question on ridge beam and platform framing

[dablack]

We are talking about a story and a half.  Full upper story.  The building is 26' wide.  I'm going to use a beam down the middle of the building to support 13' long 2nd floor joists.  Upstairs, I'm thinking ridge beam.

Question 1.  If I use a ridge beam, can I platform frame the 2nd story wall?  I'm 99% sure I can but just wanted to be possitive. 

Q2.  Instead of posts that support the ridge beam, can I have a load bearing wall that runs the length of the building under the ridge?

thanks
Austin

[Don_P]

Yes and Yes.
The balloon framing that you often see here on sidewalls is actually quite rare in conventional framing. It is typical to platform frame the second floor and use tied rafters or ridgebeam construction, or... (Q2) a wall under the ridge. If there is a load bearing wall supporting the ridge there does not need to be a ridgebeam, the wall is carrying the load.

[dablack]

I'm assuming a load bearing wall would have to be pretty stout.  I'm assuming 2x6?  I'm going to have to be busy on google tonight.  I haven't seen much info as far as design of a load bearing wall.  I know how much load per linear foot it has to take but I'm not sure how to apply that information. 

Also, if that wall is transfering that load down into the 1st floor ceiling beam, then those beams have to carry the roof load as well as the 2nd floor load.  Correct?  hmmmmm   That is the first time I thought of that.  Right now 10x12 pine beam 13' long will handle what I want to do.  I don't know if it will handle the roof loads too. 

Thoughts?

Austin

[Squirl]

I'm assuming a load bearing wall would have to be pretty stout.  I'm assuming 2x6?  I'm going to have to be busy on google tonight.  I haven't seen much info as far as design of a load bearing wall.  I know how much load per linear foot it has to take but I'm not sure how to apply that information. 

Nope.  It has to be the same standard as any exterior wall. 2x4 24" O/C to support just the roof, 2x4 16"  or 2x6 24" O/C to support the roof and a floor, etc.

A bearing wall is a bearing wall.

R602.4 Interior load-bearing walls. Interior load-bearing walls shall be constructed, framed and fireblocked as specified for exterior walls.

TABLE R602.3(5)
http://publicecodes.cyberregs.com/st/ny/st/b400v10/st_ny_st_b400v10_6_sec002.htm

Question 2.  Yup.

TABLE R502.5(2) GIRDER SPANSa AND HEADER SPANSa FOR INTERIOR BEARING WALLS (Maximum spans for Douglas fir-larch, hem-fir, southern pine and spruce-pine-firb and required number of jack studs)

http://publicecodes.cyberregs.com/st/ny/st/b400v10/st_ny_st_b400v10_5_par018.htm

[Don_P]

Squirl typed faster but here's where I was going

2x4 with drywall or some form of sheathing up to ~10' should be fine for normal loads. The sheathing braces the studs in the weak, 1.5") direction. Above about 10' the potential for buckling out of plane, in the 3.5" direction, would make 2x6's a better choice.

The 2nd floor girder needs to be able to support the 2nd floor and the roof load. From the description it sounds like that beam has a 13' span under the ridge and is supporting 13' of floor and roof width perpendicular to the ridge (half of the 26' building width).

13x13=169 sq ft
Floor 169 Sq Ft x (40 lbs/sq ft Live load + 10 psf Dead load) =8450 lbs
+ (I'm assuming 20 lb wind or snow)
Roof 169 sf x (20 psf LL + 10 psf DL)=5070 lbs
Total=13,520 lbs uniformly distributed load.

http://www.timbertoolbox.com/Calcs/beamcalc.htm
Inputs;
13520
156
10
14
850
1.2
165

I'm coming in at 10 x 14. You can use a ridgebeam, over the wall, to support the 5070 of the roof if that is a problem. The load will go first to the floor girder through the wall but as it deflects the ridge beam will accept its' portion of the load.

[dablack]

thanks guys.  I really appreciate the detailed responce.  With that being said, I think I will go 2x4 16"OC just to be safe. 

Don,

You nailed the numbers except since we are at the top of a hill I was worried about a strong wind load.  I can't remember what I came up with but it was more than the normal 20 psf LL.  Yeah, that beam is getting pretty big at 10x14.  I think if I re-ran the numbers with the higher wind load, it will get even bigger.  I don't think I have any trees that will give me that.  I think I'm going to look at a built up beam made of LVLs for the girder.  That will work. 

thanks
Austin

[Don_P]

chapter 3 of the IRC gives some guidance on wind loading.

For prelim design, the supplier will do the final engineering, here are some design values to plug in for LVL's
Fb, bending strength, use 2900 psi instead of the 850 psi of SYP
E, Modulus of elasticity, stiffness, resistance to deflection, use 2.0 vs the 1.2 of SYP
Fv, Horizontal shear, the resistance to splitting lengthwise under bending load, use 285 psi vs 165 psi in SYP

LVL's come in 1.75" thick increments

Edit;
Nothing to do with this example but a good opportunity to explain something. I try to be careful to qualify loads as "uniformly distributed" vs "point loads" and there is a very important reason. In a simple beam the first check is the bending stress, or moment, on the beam. If the load is evenly distributed along the length of the beam the mathmatical formula is (total load x span)/8= maximum bending moment. However, if that same load is concentrated in the center of the beam, say from a post landing at midspan, the formula becomes (total load X span)/4... the maximum bending moment, the stress the beam must be capable of resisting, just doubled. That is a huge difference in the stress on the beam and happens to be the entire factor of safety.

