Given: We have an 8″ thick slab. You should estimate construction live load as 50% slab weight and
construction dead load (i.e. formwork weight) equal to 10% slab weight. The contractor has metal
shoring towers that are erected on a 5‐ft x 5‐ft grid. So that’s our shoring spacing and stringer spacing.
The plyform on the jobsite is the nice stuff, Structural I, not Class I. This will change your section
properties (Table 4.13) and your rolling shear stress (Table 4.14). The plywood is 5/8″ thick oriented in
the strong direction. In general, all of the lumber and plywood on the job has been used substantially in
the past, so you’re going to be conservative and not take credit for load duration. In other words, be
safe and use CD=1.0 for the entire design.
a) Since the stringer spacing and shore spacing are given up front, let’s immediately do a check to
ensure the metal shoring towers are okay. Compute the load on the shores and compare to the
tower capacity. For reference see your textbook, Fig. 4.70, pg. 4‐46 and Fig. 11.114, pg. 11‐38 to
see what we’re talking about. They are also described in detail on page 11‐38, section 11.9.6.
Suppose we look up the particular brand online and find out they are rated for 11,000 lb
per leg at full extension. Are we okay?
b) Specify the required joist spacing. Consider 3‐span continuous. Check bending, deflection, and
shear. When checking deflection, limit deflection to the lesser of L/360 or 1/16″. Notice in the
equation for max = 1/16″ the stuff inside the radical is raised to the 1/4th power, not cube root.
c) The contractor wants to use 2×4’s for the joists (preferably), but if they won’t work he/she can
use 4×4’s since there are a lot of these on the jobsite already. Basically they have a large
shipping container on site full of DF‐L, Construction grade, 2×4’s and 4×4’s.
What joist size should we specify to the contractor? Limit deflection to L/360 and
remember we’re not taking credit for CD. Also, your stringer spacing is already set at 5‐ft
o.c. Check bending, deflection, and shear.
d) Now we have to specify the stringers. With 5‐ft tributary width we know the 2×4’s probably
won’t work, so let’s check to see if 4×4’s will work. Same inputs as the previous step: L/360,
CD=1.0, shore spacing is 5‐ft o.c.
Check bending, deflection, and shear for 4×4’s. Are we okay?
Out of curiosity, what sizes is construction grade lumber available in? Use Table 4.3 to
answer this. List the sizes.
e) Another option for the stringers is steel or aluminum.
Suppose we went with a steel section and we’re using W8x10. What is the applied
moment on the stringer compared to the allowable moment? Assume simplysupported.
Use basic steel design tables to figure this out. And don’t make it
complicated – you may need a refresher but you’re going to be looking at tables, not
doing crazy math. Ask if you need help.
f) Finally let’s consider an aluminum option. See the attached PDF entitled “SAFWAY shoring
systems.” Take a look at the aluminum joists listed at the bottom of page 9.
Can we use these aluminum joists on our job? Compare applied load to allowable load
(plf).
g) A comparable wood member in this scenario might be a 4×8. Compare the weight per foot of
these three members.
What is the weight per foot (plf) for W8x10?
What is the weight per foot (plf) for a 4×8?
What is the weight per foot (plf) for the aluminum joists in the PDF?
h) Will all three options noted above fit in the U8 headpiece listed in the same PDF, sheet 8?
Problem 2
Design the formwork for a 10‐ft high concrete wall. The rate of pour is 6 ft/hr at 50°F. Assume normal
weight concrete (150 pcf) with retarders. All lumber is Construction grade, Douglas Fir‐ Larch. Assume
7‐day construction loads for the load duration factor. Limit deflection to L/360.
a) Specify the stud spacing using 7/8″ sheathing (Plyform, Class 1, B‐B used wet) with face
grain perpendicular to span (i.e. weak orientation).
b) Specify the stud span (wale spacing) if 2×4’s are used?
c) Using double 2×4 wales, specify the tie spacing. For the purposes of this problem, round
your value down to the nearest even number.
d) If the safe working load of the ties is 4,000 lb, are we okay with this design?
e) Check bearing between studs and wales, compare to allowable. Use Cb only if needed.
f) Check bearing between wales and washers. See illustration below. The width of the plate
washer measured horizontally is 2″ and it is 4″ measured vertically. The gap between wales
is 1/2″. Use Cb if needed, see table 7.4 in your textbook
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