Concrete Pressure on Inclined Formwork 1

[Bridge100]

Could somebody please direct me to a good concrete formwork publication or help me with the following problem? I’m designing formwork for an inclined concrete column but I can’t find anything that deals with this problem. The contractor would like to pour the entire 40 Ft tall column in one day.

Looking at the design of the underside incline bulkhead I am considering the following:

All vertical and lateral loads must be converted to force components normal to the forming face. (See attached method B)

The shear components of the vertical and lateral loads will not be resisted by the form since the form does not have shear connectors or friction with the fluid concrete (See attached method A). Therefore, the shear component of the vertical concrete load will travel through the previously cast pier stem below.

The lateral load would be based on an assumed allowable form pressure. I know that ACI 347 suggests designing for full liquid head for columns this tall; however, it has been the contractor’s experience to use penetrating rods to determine when initial concrete set has occurred.

The vertical normal component would vary from zero at the top of the form to 150pcf*40’*cos2(A) at the bottom.

The lateral normal component would be a uniform load of 150pcf*Liquid Head (Ft)*sin2(A).

Can anybody see problems with this approach?

 

[BAretired]

If you consider concrete as a liquid with unit weight of 150 pcf, the pressure is 150*H where H is the head. The pressure acts normal to any surface.

If the rate of pour is slow enough to allow the concrete in the lower part to attain initial set before the upper part is cast, the form pressure will be reduced by a factor which depends on the rate of pour. I don't know what that factor is, but if the contractor can determine when initial set has taken place the pressure on the sloping form should be no different than that of a vertical form, should it?

A book on formwork would be a good source of information, but in any case, a little conservatism seems warranted.

BA

 

[Bridge100]

So you believe that the majority of the concrete dead load above the bulkhead is acting on the lower stem more than the bulkhead itself?

 

[BAretired]

If the concrete remains liquid throughout the pour, the weight of concrete acting on the bulkhead is the full weight of the shaded triangle on your lower diagram. This must be transferred to the pier by compressive stresses in the bulkhead varying from 0 at the top to maximum at the bottom.

If the concrete has partially set up in the lower part when the pour is completed, some of the load will be transferred to the pier by shear in the set concrete. The pressure on the bulkhead at any given time will depend on the rate of concrete placement.

BA

 

[jberg]

BA,

You are correct about the rate of placement affecting the lateral pressure on the formwork. Attached is a link to a Dayton Superior forming design guide. If you refer to Page 14 of the attached PDF, they list lateral pressures based on the rate of concrete placement.

Hope it helps!

JWB

 

[BAretired]

Thanks, jberg that is a big help and not something that could be calculated from scratch.

With a height of 40' and the requirement that it must be poured within a day, it would be necessary to place concrete at a rate of 40/24 = 1.67 (say 2) feet per hour. Assume a temperature of 50⁰F.

Then P = 150 + 9000*R/T = 150 + 9000*2/50 = 510 psf maximum.

But this would run into overtime. If the whole pour had to be completed in 8 hours, R = 40/8 = 5 feet per hour. Then, P = 150 + 9000*5/50 = 1050 psf max.

If the contractor really wants to speed things up, let us say he wants to complete the pour in 4 hours. Then R = 40/4 = 10 feet per hour. Now, with this high rate of pouring, we have a different formula for pressure, namely:
P = 150 + 43,400/T + 2800*R/T
= 150 + 43400/50 + 2800*10/50 = 1578 psf max.

The rate of placement will affect the pressure, P but whatever value we assign to P, it must be applied normal to the bulknead. It is not necessary to consider the vertical and horizontal components separately.

The cost of additional formwork pressure versus savings in time are factors for the contractor to consider.

BA

 

[Bridge100]

BA,

You stated earlier, "This must be transferred to the pier by compressive stresses in the bulkhead varying from 0 at the top to maximum at the bottom." This is the main point of my question: How can there be a compressive force in the bulkhead if the loads are applied normal to the bulkhead form face without applying a shear force?

I am breaking my loads into vertical and lateral components because I feel that the vertical load cannot be reduced regardless of the adjustment to the rate of pour, unless we consider the shear strength of the initially set concrete. I would not dare do that! Only the lateral component can be adjusted based on rate of pour.

One additional item of infomation that I should have mentioned before is that the pier is a mass concrete pour. All guidelines that I have seen recommend full liquid head design; however, the contractor has seen setup in previous jobs by using tamping rods and load cells within the form ties to guage pour rate.

 

[hokie66]

BA is correct. Pressure by concrete is in all directions. The pressure should be taken normal to all surfaces, and it is not necessary to resolve the pressure into components.

A separate issue is that the formwork must be able to support the gravity weight of the concrete globally between the formwork supports.

Don't confuse or combine the two loading conditions.

 

[BAretired]

Bridge100,

How can there be a compressive force in the bulkhead if the loads are applied normal to the bulkhead form face without applying a shear force?

I assumed that your walers would carry only horizontal forces, leaving the bulkhead to provide static equilibrium.

Your "column" is more like a trapezoidal shaped wall. I am assuming that the trapezoidal sides will have walers and snap ties through the wall to hold the bulkheads in place. Say that 'S' is the waler spacing and 'W' is the width of column or wall.

The sloping bulkheads will have short walers spaced at 'S' vertically (S/sin70 diagonally) and held in place horizontally by the long walers which act in tension.

The sloping bulkhead normal force per waler set is F = P*W*S/sin70 where P is the concrete pressure. This can be resolved into two components, one horizontal, the other, parallel to the bulkhead. So the tension carried by the two side walers is P*W*S/sin²70 = 1.132P*W*S and the axial force added to the bulkhead is P*W*S*cot70/sin70 = 0.387P*W*S.

