The New World Dutch Barn (Timberbee)

From the Quicksilver Metaweb.

Traditional Structure must Holistically resolve the forces present within it. Most said buildings are in a general state of rest. A good example of this is a typical H Bent, aisled Dutch barn;

• It’s main section Consisting of Two Purlin Posts, stretching from Purlin Plate to Long sill, separated by, an often massive, Tie beam, which is generally braced with an opposing pair of asymmetric braces (60/30, 3/4/5, etc.).

These 5 elements; posts, Tie, Braces, form the Principle “Bent” of the Barn.

The Bent’s are connected, above, by continuous Purlin Plates, which run the length of the Building, and, below, by a grid of sills, upon which the barn is set, and by which, the flooring is supported. There are also girts which run from one Purlin Post to the next, and a series of opposing braces which run from the Purlin Posts to the Purlins, the larger the brace, the stiffer the frame.

Braces help to allow the frame to move as a whole when any One element of the structure is acted upon.

In addition, the more connections between the individual components the more efficiently the structure resolves issues of force.

The Barn also consists of two “Side-Aisles”, their width, and height varies, as do all pertinent dimensions, upon the buildings intended use, builders origin, materials on hand, and budget of the owner – There are a great many, subtle, variations in Dutch Barn geometry.

Generally the Common rafters are joined with a tongue and fork, or a variation of a half-lap at the peak, and rest, upon, both, the Purlin Plates and the Wall Plates – the wall plates being an element of the Side-Aisles.

The Side-Aisles generally have no sectional bracing themselves, indeed, not any bracing at all, neither sectional nor longitudinal, receiving all their bracing from the Central Aisle, or, the “H” Bent, for, the Main Aisle greatly resembles an H.

But, the Aisles themselves receive little but purely downward thrust from the roof, as the rafters are, near perfectly, balanced upon the Purlin Plates. In fact, when the Side-Aisles must be removed, as happens from time to time over the course of the buildings life to allow for, either the aisle posts to be replaced, or for the sills they sit on to be replaced or repaired, the roof Could remain intact, and, Relatively unaffected.

Dutch Barns handle stress fairly well. Existing in a fairly Balanced State. The great bulk of the static load of the roof is borne by the Purlin Plates, the Purlin Plates, then, efficiently, resist, And transfer the load Straight down the Purlin Posts, to the sills, with no intermediaries, thus resolving any outward thrust.

The only area of weakness is the manner in which the frame handles dynamic loads, multiple braces, and connections between major elements in the longitudinal direction – in the direction of the Ridge (or, down, what We think of as, the length of the Barn), handles lateral racking.

There is little bracing in the cross section, and, though the H Bent is buttressed by Side-Aisles, these aisles are, themselves, generally not braced, and so, do little if anything to resolve racking, though, they do offer some resistance, particularly on the Gables, due, in great part, to the sheathing, which, in these buildings is Almost Always Horizontal, Almost. The presence of the sheathing does offer some small bracing.

Beyond the presence of the Two Bent braces – remember, in an opposing pair, only One brace is ever engaged, as they truly only function in compression, Not in tension. In addition to these two braces, is the enormous Body of the Tie itself.

In this case it is not mass which resists racking, it is the depth of the Wedged, Half-Dovetail, Through Tennon, of the Tie, engaged within the Post’s Mortise which acts to withstand the load. This joint, in addition to being wedged, is also pegged, generally with three, rather large pegs.

The post itself also acts to resist racking, partly by the nature of it’s shape. Both the post and the Tie are Typically Rectangular. The Posts narrowest dimension is measured down the length of the Tie, it’s greatest dimension measured down the length of the Purlin Plate. This makes the post most resisting to bending in the longitudinal section. Multiple connections, in this plane, stiffen the post even further, limiting deflection.

The Posts most flexible dimension is in it’s cross sectional plane. In addition, in this direction the post also has an Extremely large bending moment. There is No bending moment longitudinally. This is actually an aid, as opposed to a flaw.

Picture the Barn as a scale model, the wind as your hand. Press gently on one side of the roof. You now begin to see the building lean away from you, compressing, and distorting as it does so.

Looking inside the structure, while continuing to apply pressure you see a number of things happening; * The girts, which connect the side-aisle posts to the H bent, distort; their angles opening up. * The brace nearest you becomes slack, * The brace to the far side of the Tie Beam is engaged, thrusting upwards against the Great Bulk of the Tie beam, and downwards against the more flexible Post, attempting to bend it. * The girts, connecting the Far side-aisle posts to the H Bent, distorts; their angles seeking to close in upon themselves.

• The Purlin Post nearest you, distorts, attempting to bend -- from the purlin plate to the tie, towards the opposite Purlin Post.

• The top of the Purlin Post furthest from you, it beingOnly connected to the opposite post, through the Tie Beam; which is at, or near, it’s halfway point, and the Sill; which is lower still, being at the very bottom of the posts, is Mainly Only receiving thrust from the roof.

• As you continue to press, you should notice that the roof is Also, continue to flatten,

• All of the Purlin Posts ,, indeed, All of the posts; Side-Aisle posts as well as Purlin Posts, on the near side of you, are Bending away.

• This is causing the roof on the near side, i.e.; under your hand, to be depressed.

• Meanwhile, All of the Posts on the far side, are bending away from you as well.

• But Force is acting differently on the posts throughout the whole building.

• The shorter, side-aisle posts, on the near side, are being directly pushed, by your hand, as well as being Pulled along by the movement of the Roof, and by the movement of the H Bent, from which they take their strength. Remember What happens to the H bent, happens to the side-aisles.

• The side-aisle posts away from you, are affected slightly different. For them, the Bulk, the greatest source of bending is coming from the Tie itself, transmitted through the girts, which act to collapse, since they contain no braces, and thus, cannot offer resistance.

• The far side-aisles are being crushed together, and pushed over, while the Near side-aisles are being Pulled along.

• What is happening above the Ties, is a slightly different matter. The Ties themselves are so Massive, and Deep that they resist Any Bending whatsoever. The Ties will bend, Only after the greater portion of the Barn has collapsed in a massive, and horribly, catastrophic failure, and, even, then it is wildly unlikely.

• So, the Ties are, very efficiently transferring load to all the other elements in the barn in their direction. Not so with the purlin Plate, above the level of the Tie.

• This is bending.

• As mentioned before. The Purlin Posts act independently of one another. They also act as Giant Springs. And are Extremely inefficient at transferring load, transferring it to the opposite post Only by first receiving the load via the Purlin Plate, and transferring that load to the Tie below.

• But before this can happen, the post undergoes Bending, A lot of bending, and so absorbs some of that load.

• The purlin post, on your far side, receives load from Two points, the Tie, which receives it from the opposite Post, and the Purlin Plate, which receives it from the opposite roof plane.

• This post can bend in Two directions, away from the force of your hand, or, towards it.

• The Tie is wanting to push this post Away from you, but, in order to do this, the post needs to, Literally lift the far roof plane. As the post is weak in this direction, and highly resistant to bending, most of the force exerted upon it by the Tie Beam is forcing it to bend between two, fairly stable points, the Sill, which is Fixed. and the Purlin Plate, which is in motion, but, as the roof is Not rigid, the motion of the Tie is As great, or even Greater, than the motion of the roof, whose energy is being absorbed by the Post nearest your hand.

• Still with me?

• What you should see happening is that the Top of the Post, nearest your hand, is bending inwards with more force than is, then, transmitted to the Tie Beam.

• The Movement of the Tie beam is pulling the side-aisle nearest you out of shape, acting to Push over the far side-aisle

• And

• It is Bending the far Purlin Post, roughly, in half.

• The Result is that the Upper half of the Far Purlin Post is moving more, inward than outward.

• And, force is being resolved more through absorption, than by resistance.

The difference between the Willow and the Oak (English Framing)