Post load??

[okie]

Ok, I asked a question on here yesterday and I dont think I gave enough info so I did my best at figuring the total load on the posts using methods in a couple of books that I have and heres what I came up with.

The total load on outside posts that will support 1st and loft floor joists as well as 1/4 of the roof weight will be just shy of 6,000 lbs each post (That is a heavy estimate).

The total load of the central posts that will carry 1st and loft floor joists as well as 1/2 of the roof weight will be just shy of 11,000 lbs (also a heavy estimate).

I figured this with greater snow load than I will ever see here in OK and a greater live load on joist than the structure will ever see but I would rather overkill during construction than have to rebuild or replace structural elements due to cutting corners.
So on with my revised question:   What size square upright post would I need to support 11,000 lbs and what size would I need to support 6000 lbs? I will be using Oak, both red and white and possibly honey locust for my posts and each will be on 8 foot centers.
one of my books says that a round fir post with a 6in tip dia has a working load of 8,000 lbs but can support 12,000 lbs with carefull engineering whatever that means.

[Jim_Rogers]

There is more information needed.
How tall are the posts?

[maineframer]

Oakie,

The posts or vertical members in the frame are not nearly as critical to "oversize" as are the beams or the horizontail members. The main reason posts are sized 8x8 or larger is to allow for long enough tenons, good joinery and ample pegging opportunity.

Be sure to have your design checked by a competent engineer, it will be worth it in the long run.

David

[okie]

Jim, The perimeter posts will be right @ 15' give or take for grade variation and the center posts will be right @ 23' also give or take. This building is not going to be a timberframe with posts and beams but more of a pole barn style with girts to support the floor joists and rafters. The maximum unsupported joist span will be 16' in the loft area and 8' on the main floor. Rafters will have a 8' rise over a 16' run or 6/12 pitch with a hip style roof. I really appreciate y'alls input on this.

[Don P]

Give this a try, this is just a vertical load, any lateral loads are handled through something other than the post;
https://forestryforum.com/members/donp/columncalc.htm

#2 Red oak has an Fc of 350 psi, E of 1 million psi
#2 WO Fc 400, E .8
Honeylocust is not in the book, I'm going to guess it would be close to the Beech/Birch/Hickory class #2 Fc 425, E 1.2

For Dave,
#2 EWP Fc 325, E .9
#2 Hemlock Fc  400, E .9

Careful design most likely means that there is a stricly vertical load centered on a well mated surface with no side forces inducing buckling other than the perfect axial load. Sounds like you're eating into your safeties to me if you go there  ;).

There is a newer way of doing the math that was introduced with the '05 version of the NDS. The difference for most situations is whether the fly is flapping or has landed though.

[okie]

Don, THANK YOU. If I done that right I can frame the exterior walls with 6x6 with room to spare on all accounts, if I use a 8 x 8 for the center posts it barely passes compression so if I up it to a 9x9 it also has room to spare. I really appreciate you taking the time to post that calculator link. If you dont mind me asking, where did you come up with the info on the wood elasticity and allowable compression?

[Don P]

glad to be of help.
The National Design Specification for Wood Construction is referred, or more correctly, deferred to by the ICC's national codes. Those numbers came from table 4d, Reference design values for visually graded timbers (5x5 and larger). The design values are from the '05 (latest version) the math came from the '01 version.

On any of my calcs you can read the source code by right clicking while on a blank spot on the calc page and then clicking "view source". The math is in the upper javascript part. I can also walk you through it if you would like to understand how it's done.

[Don P]

Well nobody's asked, but its snowin and they say if you can teach something, you own it, so you're in for it now  :D

A column is a compression member that is usually many times longer than its width and depth. Wood columns usually are one of three construction types, solid, spaced, or built up. A solid column is a single piece of wood or a glulam. A spaced column typically has two or more continuous pieces seperated by end and middle blocks. A built up column is individual full length pieces joined by mechanical fasteners.

