The Geometry of a Corn Crib

Our corn crib:  built c. 1810, photographed c. 2000. 

Well used, in need of attention, like every building on the farm.

 

 

In time we replaced the roof. Then we removed rotten siding.

 

 

 

  

 

We were surprised!The crib's post and beam frame was scribed! It has neat, precise marriage marks which, in this part of Vermont, are found on frames which were built 1800 -1820.

On the left are 'll" on both post and brace. On the right the marks on both post and brace are "llll".

 
 

 When our house was built  in 1810, the corn crib may have been here.

It has been moved at least 3 times.  It has been repaired, reinforced; its interior reconfigured. It has held corn, feed, tools, chickens, furniture, and been home to many birds.

Its plan:

A rectangle divided into 3 equal bays. The lengthwise framing in the larger space labeled 'corn' may have been flexible partitions. The tool and grain storage space has a low ceiling to allow more head room in the loft above it.

 

 

 

A cross section of the crib at the partition 

It is about 15 ft. wide at the base. Today its sill is about 24" above grade on the south end, 12" on the north.

 

 

 

 

Of course I was curious about the geometry. 

A corn crib was useful, and utilitarian. It served a purpose without frills. 

What was the simple, direct way to lay out its frame?

 

The floor plan: a rectangle with 4 corner posts and 4 evenly spaced intermediate posts. 

The dashed line shows the slope of the walls. The slope keeps rain off the corn. It's possible that the angle also keeps the cobs from locking in place. 

The slope was determined by a simple choice - see below*.

 

Today a carpenter would lay out the plan with a tape measure and a steel square. In 1810, these tools did not exist. Instead the carpenter used geometry.  

Here is the basic layout for a square as drawn by Audel in 1923.  This could have been used for the corn crib.

The layout using the geometry of a circle:

A-A is the width. Its arcs become the radius of the circle. They cross at the center of the circle. Then the circle is drawn with marks where 6 arcs cross the circle. 2 of the arcs cross at B and B: 2 at C and C.

 Draw 2 lines:  A-B and A-B. They are the sides of the  plan. 

2 intermediate posts are located at C and C, where the circle's arcs cross the lines A-B.

 Swing 2 more arcs from the corners of the plan: center of the arc on A, Then swing from the other A to C.  They cross the earlier arcs at D and D. The Line D, extended divides A-C in half and is the location of the next set of posts.

The arcs with their centers on both  D's, swung from the other D, locates E which marks the length of the  corn crib floor. (I drew only one.)

I can draw this layout much faster than I can write about it. Even so, it takes too long. I think a master carpenter would already know the geometry which I've laid out here and would have used short cuts.

 

He would have had a framing floor, probably in a barn.  

He would know the general plan. He and his client would have reviewed size and any needed variations. The work just  needed to be laid out. He would have used twine and awls, a chalk line to mark the lines on his framing floor. 

I  have drawn the chalk line set to both points, and a squiggle where the line is not held tight

A= the width of the corn crib, the ends of the rectangle.

Both sides need to be perpendicular to A: the 3/4/5 triangle  - B - sets the corner. The carpenter could then snap his Line - D - on both sides. Folding his Line -A- in half he could lay out 3 lengths along his Line, and then connect the 2 sides. The plan for the crib was done.

 

The plan could be easily 'trued' by checking that the diagonals of the whole rectangle, solid lines - E, as well as those for the 3 smaller rectangles, dashed lines - F, matched.

 

 

 

The elevation, here laid out with a compass.  

 

 

 

 Again, I think it would be easier to lay out the frame with a chalk line, using the width of the crib as the unit of the square.

 

The square and its diagonals.

Note that the diagonals cross at the location of the  beam that carries the loft. This is the first point.

 

 

2 points are necessary to draw a Line - or to snap a chalk line. The carpenter divided his Line in half, locating the vertical center line of the square.

The diagonals on the right half cross to give the carpenter his 2 points. He could locate the beam carrying the loft over the feed bins.

