24x32 in Southern Illinois - New member.

[interex]

I understand that, I was meaning to run one ridge board at the top, instead of two ridge beams spaced 8 foot apart as initially suggested by myself.

[Don_P]

The roof you have shown is not one to copy.

All depending on how far southern, If mid southern you are probably 20 pound per square foot snow load. assume 15 psf for dead load, the weight of the roof materials, or 35 psf total load. As I remember your pier layout dictated the 2 rows of roof support beams, I'm guessing your friend doesn't know this. Using 2 24' span roof beams lowers the load per beam and the load per pier. Lets do some cyphering.

With 2 rows of roof beams, they are 8' apart. If we take a slice across the 24' wide building in the 24' long open span the left wall supports 4' of the roof + the overhang (I'll assume 2')... 6'.
6' of tributary width x 35 psf= 210 pounds per lineal foot on the outer walls.

Each roof beam supports4' of roof between the wall and the beam and halfway to the next beam, 4' for a total tributary width of 8'.
8'x35psf=280 plf on the beam

The span of the roof beam is 24'.
280 plf X 24 feet= 6720 lbs uniformly distributed on a 24' span.

If you look at the roof table for LVL's it should show that a double 14" LVL (3.5" x 14" total beam dimension) will work for each roof beam at that span and load. The supplier will do this math.

Each end of those beams will then be supporting 3360 lbs, add this to any floor loads for the load being delivered to the footing. This all seems in range.

[interex]

Okay, remember, we're working with 32' long by 24' wide.

In the illustration you made shown below...  you can see where the 6 posts would be to support the roof beams.  We have 6 post total, 4 on the south end, and 2 on the north end.  The south ones are 8 foot apart side by side, and north to south... and the north ones are 8 foot apart side by side.

The span for the southern 8 foot will be covered on the southern 4 posts, and the other 24 foot will be on the northern 2 posts.

I understand that LVL is a strong wood, but why is it better to use as opposed to built box beams, or making 4"x14"x36 (total) beams?  

Would a home built beam of that size not be able to support that weight, or the total uniform weight of the roof, and if not, why, if it could be explained better.

[Don_P]

LVL's are stronger and stiffer than "dimensional", sawn 2x lumber. 2x lumber is in turn stronger than solid timbers. This has to due in part with the grade of material used. In part it is due to the distribution of defects, one large defect in a timber controls the strength where if that same defect were sawn into boards and the location if the defect was not continuous through any one ply the beam would be stronger. And in part the stiffness is due to the glueing of the plies together in an LVL.

Fb is the value for bending strength,
In an lvl fb= ~2750 psi
In a typical SPF 2x it is ~1250 psi
In an oak timber it is about 750 psi

Stiffness is in lvl about 2.0, in dimensional about 1.6 and in heavy timber about 1.4

Horizontal shear in an lvl is about 275 psi, in dimensional about 150 psi and in a solid timber about the same as dimensional.

It will take a much larger sawn timber to do the same work as an lvl.  (these numbers are approximate from memory but are close). We can size it in sawn or heavy timber, but it'll be a monster.

Depth is very critical to the strength and stiffness of a beam. In determining the section modulus used to find the shape of a needed rectangular beam width is multiplied by depth SQUARED, depth makes a beam stronger and stiffer much faster than making a beam wider.

The beam most likely can be 36' long, the supplier of an lvl can do all the checks required. They will be able to give you a printout showing the load reactions on each of the 6 posts to aid in your load trace.

Basically the short answer is that it is not going to make much sense to try to fabricate a beam of that size on site. The lumber would be very special order.

Edit;
I just checked it very quickly on Ilevel's Forte software;
A double 14" lvl checked for me if 36' long but required a triple if it is just spanning the 24'. Interesting if you think about it, the 8' backspan is helping to counter the load on the 24' span. this comes with a cost, the reactions are -109 lbs at the south post in the loft, the beam is actually pulling up slightly. The interior loft edge post is experiencing a ~5000 lb load and the north end posts carry ~2400 lbs.

No solid sawn combination passed, a 6x14" glulam also checked.

