem is go into Design Info, click on Bearing Design Options, and check any of the options available. These options include upgrading lumber, using a bearing block, or using a truss bearing enhancer. If you have these options checked, the program will attempt to fix the undersized bearing problem. If the bearing is still too small you may need to either increase the actual size of the bearing or add a ply to the truss.
If a vertical member comes down directly above a bearing, like a web at an interior bearing or at a raised heel, you may be able to run the vertical through the bottom chord so the end grain of the vertical member is sitting directly on the bearing, which gives a much higher crushing value. Be aware this will fix the crushing on the truss only and not the top plate of the wall. The program assumes you are using the same material on the top plate as the bottom chord of your truss unless set up otherwise.
When using the Bearing Block option to fix an undersized bearing, care must be taken on trusses with long scarf cuts at the heel. The Bearing Block ignores the scarf cut when calculating the nailing requirements. The nailing pattern and quantity of nails shown may be impossible to achieve. Also note bearing blocks are assumed to be the same lumber grade and species as the bottom chord. When in doubt, have these trusses reviewed by an engineer.
4. Gable end trusses run as interior wind zone.
If a truss is at the end of the building, be sure the roof zone is set to exterior or gable end zone in the wind.
5. Corner girders missing load.
Many times, on cantilevered corner girders, you will need to make sure additional load for framing and fascia loads are added at the end of the girder or cantilevered section. Since framing is often used instead of trusses, layout does not put this additional load on the end of a corner girder. If a structural fascia is being used, a concentrated load will need to be added at the end of the corner girder and also at the end of the other truss which will be supporting the other end of the fascia. This can be achieved for symmetric corners with Girder loading in the engineering software by checking the “cantilevered corner” box in version 6 or later.
6. Reducing the spacing on Girder trusses without increasing the dead loads.
Do not lower the truss spacing on girder trusses. If you must do this, be sure to check the dead loads on the girder to be sure the truss has been checked for the proper loading. An easy way to do this is to run the truss and preview it to see the weight of the truss. Take the weight and divided it by the span of your truss in feet. Then divide that number by the truss spacing. This number will be the minimum amount of total dead load you will need to have to account for the weight of the truss. You will also need to add any additional loads for material such as sheathing, shingles, drywall, or insulation. Increase your TCDL and BCDL to the appropriate number based on these checks. Again, it is much easier to simply leave the girders at the normal truss spacing.
7. Thermal factor needs to be selected correctly.
Trusses that are designed for IRC should generally be designed with a Ct factor of 1.1 because the IRC states that the minimum insulation to be used in normal heated residential structures is R-30. ASCE says that if an R value of 25 or greater is used between the heated and ventilated space, a Ct factor of 1.1 is to be used. Thermal factors should only be set to 1.0 if the roof trusses will be heated and kept above freezing at all times. This may occur in some commercial buildings. Please also be aware that if a Ct factor of 1.0 is used, a 2X overhang snow factor is then required. The same discussion holds true for agricultural trusses. If the structure will be unheated at some point in its lifetime, it should have a Ct factor of 1.2.
Additional Areas of Concern
8. Manually changing heel conditions such as wedges or sliders (turning heel solving off).
9. Raised heel valley trusses.
Valley trusses will not have sheathing applied to one of its faces to stabilize it so be sure to add a web and triangulate at least one end of a valley truss with raised heels.
10. Flat trusses with a parapet.
If a truss with a flat top chord is on a structure with a parapet surrounding it, additional loads should be added to the top chord for snow drifting.
11. Large trusses not triangulated at the heels.
In order to cut costs, sometimes designers leave out webs that form the triangles which make a truss structurally sound. While these trusses may run, some designs may not be structurally viable and should be avoided or reviewed by an engineer. To avoid this, add webs to triangulate trusses, especially at the heels if the truss span is 20’ or longer.
12. Long bearings that end in the middle of a truss panel.
If you have a long bearing where the end of that bearing is not located at a joint (i.e. the end of the bearing is in the middle of a panel and there is no joint number associated to the end of the bearing), the program may not be checking the truss for induced shear and bending at that location.
If you see one of these long bearings with no joint number at its end, it may be necessary to add an additional smaller bearing at the end of the larger bearing. This is especially true for girder trusses. An example of how to check this is as follows:
If you have a 6-0-0 bearing ending in the middle of a truss panel, you can input a new 0-3-8 bearing at 6-0-0 and aligned at the end of the longer bearing.
The two bearings will actually overlap as you can see in the second picture. You will notice a new joint number (15) being inserted at the new bearing location and it will now give a reaction at the end of the bearing. An easy way to avoid this is to run a web down to the end of a large bearing.
13. Top chord bearing floor trusses.
When using top chord bearing floor trusses, it will usually be required to have a double top chord above the bearing. Be sure the stacked top chord extends far enough into the truss so it can be attached with at least 2 plates. If it does not plate the stacked member with at least two plates, extend it in Versatruss until a second plate is added or simply run it back to the next vertical web.
14. Interior Raised bearings.
When you have an interior bearing located on the upper portion of a step-up in the bottom chord running just beside the vertical, you may want to run a web down to the bearing to help the shear at that location. Usually you can accomplish this by reversing a diagonal web or adding another vertical web along the existing vertical and above the bearing.