 

 

 

 

 

Next the carpenter needed to set the wall height. He added one diagonal on the left side; then the diagonals for the upper half of the square.

He had 2 more points whose Line (dashed here) located the plates for the side walls and the roof.

 

 

Finally he used half his unit (the width of the crib between walls) as the height to the ridge of his roof.  Here I have drawn it as an arc.

 

 

 

 

 

 * Now he was ready to layout the slope  for the walls.

A straight line (with arrows) from the outside of the sill at the bottom is on the inside of the plate at the top of the wall. This determines the slant of the walls.

I didn't discover this myself. Someone showed me. Unfortunately I don't remember who and can't give him credit. 

 

 

 

The 'front' of the corn crib today. We added a window, matching  one already in the crib.

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The 'back' of the crib.

We moved the door to the 'back'  as we no longer unload from a wagon.

The hollyhocks self-seeded here. They approve of our work.

 

 

Post Script:  Images of the frame

 

 

Post, plates, and rafters in a corner of the crib

 

 

 

 

 3"x4" braces cut by a sash saw, pegged.

 

 

 

 

The original rafters are 3" x 4", cut by a sash saw.

Later logs were added between the rafters and butted together. Many of the logs are birch.

 

 

The rafters were mortised and tenoned at the ridge.The large peg is visible here.

Knob and tube wiring and electrical cable are still in place though today the crib has no electricity. 

At one point there was also telephone line.

Corn Cribs for the Indiana Barn Foundation Workshop and Keynote

July 19-20, 2019, I will give a workshop and the keynote address on Practical Geometry for the Indiana Barn Foundation at the Oakwood Retreat Center in Selma, Indiana.
Their website is : http://www.indianabarns.org/




The picture they used for their announcement is the one I use on my website: It's a farm house north of Boston built about 1830 which was designed using a wonderful combination of circles and squares. I hadn't planned to include it in the talk but now think, of course, I will.

One post about this house is here: https://www.jgrarchitect.com/2014/09/how-to-construct-square.html

I might have chosen this picture of my corn crib as it is a barn out-building - and because it is red!

My post about the corn crib here: https://www.jgrarchitect.com/2018/09/a-corn-crib-seen-as-trapezoid.html

2 more corn cribs will be part of the lecture and probably the workshop.
They are from different construction traditions in different parts of the States. utilitarian buildings, built without frill, their forms easy to understand and read. They all use geometry differently.


The front of the Knight corn crib in Flatbrookville, Sussex County, NJ, shown here, is one of 3 HABS photographs of the crib. taken in 1969.

This is the quote which accompanies the photographs and drawings.
"Within this farm group that developed between 1826 and 1910, two significant structures are the corn crib and hog barn. The corn crib is an excellent example of early corn cribs in the Delaware River Valley, featuring a frame that is hewn and pegged."
This corn crib in New Jersey is therefore about the same age as mine in Vermont; they are not the same.


Here are the HABS drawings.

 It is so different from  my Vermont corn crib that if the sides did not slant I would have thought it just a shed.

The layout is governed by its width. The posts are placed  by 2 red squares and the arc of the squares' diagonals.
The same sized  2 red squares and the arc of the diagonal determine the height of the crib and its roof.
A crew could lay out the plan on site with twine,
then layout the section of the crib on a framing floor with twine,  cut the frame and assemble it on site.



The slope of the walls is set by the square: the sills - noted in red - are located inside the lower corners of the square, the mid-plates - also red - are located on the outside of the upper corners. the slope continues up to the height of the upper square.  The red line on the right indicates that the height of the wall is divided in half by the center girt.

 The crib design could have begun with a line and its perpendicular. The line could be the width; the perpendicular could have been one wall. Then with twine the arcs - in red - could have located the posts - also in red.

The upper brace in the section falls on the point where the diagonals crosses the arcs of the side of the square - noted in red. 
I have seen this only once in New England. The center brace is on the line of the squares.
The lower brace is not located on the arcs. Instead it is centered between the floor and the mid-brace.

To see the HABS photographs and drawings, type the name and location "Knight corncrib... " into your browser.