[Squirl]

I thought you had a slightly higher snow load of 30 psf.  I had worked through a few calculations, but do not have the knowledge surity to do the calculations that Don_P does.  My calculations were coming up with 10,000 lbs per beam.  Or 5,000 lbs per foot with additional weight of two floors.  I re read the beginning and the concrete piers bear on 3 sq. ft. of soil.  If you have very low bearing capacity to the soil, with the floor load this could still be a problem.  Just something to double check.

BTW don, which product guide are you looking at?

LVL perform a lot different than glue lams.
http://www.americandreamconsulting.com/Charts/AnthonyGlueLam.pdf

[Don_P]

Squirl,
I have GP's residential floor & roof systems product guide, 2.23 mb if you google it should pop up.
I was also playing with it on Ilevel's Forte software, a free download.

The codebook snowload map shows 20 psf from around Rockford south.

Your Anthony products link is a good read and compares other materials I didn't above. Go to pg 11 and scroll down to 24' and over to better than 280 lb total load. A 3.5"x 15.25" or a 5.5"x 12.375" passes there. I'd step to the 13.75" deep beam, it was close with the 12", deflection would be getting heavy.

[Don_P]

This gnawed at the back of my brain while I was working on this yesterday and I ignored it. I had an email from PEte that I agree with. I'm late at the moment and have the client's in so may not get back to this for a little bit but this is worth posting to think about.

RE: 24x32 in Southern Ill., reply #101
Take a look at your AITC Timber Const. Manual, at the beam formulas for a symmetrical two span continuous beam, your 12' rafters, probably formulas #29: I would leave the ext. wall reaction at (2'+4')(horiz. projected roof load), that is their R1 or V1. This allows for some ice build-up weight on the overhang. But, notice that the center reaction on the cont. rafter is actually (10wL/8), their R2 = 2V2; which is the load on the LVL you are talking about. Furthermore, the location of your LVL is just about at the location, w.r.t. the ridge, where a snow drift is deepest on the leeward side of the roof. I'd bump that LVL loading up a bit. Finally, watch out for the gable wall post on the short 12' loft span and the possibility of uplift from the LVL beam. Then design the rafters for the full 12' span, or maybe 10', and then allow a small birds-mouth over the LVL.

[interex]

I'm seriously thinking about scrapping the roof beams all together.

I think it would be too much on weight, and work to have them.

[Squirl]

Engineering, more advanced than I.  Normally engineers charge out the wazoo for information that Don can give.

So I will describe my original concern about the piers a little better and hopefully someone can weigh in on my calculations.  Don_P Came up with an interior loft post carrying 5000 lbs from the roof load.  Now for the loft floor, it looks like it carries 4 ft evenly around each posts for the loft.  So each post would carry 4ftx8ft area or 32 square feet. At a load of 40 p.s.f. that would be an additional 1280 lbs per center post for the loft.  Now to add in the first floor.  This look like it carries an even load of 4 ft around but the floor is complete and a half like the loft so an additional 2560 per post.  This adds up to 5000 + 1280 + 2560 = 8840 lbs per concrete pier.  With each pier bearing around 3 square feet on the soil, that is roughly 3000 pounds per square foot in my equations.  You would have to have pretty good soil, at least better than the clay soils I deal with.

[Don_P]

Interex, describe plan B, the load isn't going away, how are you thinking of building it?

[CjAl]

The way ive seen this.done in timber frame with trying to keep the beam size down is to.use three.beams. One in the middle and one each side of it half way to the wall.

It would help to.extend the loft out to the middle point of the building so you had a central support making it 17' its always a tradeoff to have that big open cieling. I was watching a house.build the other day and i swear the ridge beam had to be 8" x 24" or more.

[interex]

Interex, describe plan B, the load isn't going away, how are you thinking of building it?

Honestly, I'm not concerned with the load/weight of the building on the piers.  The piers held a 2x6 wall constructed 1 story 2330 sq ft double wide with no problems at all, granted I know it's not the same thing as what I'm doing, but the piers seem solid, the soil seems solid, and I really don't see anything sinking.  

So, I'm going to move away from the piers/soil concerns.  

I was wanting to run two beams down the center of the building, but since that 24' open span came into question, I am going to have to scrap that idea and just go with plain rafters and collar ties, like this....