[dablack]

Hey Don,

I really enjoyed taking statics my freshman year and I always enjoy your refreshers.  I've forgotten almost all my statics and for sure all of my dynamics.  Your refresher courses are, well, refreshing.  HA!  I still have all my books but they don't have all the info for the physical properties of WOOD!  HA! 

thanks
Austin

[Don_P]

LOL, Cool, I never took any while in school, learned it all the hard knocks way and taking short courses after I realized I needed that stuff for what I was doing in the field. I wish they would start teaching this in conjunction with HS drafting and shop... I always wondered in math class "what am I ever going to use this stuff for?"

[dablack]

Just going back and reading this thread.  I'm still working on the plan.  Everything is a compromise between cost, time, and meeting code. 

It is amazing what you have picked up and put together on your own.  I think they could do a very basic level statics class in highschool but I'm not sure where they would put it.  You touch on all of this in physics but most kids don't get to physics in highschool or at all.  In college, Statics was a 1st or 2nd year class.  All engineers have to take it.  As a mechanical engineer, the next stop after statics is dynamics.  It is usually a Jr. level class and is a weedout class.  We had a couple of 4th and 5th year guys taking dynamics for their 3rd time.  Finally, two of them had to go to another school and get the credit and transfer it back.  Not sure how they got away with that.  I made it through with a C and was glad.  It was just a 3 hour class and I was taking a total of 15 hours that semester but dynamics took about 1/2 of my study time.....

The ridge beam didn't work out.  I was trying to keep the building "short" so it wouldn't be too big next to a true two story that I'm going to build next door later.  Well, a 1 story wouldn't work.  We have four kids and need more height upstairs.  Attic trusses won't work because to get any space, you have to go with 10/12 pitch or 12/12 pitch.  Been there, done that.....not again.  Most roofers wouldn't even bid it and the rest were trying to buy a new truck with the money they wanted to charge me.  That left me with some sort of kick wall and a 8/12 pitch roof.  With out rafter ties, I'm not in code.  Making the kick wall tall enough to put the rafter tie in the lower 1/3 of the roof, made the building too tall.  So, round and round I go.  I even thought of simple sisor trusses but they aren't much cheaper than attic trusses and you don't get a bottom cord.  After buying Ijoists to span the 26' the sissor trusses cost more than attic trusses.  So, I've had to just forget about my self imposed height restriction.  I'm going to build a 4 or 5' kick wall so the rafter ties will be in the lower 1/3 of the roof.  I think I might step it down to a 7/12 pitch roof to make it a little easier on me to roof up there.  Oh well. 

thanks
Austin

[Don_P]

Austin,
I'm getting a passable ridgebeam with a 13' span on a 26' wide building having a 30 psf live load&10psf dead load by using a built up 3 or 4 ply 2x12 ridgebeam, measuring 6x11.25", is that workable? By building it up from multiple members the defects are better distributed, the Fb goes from 850 psi to 1078 psi. This is in #2 or better. I recall you mentioned sawing with the FF guys, if you have trees that will produce 2x12's pull the best of the boards for the beams. The reason I said 3 or 4 ply is it depends on the thickness y'all are sawing, the overall 6" thickness is what I ran. It could be done with 4 store bought #2 SYP 2x12's to get that same 6" thickness.

Wish you were closer, I sawed some white oak yesterday that is Fb 1725 psi, dead clear. If the crew are running trucks in your direction some of them might be willing to throw on a few of the higher stress timbers from their neighborhood.

[dablack]

Don,

Thanks so much for your help.  When I said, "the ridge beam didn't work out", I didn't mean, it wouldn't work.  There was more than one reason why the beam didn't work.  One was size.  Yes, moving a 10x14 beam would be tough and yes three or four 2x12s would be much easier.  2nd problem was the foundation improvements for supporting the beam.  I would need to cut three holes in the slab, dig it out deeper, add rebar and pour in footers for the posts.  I know it is completely doable but it was going to be work that I wasn't really wanting to do if there was another option. 

What I originally wanted to do is build a one story with a 8/12 pitch roof and put the four kids in the "attic".  I figured I could put a 1/2 bath up there and it would be great.  They would love it.  The house is 26' wide so that gives a peak height in the attic of about 6' or 7'.  They would have tons of room and it would be fun.  The problem with that plan is there is no plumbing in the slab and I would either need to jackhammer out some slab to add a drain or build a raise platform for a downstairs bathroom. 

After working on the plan for a while, we decided it would be easier if all the bathrooms were upstairs and that meant the master bedroom upstairs too.  So, to get adults up there I needed at least a 1 or 2' pony wall up there to get a little more head room.  That is what pushed me to a ridge beam. 

After working on that plan for a while, after doing all the work of a ridge beam and foundation improvements, it would be easier to just forget about the self imposed height restrictions and have a platform framed 5' pony wall that would put the rafter ties in the lower 1/3 of the rafter.  So, that is where we are now.  All three bedrooms, and two baths upstairs.  The 5' pony wall allows us to use the complete 26' width of the upstairs.  It isn't ideal but without lots more money that is the best I can come up with.  Also, with all the bedrooms upstairs, I have room downstairs for a single 26' deep, 13' wide garage bay.  We are playing $100 a month for storage of my garage stuff that wasn't lost in the fire.  If we can get out of paying that we will be in that much better of shape.  Also, this design gives me a little room to run vent pipes up in the attic and have a little storage up there too. 

NOW, with all that being said, the original plan of a one story with the kids upstairs is still a good one.  HA!

thanks again,
Austin