Concrete anchors at each face of wall

[Besi85]

Hi all,

Does anyone know of any guidelines on how to check (2) anchor groups [2 anchors per group], that are mounted on the outside and inside faces of a concrete wall? Both anchor groups are exposed to combined shear and tension.
Edge distance is not an issue.
Anchor groups are identical to each other and are at the same elevation.
Anchor spacing of anchor group = 399mm
Embedment of each anchor [4 tot anchors] = 300mm
Wall thickness = 16"

Anchor groups are placed in such a way that one anchor of anchor group X is equidistant to the anchors of anchor group Y. I.e. center to center spacing of anchors = 399mm/2=199.5mm and as I said earlier anchors are on the same plane.

For a picture of the anchor group look at:

http://www.perijapan.jp/en/products...entimage/5/productcategory_ID/11/app_id/0.cfm

Just to give a global picture, these anchors support ACS-R or P Peri brackets / work platforms which in turn make up a self-climbing formwork system usually used for vertical wall pours in high-rise buildings.

 

[RHTPE]

Besi85 - How about posting a sketch showing a section thru the wall. From what you stated, the anchors extend into the wall almost 12". From that I would say they can't be in the same plan location and at the same elevation on both wall faces, as they would interfere with each other. If they do align in plan & elevation, I would consider a different style of embed.

You will need to consider the loads applied at each stage of the operation of the self-climbing forms and also consider when the forms on one face are moved relative to the other face. Unlikely that both wall faces get jumped at the same time.

I would put the responsibility for anchor design on Peri - it's their system and they should have some previous experience at demostrating how the anchor design can be proven.


Ralph
Structures Consulting
Northeast USA

 

[Besi85]

RHTPE,
Thanks for the response!

I have attached a sketch and hope it explains the situation better.
Yes, the anchors extend into the wall almost 12"
All inserts are in the same elevation [they sit on the same horizontal plane].
Within the plane they are staggered-see sketch.
They do not interfere geometrically.

Yes, applied loads are considered at various stages of operation.
These are the distinct cases:
1. In-operation = working
2. Climbing
3. Out-of-operation = storm

The adequacy of the platforms and brackets as been verified.
The engineering of record will verify that the forces are successfully transmitted into the building structure.

I need to verify the strength of the embeds during cases 1-3. Hope this clarifies a bit.

The platforms are independent of each other. If one moves before or after it does not matter, so long the forces on the embeds are accounted for.

Peri has no guidelines/bulletins on the proximity of opposing anchors. They do have interaction diagrams for the anchor groups [which are meant for internal use only] but they do not publish any reduction factor formulas for anchor spacing and such. To make matters more difficult these anchors "see" tensile forces in opposite directions and their cones of failures overlap.

 

[RHTPE]

Besi85 - Let's examine what you have in the context of a wall forming system application.

- shear force from forming system dead load would be applied simultaneous on both wall faces.
- shear force from construction live load will vary from side-to-side, but is unlikely to be at full magnitude simultaneous on both wall faces.
- depending on the forming system you probably will not carry any loads from concrete placement into these anchors.
- wind loading is probably the highest load applying tension to the anchors. It is not going to occur simultaneous on both wall faces.
- I would also carefully examine the shear cones on each wall face to determine if they will indeed overlap. It may only require a small dimensional change to avoid the overlap.

Do you work for the entity providing the forming system? If not, I would put the burden on them.


Ralph
Structures Consulting
Northeast USA

 

[Besi85]

RHTPE

-agreed
-agreed as far as the probability goes, but the full live load has to be applied on both sides
-agreed, this type of formwork has the ability to resist forces from concrete placement [wall forms are tied with threaded rods, which take all the lateral wet concrete forces in tension]
-agreed, wind can blow only one way, so it will induce additional tension to only one anchor group, conservatively the total tension is applied to each opposing anchor group
-The failure cones do overlap. It turns out that it is a big deal to relocate these anchors.

I do not work for PERI, we are reviewing their comprehensive formwork calculations and drawings and we make sure that the anchors can take the applied forces by specifying the minimum concrete compressive strength needed for working, climbing, storm.

After talking to a statics department engineer in Germany, it turns out that he had done some tests on this situation a few years ago. The tests showed that the overlapping cones between the opposing anchor groups actually help in capacity because there is a good chunk of concrete in the cone intersection that is compressed. This test would be if both the opposing anchors are loaded at their max. I am awaiting his test results and the "theory" behind it as he told me.

Obviously if one of them is loaded, this is no different as having one anchor group.


Furthermore, the walls are reinforced. There is no question about the big capacity boost you get by the reinforcing steel, it basically makes the concrete so strong that the only failure mode becomes the steel shear/tension failure.

The individual anchor group is adequate to carry the max applied forces. So having the anchor groups closer together in opposite faces of wall, only helps their capacity. This would be my conclusion. I am interested to see his report.

Thanks for all your input!

 

[RHTPE]

Besi85

I'm assuming that your sketch of the anchors was a plan view.

See my sketch attached. Shear in the anchors should not be an issue, as it's probably more dependent on the strength of the steel. It's the safe tension load on the anchors that may suffer as a result of the close spacing. As I said previously, it is unlikely the anchor group on BOTH sides will feel tension at the same time, so a reduction in safe tension capacity would only be required because of the embed spacing.

ACI 318-05 Appendix D does not address a condition with anchors on opposing faces of the same wall. Figure RD.4.1, i & ii show the likely failure modes.

I would be interested to see what the folks in Germany provide. I can see how the concrete within the overlapping failure surfaces would be in compression. I would be inclined to move the failure surface to the middle of the overlap (thus reducing hef, or h'ef in 318 Ap D) and run the numbers. If things check out with a reduced failure surface area, I would probably stop there.

If more capacity is needed, then I'd want to see the test results AND the write-up & diagram on how the testing was done.

Ralph
Structures Consulting
Northeast USA