BA

 

[Grahammachin]

I have a question regarding the pouring of the concrete itself rather than the formwork itself.

How is the contractor planning on pouring the concrete? Will he be pouring from the top so that the concrete will be hitting all the reinforcement on the way down? Or will he pouring from the bottom up through a tube which he will be pulling up as he fills the column?

I believe there is a rule of thumb about not allowing concrete to free fall more than 1.5-2m (5'-7') through a reinforced cage as this will cause segregation of the concrete.

Also is the contractor going to guarantee that the concrete at the bottom of the formwork will be sufficiently compacted so as to minimise the risk of honeycombing at the bottom of the pour?

 

[BAretired]

Another option to avoid segregation is to pour concrete through "elephant trunks", a flexible hose made of canvas which prevents the aggregate from colliding with reinforcement. This technique is sometimes used when pouring deep piles.

BA

 

[Bridge100]

Grahammachin - The concrete will be placed with a pipe from within the form. The pier form is plenty wide to fit crew members inside to work the concrete from bottom up.

BA - I can't figure out your last 2 equations. Do you have a reference that you can provide? I am still confused as to how we have an axial load on the bulkhead. I have attached another sketch to try to help clear up my question.

 

[BAretired]

Bridge100,

I don't have a reference offhand, although I might be able to find one. The principal is essential for you to understand if you are designing formwork. Let me try again and if anyone else can explain it more clearly, please join in.

Suppose the elevation on your attachment shows an aquarium built with glass walls. The triangular area shown hatched has to be carried by the sloping wall. Its area is H²tan20 = 0.364H² where H is the height of aquarium. The volume of water above the sloping wall is 0.364H²*W where W is the width of tank. That load must be carried entirely by the sloping wall because there is nothing else available to carry it.

The axial compression at the bottom of the sloping wall must be:

0.364H²*W*[γ]/sin70 = 0.3870H²*W*[γ] where [γ0 is the unit weight of water.

If you consider any vertical section within the shaded region, suppose the depth of water at the section is z. Then the axial load in the sloping wall at depth z is 0.3870*z²*W*[γ].

Are you okay with that, so far?

BA

 

[Bridge100]

BA,

One minor correction on the area equation should read: A=.5*H^2*tan20.

Besides that, I would agree with your final equation for the axial load.

Go on....

 

[BAretired]

Bridge100,

You are correct about the factor 0.5 for the area of a triangle. So the axial compression, C at the bottom of the inclined form is:

C = 0.5*H²*W*[γ]tan20/cos20
= 0.5*40²*W*150*0.387 = 46,500W#

That is a very substantial compression. It is based on the assumption that the concrete remains fluid during the entire pour and also that the side forms are capable of resisting horizontal tension only where they connect to the end forms.

If the contractor adjusts his rate of pour, the lateral pressure and hence the axial force will be reduced accordingly.

BA

 

[doka1]

When we form battered walls we stab coil rod into the footing to have somewhere to hold the form down. Columns are tricky because people like to fill them up quickly, but if they take there time and rod the concrete they wont have any problems. Rodding is the method by which you can determine how the concrete lifts are setting up. Formulas are great for the initial design, but the contractor should follow this procedure. Concrete sets up differently depending on the temperature and admixtures. Not sure what MK Hurd's book has in reference to battered wall formwork design, but there are adjustment factors for temperature etc.. But your typical batter is 1:12, with a coil rod at 3/4" spaced at like 3' horizontally (conservative), that would be a good place to start calculations. The weakest part of the system is the attachment point on the forms.

 

[cap4000]

I have done a battered concrete dam holding back water which the structural analysis is similar. I used 10 foot concrete pour based on 150pcf. That 10 feet must set and then another 10 feet could get poured. I get the two force components based on 1 foot width of formwork Fy= 10.9k and Fx = 30k. The resulatant is 31.9k. with the angle between the component as 20 degrees which checks with your 70 degrees. Clearly a lot of load must be ditributed to the formwork and or wall ties. Good Luck.

 

[SAIL3]

Agree that press. load is normal to all surfaces. If consider the
conc. as a liquid, then by what mechanism does this axial load
end up in the sloped side, since there is no friction between the
sloped side and the conc.
Suppose one has a liquid container shaped like a square box.
Case#1...All sides vert...fill with liquid.
No axial load in sides, just bottom perimiter load.
Case#2...slope one side, say 60deg...fill with liquid.
sum up reactions at base...results in unbalanced moment.
The two sides(closest to the sloped side) react this
moment in bending. This results in a stress distribution
of tension in portion furthest away and compression
in portion closest to the sloped side. The sloped side
will have hoop tension and shear normal to the face.

I could be all wet on this one(excuse the pun).

 

[BAretired]

Case#1...All sides vert...fill with liquid.
No axial load in sides, just bottom perimiter load.

Neglecting wall dead load, axial load in walls = 0 vertically, variable horizontally, uniform water pressure on container base.

Case#2...slope one side, say 60deg...fill with liquid.
sum up reactions at base...results in unbalanced moment.
The two sides(closest to the sloped side) react this
moment in bending. This results in a stress distribution
of tension in portion furthest away and compression
in portion closest to the sloped side. The sloped side
will have hoop tension and shear normal to the face.

Pressure on base remains same as Case #1.

All four walls act like a hollow section of varyiable depth to carry the additional eccentric mass of water. Sloping wall acts in compression along slope. Opposite wall acts in vertical tension. Side walls act in flexure with variable vertical stress. All walls carry horizontal tension.

BA

 

[BAretired]

The container in Case #2 does not perform quite the same as formwork. The walls and base of a container are connected together to form one rigid body. The side bulkheads of this thread are constrained to resist only horizontal tension from the inclined bulkheads.

BA