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Lateral Pmax on Formwork


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#1 Construction PE

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Posted 15 October 2011 - 01:05 PM

Sample Problem:
Due to a unique jobsite condition, a 20-in square column 12-ft high is to be concreted in 1-hr at a temperature of 60F (concrete). The maximum lateral pressure for the formwork is most nearly:
a- 1400psf
b- 1600psf
c- 1800psf
d- 1950psf

According to School of PE, the answer is:
Rate of placement is 12ft in 1-hr, so since R>7ft/hr use equation:

Pmax= (150) + (43,400/T) + (2800 R/T)
This gives an answer of 1433psf (so the correct answer choice should be A)

However, I have my doubts...

If you look at ACI 347, yes, the equation they used is for a rate of placement greater than 7ft/hr (ACI 347 eqtn 2.4) however, this equation 2.4 is for concrete placement in a WALL (problem states you are pouring a column).

ACI 347 provides a different equation to be used for concrete poured in a column... ACI eqtn 2.2:

Pmax= CwCc(150 + (9000R/T))
This "column" equation appears to be for all columns poured (not dependent on rate of placement)

If column eqnt 2.2 is used, the answer would be 1950psf (which would mean answer d is correct).

Lastly, if you look this topic up in CERM page 49-7 eqnt 49.3, CERM seems to agree with School of PE as they only present 1 set of equations (for placement <7ft/hr and for >7ft/hr). Neither CERM or School of PE make a distinction on whether a wall is being poured or a column.

I have come across a couple of similar problems and get them "wrong" for using the ACI 347 equation for columns.

Does anyone know if the PE exam would place more importance on rate of placement vs. what is being poured? Just wondering if I do need to use the "wall" equations to solve column problems during the test.

Any insight would be appreciated!
Thanks!

#2 civilized_naah

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Posted 15 October 2011 - 09:40 PM

There are a couple of small differences between ASCE 37 and ACI 347 on this topic. I think they are both on the references list. However, both are agreed on one point, i.e. the tabulated values for lateral pressure of green concrete canot exceed the hydrostatic pressure, p = gamma H, which is this case would be 150x12 = 1800 psf

#3 Construction PE

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Posted 16 October 2011 - 12:00 PM

Civilized, you are so right! I forgot about the p=gamma H maximum when writing the original post.

So we would be looking at answer choices:
c- 1800 (using the column equation) or
a- 1400 (using the wall equation with a rate of placement R> 7ft/hr)

#4 Jayman_PE

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Posted 16 October 2011 - 07:38 PM

I do not have the ACI 347 in my possesion at this moment but looking at ACSE 37, the rate of placement eqn. 4-4 applies for walls, where the rate of placement is 7 to 10 feet/hr. In this case you have a column poured 12 ft/hr. As alluded to earlier here maybe ACI 347 clarifies this.

Having said that the question asks for maximum lateral pressure on the forms. My choice would be c (1800), for ACSE 37 4.7.1. provides: "Unless the conditions of Section 4.7.1.1 or 4.7.1.2 are met, formwork shall be designed for the lateral pressure of the newly placed concrete given by Equation 4-1. Maximum and minimum values given for other pressure formulas do not apply to Equation 4- 1."

C c = w x h

Since we have what the question terms a "unique jobsite condition" by pouring a column at 12 ft/hr, eqn 4-1 should goven.

That's my 2 cents.

thanks,
Jason

Edited by Jayman_10x, 16 October 2011 - 07:40 PM.


#5 andyrich

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Posted 22 February 2012 - 03:35 PM

In the equation 49.3b (CERM), there is a caveat that R< 7 ft/hr for that formua to apply.

It also notes that the max pressure need not exceed 150h, or 2000 psf for walls ans 3000 psf for columns. The implication of this is that the same formula applies to both columns and walls. This section is based on ACI 347 requirements.

Since R>7 ft/hr, and based on this section (only), you would only have the 150h and <3000 to use. Since R>7, the rate of pour is pretty fast, and the concrete really is not set up much, so it acts much ike a fluid than a solid. Hence, I would agree with the 150h = 150*12 = 1800 <3000. The answer is C.

#6 andyrich

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Posted 23 February 2012 - 04:01 AM

I found out from the errata that the equation cited in CERM 49.3b applies when R>7, so the formula 49.3b does apply (which is the same as the ACI formula noted in the original post), and so I agree with the School of PE's answer.



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