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]

Oops, I didn't attach my sketch, so here it is.

BA

 

[Bridge100]

BA,

I like the sketch that you included in your last post - it added a new perspective to the solution.

From your sketch, are you saying that there is an axial load on your inclined form because of the load or because of the tie? This is the most important question. You show a tie component in the lower sketch but the upper sketch does not show any ties; it only shows reactions normal to the inclined form.

I have included some analysis of my own based on your wedged mass. The analysis on the left is similar to your sketch. I created a triangular concrete element that is 10 Ft Tall by 3.64 Ft Wide 1 Ft Thick. The summation of the reactions on the inclined form makes sense. I used 3.64 Ft to keep the 70 degree incline.

The total weight of the element is (0.150 kcf)(0.5)(10')(3.64')=2.73 kip

The summation of the inclined reactions = 2.73/cos 70 = 7.98 kip or 3.64+4.34=7.98 kip.

The second case uses a linearly distributed load placed normal to the surface. This case uses full liquid head of (0.150 kcf)(10’) = 1.5 k/ft. The summed reactions of the inclined forms are agreeable between cases 1 and 2, although the individual reactions differ greatly due to the varying locations of the load centroids.

The pressure within the form will be constant at any given depth and will act normal to any surface if we are considering a fluid. I would like to refer back to my attachment from my 8 Feb 11 9:02 posting. The pressure at the bottom left is the greatest, not uniform. I would attribute this to the initial setting of the concrete. The maximum pressure from the result view shows a maximum pressure of 120 psi. If the concrete could support 120 psi well before it actually experienced this loading, then the forms will not see this value applied because the concrete is strong enough to support it. If not, then the forms will experience 120 psi or 17,280 psf which is well beyond the 1200 psf design pressure that many forms are designed for.

 

[Bridge100]

Now I forgot the attachment - Here it is.

 

[Bridge100]

I would like to make one more point. I have attached a sketch of an external frame. If the force on the bulkhead is normal only and does not apply an axial load, as I am arguing, then the inclined member of an external bulkhead support frame could actually experience an axial tension force from the overturning forces on the frame. Let's say the form ties (not shown) are installed normal to the inclined form. They do not support any self-weight of concrete - they only help to hold down the other form on the top of the inclined concrete pier stem.

 

[BAretired]

Bridge100,

As shown in your latest sketch, the statics of this problem are very dependent on the method chosen to support the inclined form. You show a framed retaining wall which develops tension in the inclined form and compression in the vertical member. Under liquid pressure, the moment about the toe is readily calculated as are the three forces shown.

Under liquid pressure, the total force on the bulkhead is normal and does not stress the bulkhead axially. If the inclined bulkhead could be held in position in some way, it would have only normal pressure to resist but the vertical component of that normal force would precisely equilibrate the liquid weight. The forces acting on the bulkhead supports depend on the design selected.

If we consider modifying the liquid pressure as recommended by M. K. Hurd, the horizontal pressures are reduced but the vertical pressure remains the same. Assume your client pours at a rate of 5 feet per hour at a temperature of 50F.

At depth z, the form supports a pressure of 150*z vertically but only 1050 psf laterally if z > 7'. In the top 7', the pressure varies from 0 to 1050 psf, a substantial reduction from liquid pressure. This greatly reduces the moment about the base and results in much smaller forces for the bulkhead support structure.

BA

 

[Bridge100]

BA,

So if we have a 40' Column on a 20 degree incline and 1050 psf lateral load, what would you say the maximum normal load is to the form at the lowest point? I come up with 1630 psf. What do you think? I think we're almost done here!!

 

[BAretired]

Sounds right to me, Bridge100.

BA

 

[psmaxtor]

BA,
Appreciate your posts/sketches. I agree that concrete axial force does not transfer thru bulkhead form. Your vector method and Bridge 100 cos^2 method produces the same results.

One last thing..Looking at the liquid Concrete sketch you provided the Normal force (N) shall read:
150h^2/(2 sin alpha). Not (2 cos alpha).

PS

 

[Bridge100]

From what I have seen, we need full liquid head applied normal to the inclined form to account for the full weight of the concrete above. If you reduce the lateral pressure to a value less than that figured from the full depth, then we would not see the full load of the concrete on the form. Does everybody agree?

 

[BAretired]

psmaxtor,

You are correct about the value of the normal force. Thank you for pointing that out.

Bridge100,

What would happen if, instead of an inclined bulkhead, you used a stepped bulkhead with risers of 12" and treads of 12tan20 = 3.5"? Would you not get 1050# pushing against each riser and the full head of concrete bearing on each tread.

Is the assumption of zero friction between concrete and inclined form valid, or is it possible that the bulkhead feels axial load resulting from friction.

I just don't feel comfortable saying that freshly placed concrete can be expected to take substantial shear stresses before it has reached its final set and has had time enough to make strength gains.

BA

 

[Bridge100]

BA,

I agree that if you used a stepped bulkhead as you describe that the concrete pressure would bear normal to the riser and tread with the lateral and vertical forces. You would have axial load on the bulkhead from these forces.

I share your same concern about the zero friction value between the concrete and the inclined form. However, if one did assume some amount of friction, then the friction force would be the result of the friction coefficient times the normal force to the form. The friction coefficient should be small, if any. I have seen an example of a grain bin design with sloped walls where the grain pressure acts normal to all walls of the bin and the friction force is due to the coefficient of friction between the grain and the wall material.

I am currently considering the following for the pour:
1) Smaller, plumb piers will be poured first with the same concrete mix. This will give us a chance to study the concrete strength.
2) The crew will rod the concrete as the pour advances.
3) We will install load cells in line will several of the side form ties to verify the expected pressure.
4) We can take early concrete cylinder breaks to check concrete strength. If strength is lacking, we will have quite the pour and use a construction joint half way up.

 

[BAretired]

B100,

I found the following link on the coefficient of friction between wood and concrete, but I imagine they are talking about hardened concrete on non-oiled wood.

http://www.engineershandbook.com/Tables/frictioncoefficients.htm

As you can see, the coefficient for static friction is listed as 0.62 but I'm not too sure whether this applies to your situation.

This is an interesting question, one that I have never really thought about prior to this thread.

BA

 

[BAretired]

It might be useful to perform a test on recently placed concrete on a horizontal form to determine the coefficient of friction between the two materials. The test should be relatively easy to perform.

BA

 

[BAretired]

After further thought, I don't think friction is critical. The attached sketch shows a bulkhead with a hinge at the bottom and a horizontal reaction as shown at height 'a' above the hinge. The choice was arbitrary. The bulkhead could be supported in a variety of other ways.

Reactions are shown. Axial force, shear and bending moment can be calculated at any point along the bulkhead.

Adding a multitude of horizontal ties instead of one changes the bulkhead to a continuous beam instead of a simple beam with cantilever.

BA