Depending on the relationship between length and cross section, its slenderness ratio, the capacity of a wood column is determined by its stiffness and parallel to grain compressive strength. This is an important concept. Older engineering texts ('86 NDS and before) think about columns in three length classes, short, intermediate, and long. A short column, the chunk of firewood you put under a car jack is not going to buckle. It's crushing strength, or the parallel to grain compressive strength is its limiting factor and the only thing that need be checked. At the other end of the spectrum is the doubled up 2x4 pump jack poles in my sheathing thread yesterday. They are over 24' long. I'll never scratch their compressive strength, they are limited by their resistance to buckling, their stiffness, or, modulus of elasticity. Between those two extremes we need to consider both characteristics when sizing a column. As they get shorter, compressive strength is more heavily considered, as they get longer, buckling is more heavily considered. With more recent developments in mathmatical modeling ('91 NDS and later texts) you generally don't need to pick a dividing point between short, intermediate or long, the formulas simply follow that curve. The newer way is pretty intimidating looking compared to the old ways but works pretty well with computer backup.

This graph from a pre '91 text shows allowable compressive strength plotted against columns of increasing slenderness ratio. It shows the older formulas and the basic ranges of each column type. That curve is reality, our modelling of it has grown more accurate with time and so the subdivisions into length classes is no longer neccessary, but you can see they were not really all that inacurate. The graph helps me understand what's going on. Imagine the chunk od firewood and jack at the left, full compressive strength of the wood. My pumpjack pole to the right, sure lost alot of load carrying ability there.


The slenderness ratio cannot exceed 1/50. For temporary construction bracing it may go to 1/75 (that is juicy! I jacked up and immediately braced off those pump jack poles before we used them)

The slenderness ratio is measured about the weak axis. A 2x4 stud wall is a good example. As soon as you sheath the 2x dimension, the weak axis that we need to be concerned with buckling in, is the 3-1/2" dimension. A pole barn with 12' walls may have girts strapping across 6x6 posts around its exterior shell. The weak axis is the unbraced 6" dimension into the barn not along the plane of the wall. An interior post in that barn may be a 6x8 with no braces in either direction, the weak axis is the 6" dimension. If there is a brace in that plane 9' off the floor, I'd check it both ways, right off the bat it would be hard to say which is the weak axis. Technically it would be the length to the least radius of gyration, which has more implications when you notch or mortise a post, that's getting ahead lets not go there now.

We should probably go into end restraint conditions just briefly because I've seen stock plans misused and weakened from the intended design. The math I use considers the end connections to be pinned. The ends are held in place but are free to rotate. This is a conservative assumption. A true pole barn, properly designed and built, often has buried poles. The bottom end condition is fixed. To visualize the difference take a piece of paper of about cardstock weight. Set it on edge on a table and with one finger on each end push together to make it bow. A uniform curve results. Now pinch each end between thumb and forefinger, fixing each end, and push together again. the curve changed. In fact there is a recurve. At the point where the recurve ends and the main curve begins is an inflection point of zero bending moment. For buckling purposes we can ignore the portion of the post outside of the main curve, thus entering a shorter "effective length" into the column formula to account for this end fixity. Now suppose you take a stock plan that calls for posts buried 4' in the ground and say "posts shouldn't be buried, I'll put mine on above ground footings" Go carefully there, the building is significantly weaker than designed. In the real world there is no truely fixed or frictionless pin condition. If none of that made sense don't worry, use the full column length in the calc and be safe, leave it to engineers to shave fractions.

That said a flagpole type condition should have its actual length doubled in the calcs, seems there's always exceptions  :-\.

Eccentricity;
The basic column calc is meant for columns subjected to axial compression loads only. That notion only exists in an ideal world. The load may be introduced to the column somewhere other than straight down the center, the column may have a slight bow, there may be an internal defect or shrinkage condition that affects the concenticity of the loading. The magnitude of this bending moment is an unknown. For most practical purposes we tend to ignore it, there are fairly large safety margins both in the design values and in the design equations. The NDS requires no adjustments for this. If you see or suspect eccentricity, side loading, sway, anything other than pure axial loading, use good judgement ... here's your sign.

[Don P]

Crossing the bridge to the 21'st century, old timers was setting in, I had worked up the new '05  math into a worksheet of sorts. You can back out and enter in any desired variables in the boxes on the calc below. I was trying to explain the new math in the process.

