Wood Header/Trimmer/King Stud Design

AZengineer · Feb 15, 2007

When designing a wood header, what assumptions are made concerning the loading of the trimmers and headers?

After sizing a header, I usually select the trimmers to provide adequate bearing area for the header and bottom plate, and assume the trimmer is taking the full gravity load. Thus, the trimmer needs to be designed as an unbraced column (unless sheathing is properly fastened to the trimmer to provide week axis bracing) and the stringent nailing requirements of NDS 15.3.3 would apply to built up trimmers.

As a practical matter, this would limit the number of 2x trimmers to 2, since 3 trimmers would require 30d nails, predrilled holes, etc, but would usually require at least 2 since the capacity of an unbraced 2x is pretty small in the weak direction. I also assume that the king stud is taking any wind load from the window/door opening that the header is spanning.

My questions are:

1. For built up headers (i.e. (2)2x8, (3)2x10, etc) subjected to bending about their week axis, is it common practice to design the nails attaching the members so that the header will not fail in horizontal shear between the members?

2. How does the IBC provide such high capacities for trimmer studs {Table 2308.9.5}? Does it assume that the trimmer is adequately nailed to the king stud so as to form a built up column? If so, the capacity of the king stud would be reduced from the biaxle bending induced (wind from one direction and eccentric header load from other direction). This would potentially require additional king studs, and therefore require a bolted column since the nailing requirements for a built up column with 4 plies would be very impractical.

3. If the king stud is assumed to take all wind load from the opening, how is this load transfered from the header into the king stud? I usually see details showind 16d nails through the king stud and into the end grain of the header, which is a fairly low capacity connection. For wide openings, the horizontal reaction of the header subjected to wind load can become quite large.

4. If multiple king studs are required for wind loading, what requirements apply to insure that they are properly fastend together to funcion as a single bending member?

5. How is the king stud typically attached to the top and bottom plates so that it sufficiently transfers the horizontal loading into the roof/floor diaphragm and foundation?

structuralaggie · Feb 15, 2007

Wow. You have really been thinking about this.

Note table 2304.9.1 specifies all fastener requirements, unless you specify other wise. I put a variation of it on all my plans. You will save you self a big head ache if you just vary from it when needed.

1. I like to show a 2x the width of the wall attached to the top and bottom of the header to help with wind loading, but nails also help.

2. I really don't like tables like this. I say calculate the loads and design the beam. A span to allowable plf table is much better. I made my own. At any rate, the trimmers are probably only be based on a bearing calculation, since no height is given for the loading. The table has no consideration for column design.

3. I think you might be right that this connection may not be adequate with nails alone based on the typical fastener schedule. But windows and doors may transfer a lot of load directly to the studs.

4. Just the fastener schedule. It also is probably not adequate to make them act compositely.

5. Also probably not adequate based on the fastener schedule.

Uh oh! They have been building things like this for years! There are all going to fall down!

To make myself feel better, I ALWAYS spec 1/2 cdx on all exterior walls. Among other things, this really helps tie every thing together and it does help transfer these loads you are worried about. It is code, so it is the minimum.

If you start doing something outrageous, like specing a simpson connector at all connections, (with exception of high wind areas on the coast) all the contractors will laugh at you and you'll run off all of your clients.

With exterior sheathing and the nails specified in the schedule, I have convinced myself that it works somehow. I guess judgement comes in at some point. Things have been built this way for so long, and they are still standing. A lot can be said about that.

There is a lot of houses that doesn't have exterior sheathing. I am convinced that it doesn't work for the components you are talking about, but more importantly it definitely doesn't work for the main wind force resisting system.

AZengineer · Feb 16, 2007

1. A 2x along the top of the header will help for shorter spans, but with long spans in tall walls, this becomes inadequate. Furthermore, longer spans will tend to see more minor axis bending since the windows and doors will have to span between floor and header instead of spreading out into the jambs as would be reasonable for small openings, so item 3 becomes a more realistic design issue.

2. So do you typically design the trimmers as unbraced columns?

3. Per item 1 above, it is reasonable to assume that windows and doors will transfer much of the load to the jambs, but in longer spans this is not the case. Prime example would be the multiple tier sliding glass doors that are bedoming more popular in high end custom homes, where an 18' opening may contain 6 doors that all slide into a pocket to create a huge opening.

4. If king studs (or any built up bending members for that matter) are connected by the fastening schedule, then they are not really going to function as a solid beam for calculating unbraced length. There must be some reduction. For example, the column stability factor Cp is reduced 25% when bracing in the built-up direction governs desing, so I would think a similar reduction would need to be taken for bending members.

5. I agree - probably not adequate...

I know that they've been buidings like this for years, but I don't think that is an excuse. Most buildings will never see the loads prescribed to their design in their lifetime, but if a jurisdiction adopts a code that prescribes given loads, it is my job to design for that regardless of how unrealistic I think they are...

structuralaggie · Feb 16, 2007

1. Agreed, but longer spans will often go to glulam because of vertical loading anyway. The full section can be used for weak axis bending.

2. No, I think you have to assume that they are attached to the kings studs and braced by the beam and the plywood sheathing and even the sheet rock. I design the king studs as a column, not the trimmers. I suppose they would help in a pinch, though.

3. Yes, but again I would imagine you would have a glulam beam here. Extra toe nails could help, but I really think between the exterior sheathing and sheet rock and nails, this connection will generally work. In special situations, I agree extra attention should be given to this.

4. I spec a lot of nails in the exterior sheathing for attachment, and then you get the sheet rock bracing it as well. The beam will support it at the level it is connected to, the trimmers help, and then you got nails too. I think this one is going to work too.

At any rate, there are redundancies that occur in wood framing. Small spans in low wind areas are usually not a problem with standard connections. The coast is a different animal though. I strap and brace everything even if it doesn't need it.

I'm not trying to discourage you from designing to code, but to think about why some stuff has been working for years. My best advice is that plywood is our friend.

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