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RE: PRE-DIMENSIONING HELP

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Hi Arthur,
	I have a simple empirical recipe that might help you. I use ASD for that
kind of structures. If you have an analysis and design software you might do
a preliminary analysis with prismatic members. At the knee and the ridge,
bending governs (usually) so I use S = M / fb and I use 23 ksi for A-36
steel and 33 ksi for A-50 steel as fb. This will give you an approximate
section modulus that you can check with W shapes and get an approximate
initial section.

	On the other hand, at the base of a frame (and in the roof beam) the depth
is going to be controlled by shear, and some practical considerations (i.e.
space needed for anchor bolts, depth must be sufficient to allow the welding
heads to clear the flanges) another thing to remember is that a column must
also be designed for wind and sometimes the wind moment may control at mid
height of the column and that may control your taper. But if you still need
an approximate section to begin, try A = V / fv and use 14 ksi and 20 ksi
for A-36 and A-50 respectively. This will give you an approximate area that
you can search in W tables to get a starting section.

	REMEMBER this are ONLY SUGGESTIONS and you should be very careful when
applying them. Also note that every frame may change due to the applied
loads and combinations.

Hope it was useful,

Francisco


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Francisco Diego
RAM International Latin America

Homero 1933  10? Piso
Col. Los Morales Polanco
11560 Mexico, D.F.
MEXICO
tel. +52 (5) 395-6782
     +52 (5) 395-3178
fax  +52 (5) 557-8222
e-mail: francisco(--nospam--at)ramint.com
URL: http://www.ramint.com
*******************************



-----Original Message-----
From: Arthur Ashby [mailto:arthurashby(--nospam--at)yahoo.com]
Sent: Thursday, June 28, 2001 13:56
To: seaint(--nospam--at)seaint.org
Subject: PRE-DIMENSIONING HELP


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Your following message has been delivered to the list
  seaint(--nospam--at)seaint.org at 12:54:40 on 28 Jun 2001.
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Hello Everyone,
Imagine a gable roof rigid frame with tapered members
seen from the front. At the left you have a tapered
column (C1), then you have a rafter from the knee to a
certain distance (R1) and then you have another rafter
that goes from that point up to the ridge (R2).

Are there any rules of thumb for pre-dimensioning of
this kind of frames?  Obviously, dimensions depend on
span, loads, materials, etc. But I would like to know
if there is a simple way to determine:
a) the initial column C1 depth
b) the final column C1 depth at the knee
c) the initial rafter depth R1 at the knee
d) the distance from the knee to the "transition
point"
e) the depth at that transition point
f) the depth of rafter R2 at the ridge

Thanks for your opinion.

Arthur


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