Leveling column with of preloaded anchors 2

[CivilEngineerUK]

I have a steel column connected to a concrete pile with preloaded anchors and grout.

What is the best way for leveling the steel column before grouting. Using normal leveling nuts will not work as the preloading length shall correspond to the entire anchor length. I have heard something about plastic nuts used as leveling nuts which will break during preloading. Any other ideas or comments?

 

[HTURKAK]

This subject in past has been discussed ..one of the thread thread507-453921.

You may consider adjustment screws or plastic nuts under the base plate .

 

[dik]

The anchor rods have to be longer and sleeved so that they can be stressed. They have to be unbonded for a length to allow the steel to 'stretch'. I don't usually use washers... the headed bolt design can accommodate this and the plate washer can be a plane of weakness to initiate a crack. I'd use steel shim plates for setting column base plate elevation.

Dik

 

[CivilEngineerUK]

Where do you normally not use washers? In the top of bottom of the anchor? Furthermore, is there any standard/rule regarding length of the anchors?

 

[dik]

I use washers on the top, above the base plate, only. The bottom threaded portion of the anchor rod is threaded slightly longer than the heavy hex head nut and then the bottom thread is peened. The AISC had an excellent article on headed anchor rods... I'm not sure what it was titled. This gives an indication for the length of the anchor rod... check with ACI, the lengths differ a bit.

I don't use a double nut and a washer at the bottom for the reason I noted... only if bearing is an issue and not normally and depending on the tensioning, it may be. I would likely leave 2' of unbonded anchor rod with an additional 18"-24" for development(can get longer with larger dia AR). Used to use Dywidag threadbar until a few years back... great stuff but connectors are pricey and with rebar profile, you can develop the length with the deformations.

Had an incident about a decade back where the plan examiner in Ottawa wanted me to sleeve the top 10' of some rock anchors I was using... so I sleeved them. They were not tensioned, so the sleeving did absolutely nothing... but, I didn't want to get into an argument with him...

Dik

 

[HTURKAK]

Headed bolts or anchor rod with a heavy hex nut will normally develop the full strength of the anchor rod in tension if sleeve is not used.

For your case full length anchor bolt sleeves are used (L=1000mm wrapped with denso) . In this case, anchor plates must be used for preloaded bolts to provide the necessary bearing area outside of the sleeve , rather than headed bolt or anchor rod with a heavy hex nut .

I remember ASCE recommends a minimum embedment depth of 12 diameters or the partial sleeve length plus six anchor diameters (sleeve length + 6do).

For your case, the failure modes should be looked together with pile concrete and reinforcement to determine the length , however, 1000 mm should be OK if the rod dia less than 50 mm.

 

[dik]

The sleeve isn't full length, but is needed to develop the tension...


The headed bolts still develop the tension and plates may not be required.

Dik

 

[HTURKAK]

Dear Mr DIK, the picture released by (OP ) shows (Prestress anchors L=1000 wrapped with denso tape )...

I understand the anchor bolt fully wrapped with a bond-breaking tape to eliminate the use of sleeve ..

 

[dik]

Just Dik... even my driver's license was signed that way... if washers are used the bondbreaker can extend to the end... the sleeve is the same as the bondbreaker. For grouted rock bolts, I usually use a neoprene 'pipe' sleeve or something of that ilk.

I usually don't use washers for reasons noted above. I prefer to extend the anchor rod and use headed nuts, only.

When I was working as a student... about 55 years back, we were galvanising some anchors. They had a helical screw on the end. I asked one of the plant engineers why the screw shaped anchorage and was informed that the anchor loads were high and Manitoba Hydro didn't want a plane of weakness at the end of the anchorage... still makes sense to me.

Dik