[peter nap]

Lord have mercy Don  smiley_brick hits_hardhat....Whatever happened to the "That's about right", formula?

[okie]

Gee Don, I used to consider myself a fairly intelligent human being but now I am not so sure  :-[ . I need to read that about 10 more times and see if I grasp it a little better. I sure appreciate you taking the time to explain that to me, now if I can wrap my thick skull around it I will be well ahead.
I was not going to embed the posts but have them about 10'' off the ground on pier footings but I gather that that greatly reduces the strength of the building but I am not sure I understand why. Would this weakness be remedied with kneebracing from the posts to the girts? If not how does that differ greatly from a timberframe mortised into a sill and top plate with knee bracing?
Now dont get me wrong I am not trying to say that a pole frame and traditional timber frame are the same but there are simularities in the fact that the loads are carried by large dimensional timbers widely spaced.
For all intensive purposes I had about talked myself into framing my house in this manner but now I am spooked. I had a different plan entirely but then I bought a book to give me ideas on storage but the book deals more on building pole frame houses than storage space and I really believe that it would cut my construction time and cost nearly in half as I will be doing 80% of the work alone.
Well I am not wure if you meant to but you have officially became my mentor.
Thank you for all your help.
Morgan.

[Jim_Rogers]

okie:
If you post is secured to your 10" high pier footing then you don't have to worry about it kicking out under bending stress, should there be any.
You just can't set it on top and expect it to sit still if there is stress being applied by the loads.
Bracing will help.
Consider an embedded strap or center rod to prevent the post from moving off it's foundation.

Don:
Nice calculator.
But again if you can add a simple pass/fail output box it will help everyone to understand if things are ok or not....

Jim Rogers

[Don P]

The pass/fail box is what's giving false passes on some browsers. Still trying to figure out a way around that.

If you saved the simple column calc, delete it and download again, there was one word wrong in a bad place. I did add a couple more lines to the very bottom to help clarify if it passes.

The embedment fixity is going over the top, I'll try to explain it better in a bit, but as long as you are checking with our calc and not relying on a stock plan while changing the post connection, you should be safe. The main thing was if you use a stock plan, connect it as the plan specifies. Guys don't take it hard at all if this is tough the first pass through. I've got 3 texts, the notebook from Dr Woeste's class and the NDS open here and big chief tablets all over the floor  ;D. Judging from the notes, I've been scratching my head on this part since 1985  :D.

We're up to 2 digits, need to go plane some wood for work.

[Jim_Rogers]

I can understand that the pass/fail box may not work on some browsers and that's a bad thing.
So if you can explain in a simple sentence what box to look at and what one to compare it too and if this number is higher than that number it fails, if it is lower it passes type of thing would work for me.....

Jim Rogers

[okie]

Don and Jim, Thank y'all again for helping me out here. A person can only beat their head against a wall so many times before something gives and so many on this forum have helped me with questions that I couldnt find or figgure out on my own. I really appreciate y'alls effort.

[Don P]

I've softened up several spots on that wall  :D
I've added those lines to the bottom of the calc Jim

Okie, I'm happy to pass on what little I've learned over the years. I can do a little mentoring on a narrow range of topics, Jim and others can speak in more depth on other parts of the puzzle. Hopefully we all mentor each other. I've been over this stuff way more than 10 times and I still see new things every time. I was trying to put a whole lot of information in a very little space. You don't have to understand all the details but I hope some of the basic concepts hit home. There's no need to be spooked just identify how you will handle the forces and build stout enough to resist them. An engineer is money well spent, giving him a close to correct, well thought out plan is going to make his job easier. With some understanding, I've caught mistakes from their end before they got built also.

In a pole barn the designer may have taken an allowable adjustment to the column length for the way an embedded column reacts as compared to a pinned column. Reread the section from yesterday on end restraint, think about the card experiment I described and look at this chart.


The condition of simply pushing on the card is labelled 1.0, it is a pin/pin connection.
The condition of pinching the ends and pushing together is the left hand mode, fixed/fixed.
Their drawing of the buckling modes is the actual shape you should see in the card stock. The pole barn designer may use the next .80 condition, a prop cantilever, the bottom of the post is buried and fixed.

