Inadequate Embedment - Looking for Ideas 6

[DMWWEngr]

Background:
I'm designing a concrete member (door jamb - 8' tall) to support a flood gate. The concrete door jamb will be oreinted vertically and will be doweled into an adjacent wall. Then, so I could sleep at night, I was also going to notch the top and the bottom of the support (into the floor and ceiling) as well as put a "healthy" amount of reinforcement into the floor and ceiling.

Problem:
The problem that I'm having is that I can't get enough embedment into the adjacent wall to get adquate support for the dowels. I've been trying to epoxy the dowels in.

FYI: Shear=3800 lbs and Pull-out=4100lbs at the critical dowel - 7" spacing, 7/8" dowel, 4" Edge distance (opposite the direction of applied load).

Does anyone have any ideas as to how I could get adequate embedment?? Top and bottom dowels recieve too large a load to be effective on their own....plus the floor and ceiling are inadequate for the forces.

One idea that was suggested at work (not by a structrual engineer), was to thicken the adjacent wall to the point where I would have adequate embedment. Can you do this or does the "slip plane" between the walls significatnly weaken the embemdent?? This idea scares me!!

Any ideas on how to get adequate embedment OR change the design to make the dowels less critical?? All comments welcome!! ---
Andrew

 

[whyun]

Have you considered smaller dowels and closer spacing?

Smaller dowels should require shorter embedment. To off-set the lower capacity of smaller dowels, just put more of them.

If the current dowel size is heavily loaded and are already close to the minimum spacing, then you might be out of luck.

 

[jeg]

If you cannot drill for the dowels, I suspect you are encountering the rebar of teh existing wall.
Can you cut back the concrete of the existing wall to expose the rebar? Then hook (or weld) the new dowels round the exposed rebar. The dowels should be detailed as links eclosing the new rebar of the door jamb. When casting the new concrete, make sure it is a strong mix with a high proportion of OPC but a very low water content. You may need a superplasticiser to ensure compaction. This will generate a lot of heat and high strength ensuring that shrinkage causes the new jamb to pull back hard onto the old concrete. Good preparation and quality control should ensure a watertight joint.
Good Luck

 

[VKG]

Welding of embeemnt to existing rebars is not considered good practice. It is hardly 'embedded' into concrete and may spall the concrete over rebar (will deform the rebar also).

You should get proper bond length. If the force required is more, increase the dia of bolts, ut take care as it also require increased bond length and increase spacing.

 

[jeg]

Yes, I do agree with VKG that welding rebar can cause problems and should only be attempted by an experienced welder with experienced supervision. Welding of rebar is permitted by BS 8110 clauses 3.12.8.17 thru 21

 

[vmirat]

Instead of using individual rebar, how about cutting a slot in the wall and install a steel shear plate?

 

[DMWWEngr]

I can drill the dowels. My problem is if I drill deep enough to acheive my required strength; the embedment depth will violate the depth allowed for my edge distance. If I drill ONLY deep enough to satsify the edge distance requirment then the strength is inadequate.

vmirat,

How do you detail the steel shear plate??

This is something I had not considered but this may work better in this situation!! What are the items one needs to be wary of when using this type of design??

Thanks to all for your advice/ideas so far!! ---
Andrew

 

[jeg]

Is the existing concrete section reinforced.
If YES, consider how one would extend the rebar. We would look for adequate lap to produce teh required anchorage bond. Edge istance is still an important factor, but is reduced by the links binding the main bars.
Can you apply similar reasoning to your dowels.

 

[vmirat]

Andrew,
Check the horizontal rebar in the wall. Once you cut the slot for the shear plate, you will develop a weakened plane in the concrete wall and induce a crack through the wall. The existing rebar must be capable of holding the wall together. I would use an angle with one leg epoxied into the wall and the other leg imbedded into your new door jamb. I would also use an epoxy to bond the new concrete to the old wall.

 

[DMWWEngr]

Thanks to all!! This gives me a few more options to look at! ---
Andrew

 

[oijammu]

Do not drill your dowels exactly horizontal.They should be off by about 10deg out of horizontal alternately above and below.That means they can act as shear restraints too,and shall not pull out.

 

[DMWWEngr]

oijammu,

That's interesting!! I hadn't heard this before. Do you know of a source where I can read more about this??

Thanks! ---
Andrew

 

[whyun]

I am at home so I do not have access to my HILTI catalogue. I do remember seeing one of their epoxy anchor using holes drilled at an angle. consult the hilti catalog if you have a copy. I will post something tomorrow, if i can remember.

 

[whyun]

The product I had in mind turned out to be HILTI HIT HY-20 for use in URM walls. They call it a 22 1/2 degree tension anchor. I am not aware of any concrete epoxy anchors installed at an angle that are tested and approved. logically, one would expect a increase in tension capacity is the slant drilling is done alternately and consider them acting in a group. however, i am not aware of such testing that was done for this.

 

[DMWWEngr]

Thanks!! ---
Andrew

 

[cage]

I would design the jam to span between floor and ceiling and reinforce floor and ceiling to suit.