[Don_P]

That will work if the rafter ties are in the lower third of roof height, on every rafter pair, across the 24' open area. If you want ties higher in the ceiling then the situation is basically the same as what I described above. The ridge now becomes a beam and needs to be sized to support half the roof. If the high ties try to restrain the roof spreading forces they do 2 things. The joint forces connecting the tie to the rafter can become very large, beyond what nails can safely do over the course of time. Look at chapter 8 of the codebook in the rafter tables. Read the footnotes.Table R802.5.1(9) details the number of nails required in the connection and the footnote shows how much to increase thae nailing to account for raising the tie through the lower third of roof height. Second, the tie is causing the rafter to bend inward under load where the tie is connected. Instead of at one end where the bending stress is low in the rafter the tie is bending it right where it is weakest. Engineerable but this is why those scenarios step outside of the normal provisions of the code.

Trusses are another way of uniformly loading just the side walls and restraining the outward push of a roof. They do make them with a clear open ceiling.

[rick91351]

Myself I would do the truss thing and let them worry about the engineering on the top part of the building.  Truss companies engineer for wind, snow and seismic where required.  Also if you still want the two beam look in the open area you could frame a couple faux beams in there.    

[Squirl]

You could always do a hybrid.  Rafter Ties over the great room and Ridge beams for the shorter 8 ft loft area. 8 ft shouldn't be difficult to span with 2 - 2x8's or 2x10's.

Rafter ties have a few advantages.  From a practical framing standpoint, they can create a platform to work from, as in the picture you posted.  This can be a lot easier than scaffolding.  I know some who incorporate this into the design to hang things like lights or fans.  Also, if you are working completely alone, you can put the roof together one board at a time as opposed to renting a crane for ridge beams or trusses.

[interex]

I got the floor done lastnight.

I need to trim around the outer edges to cut off overhang, but other than that it's pretty firm, and my 5' level shows we're level everywhere, but a few places have a little bounce to them so I'm going to crawl under and nail in some extra 2x8 pieces to "fudge" the joists where the floor isn't as firm as the rest.

Something I'm wondering about doing rafters with ties,  if I have a 10 foot wall, and my loft starts at 8 foot, and my roof is 16 foot from the floor, how low into the loft would the rafter ties be... and what kind of pitch would my roof have, being 24 foot wide?   I'm thinking 6/12 or so, but not too sure.

[MountainDon]

I don't understand why the floor feels spongy if everything from the ground up was sized correctly?   


Draw it out on a piece of paper using a scale. Use 1/4" per foot or whatever you can fit on the paper at hand. That's what I would be doing.  Remember when using rafters with a ridge board the rafter ties have to be in the lower third of the roof triangle.



Loft floor joists dropped down the wall don't qualify as a rafter tie. There are other methods like using factory built trusses.

[interex]

How many rafter ties must there be, and what size should they be?

It appears I'm going to run into the same problem holding the outer walls in, as I would have with the roof beams.

[MountainDon]

One tie for each rafter according to current code.


What thickness floor sheathing?


On a wide building you may be getting into dangerous territory with a dropped floor loft at 8 ft on 10 ft high walls using standard rafters, especially considering the snow loads possible, as well as wind loading possible.

[Squirl]

Rafter ties have to be in the lower third of the rafter.  So 10ft wall 16 foot ridge = 6 foot rise for the rafters.  6/3 = 2 feet.  So anywhere from 10 ft to 12 ft.  

Normally for loft areas people take different approaches.  They either go for a truss or ridge beam.  Trusses can be purchased or built.  I believe John has a home built truss design included with his 1.5 story plans.  Beam sizing we have covered.

Your roof pitch can be whatever you want.  The concept is, as force (wind, snow, weight of the materials) push down on the angled rafter, it pushes this force outward.  This pushes outward on the top of the wall, pushing your walls apart.  The rafter tie counteracts this by creating a triangle.

A ridge beam/wall supporting the rafter at the top and bottom stops outward force.

http://www.structural101.com/Structural-Ridge-Beam.html

[interex]

Don, my floor sheathing is 3/4" Advantech T&G.

My loft floor has (will have) 4 posts coming up from the piers in the ground, plus the outer walls, and the loft floor would be used to tie the walls together at the loft, so I was thinking I could get by without having any rafter ties over the loft, and sticking with only collar ties.

As for the 24x24 foot span over the open area, I was just trying to figure out how to keep it looking nice and keeping a semi-open look to the top.... like a cathedrial style with 3 ties going across every 8 foot.