I'm not saying he did or didn't take that increase, I was warning you to check for yourself that the columns are good for your condition. To change from a fixed to a pinned column unwittingly is to potentially be 20% or possibly as much as 35% light. You are checking your own, the calc is asking for and calculating a pin to pin length (the 1.0 condition) and assumes no fixity, so it is giving conservative output.
If you are certain of an end condition, the actual length of the column is multiplied by these factors for the adjusted column length in the calc. We shouldn't go there in my opinion, use the full length. Beefy is good.

Aside, notice in the .65 condition the inflection point where the recurve ends and the main belly of the buckling occurs. That belly is 65% of column length, its real and appropriate to use these adjustments if you are sure of your conditions. Dr Woeste told us to scratch through the theoretical line and never to use those values.

  Now..  something else has evolved during this conversation. You do need some form of sway bracing, large x's, plywood walls, diagonal sheathing, etc. Technically a TF kneebraced post needs to use another formula to account for the side bending the brace puts into the post. Remember this calc is for a purely axial load. If there is a long diagonal, say diagonal sheathing or a sheathed wall then that takes care of that direction. If there is an internal kneebrace resisting load then the combined compression with bending calc is more appropriate. This calc will give too small a post if there is much of a bending load pushing on the side of the post combined with the axial load.

[Jim_Rogers]

I like the end of the calculator now. It makes it easy to understand whether it passes or fails.

Jim Rogers

[okie]

Ok Don, I think some of it is starting to sink in. I'll look at this page some more and then check with you on an example to see if I got it right.
Thank you again.

[Don P]

Some more while going through the column chapter. We have one member who is thinking about making his own built up columns and we have talked about it here before.

No arrangement of pieces with mechanical fasteners can make a column of equal strength to a solid sawn one. This is due to the to the shear distortions that can occur in the fastenings joints. The allowable design value, Fc, is less than a solid column of the total built up dimension but is greater than the combined allowable compression values of the individual parts. There is some more info on this in the Wood Handbook downloadable from the FPL website as well, its in chapter 8 of my older copy.
If assembled as shown the strength can be expressed as a percentage of the strength of an equivalent solid sawn column.


My understanding is the cover plates on the build ups above help with the shear distortions between the individual plies, making them act together more.
For L/d ratios of 11 or greater, the strength of the column is independent of whether the individual pieces are butt jointed or full length. At L/d<11 individual pieces butted end to end fail at 75-80% of the strength of full length pieces. Look back to the graph a few posts back, this would be in the "short" catagory, look at the formula for a short column, crushing only, elasticity doesn't factor in yet. The percentage values only apply if the pieces are adequately fastened to provide composite action as an assembly. Individual pieces should not be wider than 5 times their thickness. The longitudinal spacing of spikes should not exceed 6 times the plank thickness (typically that would be a nail at least every 9" for 2x stock). Lumber must be dry to maintain contact. Spikes must go through 2 laminations and well into the third, through all plies and well into the last one is preferred.

For a typical layered up stack of lumber nailed together the books aren't in as much agreement. You cannot butt joint pieces in this type of post from my read. The NDS uses a factor of about 60% multiplied into the column formula. One text says the strength data is somewhat limited but that the column should be somewhat stronger than the individual plies added together, another says to check each lamination and add them together.

This is the best nailing schedule I found;
1. Adjacent nails are driven from opposite sides of the column
2. Nails penetrate all laminations, including at least 3.4 of the last ply
3. The end distance between the nearest nail and the end of the column is 15-18 nail diameters
4. The spacing along the column is greater than 20 nail diameters but less than 6 times the thinnest ply thickness
5. The spacing between rows across the face is between 10-20 nail diameters
6. The distance from the edge to a row is between 5-20 diameters
7. When the face width is greater than 3 times the thinnest ply thickness 2 or more rows of nails are needed. When only one row is required, stagger the nails across the face, When 3 or more rows are required nails in adjacent rows are staggered.

[okie]

Thanks again Don, I am working on changing my plans around a bit due to some of the info I gained on this post. I wondered how strong a built up column would be so Thank you much.