15. No room load on attic trusses.
When you create an attic truss be sure to define the truss as an attic under Truss Application in Truss Basics. This will ensure your attic has the required additional load added to the attic room area. If an attic truss is created using Versatruss, you will need to manually add the additional loads to the floor and ceiling area.
Also, if you have sloping walls in an attic, the program will not add any load to the room even if it is defined as an attic in Truss Basics. Load will need to be added manually to the floor, ceiling, and walls on these truss types.
16. Splicing in heel panels and two splices in the same panel.
While the program checks all of the following properly, it is still recommended to keep top and bottom chord splices out of the first panel of a truss. Try to move the splices beyond the first web so the chord member will be attached by at least two plates before the splice occurs. It is also recommended to keep from having multiple splices in the same truss panel and to avoid placing multiple splices in an Attic room. Finally, avoid splicing in the top chord of an attic truss in the slopped ceiling area of the room.
17. Maximum of one lumber size change at mid panel splices.
Never splice a member to another member that is one lumber size larger or smaller than itself in the middle of a panel. For instance, if a splice falls in the middle of a truss panel you can splice a 2x4 to another 2x4 or a 2x6, but not to a member larger than 2x6. A 2x6 may be spliced to a 2x4, 2x6, or a 2x8, but not a 2x10.
If you are required to change more than 2 lumber sized at a splice location, move the splice to the next location as shown in the example below.
18. Trusses with a notched bottom chord for ledger bearing.
If you have a truss where the bottom chord is cut back to accommodate a ledger type bearing (i.e. the end vertical sits on the ledger bearing), be sure to check the reaction at this location. Many times the bottom chord, end vertical and a web all meet at the raised bearing and if the reaction at that location is more than 500lbs you will need to rotate and/or increase the plate so part of the plate is actually above the bearing. This bearing coverage by the plate is to help prevent the end vertical from shearing.
19. Valleys and piggy backs with large vertical web spacing.
For valley and piggy back trusses, be sure the vertical studs do not exceed a 4-0-0 on center spacing. It is also recommended to add vertical webs at pitch breaks.
20. Tray ceilings near the bearing.
When you have a condition where the bottom chord has a step up very close to the heel, sometimes the truss will need to be analyzed as “tail bearing”. You will find this when the first horizontal section of the bottom chord extends out for a short distance and then steps up to intersect the top chord and upper horizontal section of the bottom chord at the same location. In situations where all three of these members meet at the same location and all three share one joint number, the vertical member will need to be analyzed as a web instead of a bottom chord. This will ensure the truss is being checked properly.
In this situation member 7-10 should be defined as a web. An alternative way to check this truss would be to run members 8-10 and 7-10 as non structural.
21. Uplift for first load case exceeds limits.
Occasionally you will get this note on trusses due to gravity uplifts that exceed 1000lbs. This is usually caused by having two bearing locations very close together on one end of the truss. The interior bearing acts as a pivot point causing the short end of the truss to want to pull up. You can either connect the truss for the large amount of uplift or you can try to eliminate the uplift by removing the outside bearing and running the truss as a cantilever. If this doesn’t solve the problem, submit the truss for review.
22. Allowable Top Chord Reaction Exceeded.
This error can occur on floor trusses with a raised bearing. When the bearing is raised, the truss will be analyzed as top chord bearing. If the first diagonal web coming off the bearing starts on the bottom chord, the allowable top chord reaction is very low. Try reversing the first diagonal web so it starts from the top chord and proceeds downward to the next joint. This will greatly increase the allowable reaction limit at that location. This will occasionally happen on roof trusses also. Try reversing the first web in this situation as well.
23. Top chord bearing roof trusses/raised bearing truss.
If you have a top chord bearing truss or a truss with a raised bearing sitting on a block, you will have two end verticals. Be sure to define the member sitting on the bearing as a block and define the member stacked next to it as a top chord. This will ensure proper plating.
24. Maximum panel lengths.
For roof trusses, use a top chord max panel length of 8’ for a 2x4 and 12’ for a 2x6. Use a maximum bottom chord panel length of 12’ for a 2x4 and 14’ for a 2x6.
For floor trusses use a maximum 30” top chord panel length and keep all duct opening to 24” maximum.
25. Insufficient Scab nailing.
If you are using the scab feature and get the note “special connection required between scab…” the scab does not work because there is not enough room to nail for the forces. Try extending the scabs length or switching the type of nail used until the program is able to call out a nailing pattern for the scab. If these options do not work, the truss should be reviewed by an engineer.
26. Web bracing does not fit.
When using T & I bracing, the program will show a web with the brace on it whether the brace physically fits or not. These trusses must be reviewed and either changed to allow the brace to fit, upgrade the web until a brace is not required, or some sort of alternative bracing will need to be called out by an engineer.
27. Incorrect dimension lines
All joints and pitch breaks must be properly dimensioned. The engineering program will automatically add the dimensions at the proper locations when creating a truss. However, if joints are moved or added or an attic room size is changed using Versatruss, the dimension lines may no longer reference the proper locations. If any of these design changes are made using Versatruss, you will need to delete the old dimensions and add the new dimensions at the proper locations.