Given: We have an 8" thick slab. You should estimate construction live load as 50% slab weight and construction dead load (i. formwork weight) equal...

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 2x4's for the joists (preferably), but if they won't work he/she can

use 4x4'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, 2x4's and 4x4'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 2x4's probably

won't work, so let's check to see if 4x4'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 4x4'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 4x8. 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 4x8?

 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 2x4's are used?

c) Using double 2x4 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