Davis, Alexander Jackson, 1803-1892
Page 1
Enquiry No. 1, "Proper thickness and depth of joist:"
Answer. For a bearing of 20 ft. or so — 3 by 12 inches.
From centers — 16 [or 10] inches apart
Remarks: By referring to my sketches of the design for
the additions at Chapel Hill, I find two cross walls on the first story
floor dividing the dormitory rooms; these walls support 2
girders, upon which, the posts of the Odeion, together with the floor of the
same, rest. There is a post between each window. The second post, from the front wall, rests upon this cross wall, or upon
a pier in the cellar (in case no cross wall was built in the cellar, (I have no plan
of cellar to tell me whether a wall was drawn there, or only a pier ,) there was certainly at least a pier.) and this pier was intended to give support
to the girders above, and even to the roof, by resting posts upon them in each
story. I had originally drawn alcoves in the dormitory rooms, and the post dividing the two alcoves was intended to have
support in the cellar and give support to girders above, which would have made a
very stiff floor.
Over
Page 2
In our part of the country, we should place joist alternating, as shown in No.
1, lapping over the hall, and the partitions of hall would be of brick, 8 to 12
inches thick in cellar, and principal floor, and this would form the floor of
the Odeion, No. 2. The two middle (black above in sketch) posts receiving a firm
support from below, and the other posts standing upon a plate resting upon the
joist of floor. These posts of the Odeion were to continue up, or be framed in
two stories, the upper one the Library, and also support both the girders of
floors, and also the roof. The roof would need no trussing, those posts bearing
the purlin and rafters of roof.
A.A. Girders supported by posts.
The partitions of alcoves may have braces to prevent the possibility of the
rafters spreading, so that no tie or iron rods would be necessary.
B posts in library, acting as queen posts, and bearing the weight of roof. The
same being so flat as not to be liable to thrust out, but lie with a
perpendicular weight upon these posts. The triangular space being wall braced.
Frame in, and secure the joists to the girders by pinning them, if they do not intertie, as 1. 2. above.
Page 3
The floor of library should have joist, 3 by 12 crossing from the wall to the
girders on the opposite side of room, 1, 2, in every 4 or 5 joist. This would
tie the walls together, and make firm work. The ties would be 30 ft. or so in
length. The joist should have a double tier of cross bracing:
between them, and be deepened with 2 inches of clay, sand, and lime.
There is still another way of laying the joist, thus, and either way would be
sufficient for good work, laying the joist length wise, instead of crosswise,
but still
depending upon the piers A A for support to be girders, 12 X 12
running across instead of lengthwise, reversing the position of the timber. This
mode is not so good as the other, as the sidewalls would not be so well tied
together. The tie however, would depend upon having the ends
secured to the wall by irons, at least in a few
places. No. 1, on the other page is the best mode provided the joist have a wall
of entry hall to rest upon. If not No. 3 is best with girders running lengthwise
and the joist over lapping. 1, 2 and thus tieing the 2 sides together.
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