On 5/22/2017 1:34 PM, Bob Minchin wrote:
Tim Watts wrote:
Very hypothetical...


It's not as easy to visualise the strength as a simple pitched roof with
tie beams.
I reckon this is a non starter for 2.5m ridge height. Note that the wiki
article shows a flat deck on which the beams bear. this is critical to
the strength as it will be in tension to support the roof weight. Even
if you replaced this with horizontal joists they would end up around
chest height.

I'm not an expert, but I am not sure this is true.

In a traditional pitched roof, the apex is assumed to be pin jointed. So
that the rafters produce an outward thrust where they rest on the walls,
because when a distributed vertical load is applied they want to "flatten".

But when you add a tie beam, you have a triangular structure which can
support a distributed vertical load, and convert it into two vertical
loads, one into each wall, without a spreading force. The tie beam does
not have to be at the bottom of the rafters.

In a gambrel roof, the fillets resist the bending moment which tends to
make the structure "flatten". How rigid it is depends on the size of the
fillets, and how well they are fastened to the main beams. Doesn't it
become equivalent to the "Tee" structure drawn below in T's. The walls
are drawn with W's

T
T
T
TTTTTTTTTTTTTTTT
W W
W W
W W
W W

In reality there will be a small spreading force in the gambrel case
because of flex in the joints.

Structural analysts please shoot me down in flames.