Maybe a look like this...

[Squirl]

My loft floor has 4 posts coming up from the piers in the ground, plus the outer walls, and the loft floor would be used to tie the walls together at the loft, so I was thinking I could get by without having any rafter ties over the loft, and sticking with only collar ties.

Are you still going with ridge beams for the loft area?  If so, you don't need a rafter tie.  At least not below the ridge beams.  I'm not so sure about the 8 ft span above.  I am with MountainDon.  Your building is wider than many posted, including the one referenced.  Also note, in the build you referenced it says that if he had longer lumber, the rafters would have been lower into the room.

Quotes from that build:
" I have no experience in construction. I am doing a lot of reading and doing all the work from information I get from the Internet. "
"I used 16' lumber for my rafter ties if I had used 20' lumber it would be sitting right on top of my wall."

Many people on here, especially moderators will make recommendations based on generally accepted building design and code free of charge.  Some such as Don_P will go out of their way to give more advanced calculations and design people would generally have to pay an engineer for.  Some people just build things not in line with generally prescribed practices.  Sometimes it works, sometimes it doesn't.  You rarely hear about it when it doesn't.  I just wouldn't expect recommendations of methods outside of generally accepted principles of framing.

[interex]

I could make my rafter ties come down lower (making them longer) if need be, as long as I could find a way to make use of 2 or 3 only and not 5-10 of them... I can have 20' rough cut log beams cut, or even make them myself.  We have several saw mills (lumber mills) in my area which I have contacted about pricing and sizing in the past.

I understand what you are saying about advice given, and structural codes... and the only thing I can think of is... those who give advice do so freely and I am reading what is said, just thinking that so much code is referenced, but it doesn't appear anyone here follows most of it.

I'm only showing pics of what I think looks nice and how I could go about doing my own build.

Also, I was planning to go with the 8 foot ridge beams over the loft.  I can easily make 2 3"x10" by 10 foot beams for over the loft.

[Squirl]

Your plan for the loft area sounds like it would work.  Don_P could give more exact calculations, but I wouldn't be uncomfortable if I were in that position.  I believe two 2x10's would work if were a single ridge beam for 8 ft, since it is two beams, a true 3"x10" should be pretty close.  Why not just size up to a 4x10 if you are going to have a 4x4 post?

I try not speak to why other people went a different direction.  They could explain that.  Sometimes they can't.  They point to someone else on the internet as to why they did it, or misinterpreting a picture on the internet. I would disagree that most people don't follow most of the code.  Most people follow most of it.  Designing roofs seems to be one of the most confusing parts.  Some people don't look into the physics of how the load is transferred.  They follow a picture of how they perceive someone else has done something without knowing the why.  Over the past few years of frequenting the site, I have noticed a few things.  Many people built cabins and houses to John's plans.  They aren't overly complicated, but he spent a lot of time working out the calculations on sizing, load distribution, and design.  Other people could see the pictures, but not these details, so when they tried to copy this, they missed some of these details.  This leads me to the two most common areas of missed details that I noticed as being beam sizing on pier foundations and roof design.  There are many who use their loft floor to combat lateral force without rafter ties.  On smaller buildings, even though it is against code, it sometimes works, because the lateral load is smaller the thinner the building.  The wider you go the outward force can rise greatly.  You can see this in nailing requirements. I believe this was MountainDon's worry because your building width is wider than what most people build on here.

Now to what you are looking for.  I believe many of the origins of the design of larger rafter ties space farther apart were from timber framing and plank & beam framing of larger rafter ties spaced farther apart with thicker plank sheathing.
http://awc.org/pdf/wcd4.pdf
p.3 of the .pdf

I'm not saying it can't be done.  I know enough that there is always something I don't know.  I do know that it would be a more advanced framing and load technique.  If Don_P and MountainDon said to put it at every rafter pair, I don't know how many other people have that advanced knowledge to be able to help.  I assume it you did the rafter tie every 8', the rafter would have to be as large as possible. It would probably even be doubled or tripled with 2x T&G lumber roof sheathing or a plywood panel rated for an 8 foot span. 

[Squirl]

Those that did follow the methods described.

Site built truss design.



Both a Structural Ridgebeam and rafter ties combined.

http://www.countryplans.com/amydan.html

Site built truss design.