RISD ARCH 2153 WOOD STRUCTURES
Spring 2016
HW 6
Reading:
Textbook: Ch 11
Problem 1
What do these letter a mean in engineered lumber...
LVL - laminated veneer lumber
PSL - parallel strand lumber
LSL - laminated strand lumber
OSB - Oriented strand board
CLT - cross laminated timber
MSL - machine stressed lumber
TJI - truss joist I beam
Problem 2
What is the bending strength in psi for Microllam LVL? 2600psi
Problem 3
What is the tension in member AB? If you are using a wood with F't = 450psi, what size wood
member do you need is square in cross section?
6ft
4ft
Step 1 Create Free body diagram, cut frame replace with parallel vectors....
6ft
4ft
P
RISD ARCH 2153 WOOD STRUCTURES
Spring 2016
Step 2 Convert Angled vectors to X and Y components
X
6ft
4ft
Y
Step 3 Sum Moments about a Point - let's pick point C...
X
6ft
4ft
Y = 750#
Sum Mc = 0 = 500# x 6ft - Y x 4ft, therfore Y = 500 x 6/4 = 750# = Y
Step 4 Do method of joints and pick that joint to sum Fx and Fy...
P
Sum Fy = 0 = -750# + 0.707P, therefore P = 750/0.707 = 1061#
Step 5 If you are using a wood with F't = 450psi, what size wood member do you need is square
in cross section?
F't = 405psi = P/A
Therefore A = P/450 = 1061/450 = 2.36 sq in
Need 1.54" sides...
Use a 2x2 member, which is 1.5" x 1.5" = 2.25sq in- close enough (or go to 3x3)
RISD ARCH 2153 WOOD STRUCTURES
Spring 2016
Problem 4
Assume a king post truss below at the bottom joint, and given that the compression in the post
is 10,000#, what is the stress in the rope if the diameter is 1/2"?
Sum forces in the Y - Direction
ADy + CDy = 10000#
Based on symmetry, ADy = CDy
Therefore, ADy = 5,000#
5000#
a
ADx
AD
Force in rope member AD is using trig...
AD sin (a) = ADy, therefore
AD (1/sqrt5) = ADy
AD = ADy / 0.447 = 11180#
Area of Rope = 0.196in2
Stress = P/A = 11180#/0.196in2 = 56,941# or 57ksi! Need steel cable.
RISD ARCH 2153 WOOD STRUCTURES
Spring 2016
Problem 5
Assuming a roof simple triangle truss with 2x members at 24” on center with 25psf snow and
10psf dead load. The slope of the roof is 3:12, what is the tension on the collar tie?
Total = 70.6 x 11ft = 777#
Sum Fx = 0 ….. T = H
Height = 11 tan angle = 2.75ft
Sum Moments about Ridge
777 (5.5ft) – 777(11ft) + T (2.75ft) = 0
T = 1554#