I Beam help needed

[drm]

Eddie,

For what it is worth, and a bit late, I will once again butt in with my opinions.

What species of lumber is stocked in PT wood out your way? I would assume that the grade is #2 but you should ask. When I worked in Chicago, some time ago before wood went through the roof, we would use Douglas Fir Larch or Hem Fir. Here in PA we get SPF or Southern Pine. It is all regional. This is important since the design values can vary greatly depending on the species and grade.

Assuming you are going to use #2 Southern Pine spanning continuous over the middle support, I would use 2x10’s. The reason I would not use 2x12’s is that I have had a tough time finding straight 2x12’s that long and the 2x10’s are a little lighter to work with. I built a platform sticking out over a slope with continuous 2x10’s x 20’. I was able to erect all but the back columns, 6x6x10’ high posts, myself and it was plenty strong to support a prefab shed 12’x20’ fully loaded. I used 3 rows of double 2x10’ with 2x6’s (12’ long) spanning across. So I effectively only had 6 2x10’s over 12’ width. Not as much load but very stiff due to the continuous spans of the 2x10’s and 2x6’s.

Next is what spacing. 16” spacing would certainly work but 12” spacing will give a bit more rigidity. It is up to you.

Most importantly is the blocking between the 2x10’s. In a two span continuous beam like you will have the largest stresses will be at the center support and a bit past 1/3 points from the ends. The center support stresses will be from negative moments meaning that the top of the beam will be in tension and the bottom in compression. The bottom, since it is in compression, will want to buckle or kick out sideways. Therefore you must have full height blocking at the center support. You also need full height blocking at the midpoints and ends as a minimum. Providing them at third points instead of the midpoints would be better but probably overkill.

Next is the attachment of the railing. I would use the blocking at third points since I would use posts at the ends and third points and blocking behind the post attachment is imperative. Spanning a railing 5 feet seems too long to me.

I would go with 5/4x6 decking due to cupping and put them tight.

Good luck and I will try to keep up with you guys.

…Derek

 

[CurlyDave]

With the change over to building a foot bridge, I was thinking that the decking could be PT 5/4's.

I have never seen PT 5/4s. I would still use 2 x 12's . I would put the crown side of the board up, and use 4 deck screws at each beam. Drill pilot holes at the ends of the decking boards to prevent splitting. Pilot holes are optional in the center of the decking. I haven't seen the shrinkage that others have seen, but then I am just an amateur carpenter.

I think your next challenge is going to be to make the railings on the bridge "feel" sturdy enough against an outward push. The attachment shows a design I have used on a similar bridge to greatly increase the stiffness of wooden railings.

 

[hilld]

Eddie,

instead of using PT for your decking, you might want to look at the "excotics" such as IPE or Cumaru. Both are Brazillian hardwoods and they are much stronger than PT and will outlast them. They should be treated when installed and might require a recoating every few years. The decking looks great and is about 4 times hard as white or red oak and extremely stong and dimensionally stable. Around here a 1x6 (no 5/4 needed) runs around $2.15 a linear foot, you can even buy it prefinished on all 4 sides for an additional 4 cents/ft. The only parts that need to be "retreated" are the cuts.

I am currently building a balcony and deck using Cumaru, lots of info on the web about that as well as IPE.

Hope that helps.

Derek

 

[buckeyefarmer]

Many moons ago, I worked summers on a county road crew. We used to build short bridges, easily 10-12' long over small streams out of rough cut oak lumber. Would use 2x12's or 3x12's, probably 16" oc, and deck over with 2x oak material. Would drive nails in using sledge hammers...
Helped redeck longer bridges with steel beams, and would use PT 2x4s on edge as the decking. would nail clips to the 2x4's that would clip over the edge of the I beam under it that held the decking down.

Also, architecture standards handbook, at local library, has steel load tables, similar to one someone posted. I think it shows how to do the load calcs.

 

[Parttimer]

Eddie, lots of interesting ideas going on. you are probably are aware already of the "new" pressure treated lumber's highers copper content. Consider what type of fasteners you use (nails, screws, anchors) are of compatable nature to the type of pressure treated lumber you use. I just went and reviewed a deck failure due to the use of a common hanger with a high copper content pt joist, and the hanger had corroded to failure (2 years old). just a reminder.

Dave

ps great discussions

 

[RobA]

Eddie,
How will you be attaching the blocking?

RobA

 

[EddieWalker]

I'm planning on using ICQ screws on the whole thing, or whatever they are called for the new PT wood. No nails at all.

I'm also going to use that flashing technique on the tops of the beams. Either roof felt or window flashing. Still not sure exactly what until I go shoping.

I also like the thought of putting the decking on touching. If it shrinks, it will leave some gaps, if not, it still wont hold water because of the seams.

The railings will be Cedar trees from my land with PT 2x6's as the toe kick and handle with copper half inch pipe as the railings. I'm gonna attach the cedar with lag bolts into the beams. My thought is one every four feet.

Eddie

 

[CurlyDave]

I'm gonna attach the cedar with lag bolts into the beams.

Consider 1/2" machine bolts with bridge washers. The bridge washers look really sharp and this system is a lot easier to re-tighten as the wood shrinks.

 

[drm]

Eddie,

Since you are going to use screws and bolts you may want to consider using stainless steel. They will hold up much better to the new PT wood treatment and not stain or corrode. They will cost significantly more but overall I think they are worth it and use them when not nailing.

...Derek