Problem from PE Review.net, Tie capacity

[lachummers]

I have been mulling over this problem solution for a while and cannot sort it.

Problem statement:

"A retaining wall is constructed by pouring concrete between temporary wood forms. The concrete is at 70EF, has a setting time of 11/2 hr, and is placed at a rate of 2ft per hr. The wall sheathing is constucted of 1-in x 6-in (nominal) boards, wales, and studs. 1/2 in diameter threaded-steel (yield strength=36 kips/in2) tie rods are used to connect the wales between the two walls. The wales are spaced 2.5 ft apart vertically. The following characteristics can be used for all lumber:

E=1.6x10EE6 lb/in2

Fb=1700lbf/in2

Fv=100lbf/in2

Fc=1200lbf/in2

If studs are spaced every 2 ft, the tie capacity in lbf is most nearly? (Note the units, lbf)

The solution says to assume that the allowable tensile stress in the ties is 0.6Fy

therefore Tie capacity=0.6FyA(tie)

The answer is 4000.

But the only way I get this is by calculating 0.6*32000*pi*(1/4)(1/4). This doesn't account for any of the spacing and furthermore is a number with lb as units, not lbf!

Can anyone explain this one to me?

thanks,

Laura

 

[lachummers]

I believe that it is a trick question meant to distract us towards the SPACING of the ties which relies on all the information above.

Tie capacity is inherent only to the size and material of the tie. Correct?

 

[ptatohed]

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