Help! Ncees structural question 508

[SF_century]

A concrete slab is continuous for six spans. The slab has a uniform thickness and is supported by 12-in-wide concrete beams at 10 ft o.c. The ratio of the suns of column stiffness to beam stiffness is less than eight to all spans. The slab experiences the following uniform loads (factored):

DL 100 Psf (includes slab weight)

LL 50 psf

Referring to aci 318-08 (excluding appendix c) and using the aci approximate method, the maximum negative moment (ft-kips) on the slab is most nearly ( a positive moment is tension on the bottom face):

A.) -0.81

B) -1.10

C) -1.35

D) -1.50

I understand what length to use but I don't understand the solution provided, because it does not include the DL or LL and just the concrete weight AND it doesn't factor the dead load. ??? Help.

 

[SF_century]

I understand this problem now. The solitons uses a trib width of one foot giving them 150 plf. sorry please ignore this post.

 

[Andy Lin]

I don't have the book but what's the answer? C?

 

[darius]

ACI 8.3.3

You have

w*ln*ln/10 = 1.5 Governs

w*ln*ln/11 = 1.36 OK but does not govern

w*ln*ln/11 = 1.25 (Not good because ratio of stiffness less than 8)

 

[SF_century]

The answer was d.

 

[kennyb04]

I had an issue with this problem initially as well before I spent a little time working through it. First, it says the load is ALREADY factored and includes the weight of slab. I missed that part and factored it again. I also initially assumed the beams and slab were monolithic, but I guess that was an incorrect assumption since the problem didnt' implicitly state that, so I ended up with the incorrect moment equation.

 

[BoilerEng]

ACI 8.3.3

You have

w*ln*ln/10 = 1.5 Governs

w*ln*ln/11 = 1.36 OK but does not govern

w*ln*ln/11 = 1.25 (Not good because ratio of stiffness less than 8)

Quick question on clear span - why do you use 10 ft (beam spacing) instead of 9 ft (beam spacing - 1/2 beam width each side)?

 

[kennyb04]

ACI 8.3.3

You have

w*ln*ln/10 = 1.5 Governs

w*ln*ln/11 = 1.36 OK but does not govern

w*ln*ln/11 = 1.25 (Not good because ratio of stiffness less than 8)

Quick question on clear span - why do you use 10 ft (beam spacing) instead of 9 ft (beam spacing - 1/2 beam width each side)?

I believe 9 ft would be used if the slab was built integrally with supports (ACI 318-08 8.9.3). However, the problem does not state whether it is built integrally or not.

 

[BoilerEng]

ACI 8.3.3

You have

w*ln*ln/10 = 1.5 Governs

w*ln*ln/11 = 1.36 OK but does not govern

w*ln*ln/11 = 1.25 (Not good because ratio of stiffness less than 8)

Quick question on clear span - why do you use 10 ft (beam spacing) instead of 9 ft (beam spacing - 1/2 beam width each side)?

I believe 9 ft would be used if the slab was built integrally with supports (ACI 318-08 8.9.3). However, the problem does not state whether it is built integrally or not.

Thanks for clarifying. I see now that ACI 8.9.1 and 8.9.3 specify exactly what you're saying.

 

[ptatohed]

I'm still trying to figure out why this thread needed the exclamation mark. :dunno: