Post Welded Base Resting on Levelling Nuts

[Syed Adnan Aqib]

We have a case of PEB structure, during erection, post welded base plate resting on centrally placed anchor bolts (embedded in concrete), found bending from edges. 

What could be the possible reason. 

 

[UmarMakhzumi]

HI Aqib,

Generally, when erecting steel structures bases, shim stacks are used that are placed under the base plate to avoid any base plate bending (as the base plate would  directly rest/ sit) on the shim stacks (compressive load) and once the structure with major members and bracing (in-place) is installed, the grout is done. The reason looks very simple that base plate has not been design for erection loads that match erection procedure or the engineer designing assumed the contractor was qualified and would install per industry standard. Keeping the column on levelling nuts is big mistake.

Out of curiosity, how long was the structure left like this before the base plate bending was observed. Also, why hasn't this been grouted? 

Thanks.

[Syed Adnan Aqib]

Hi Umer, 

Thank You for replying.

We cannot grout until main framing erection has been completed and building plumb is checked. 

During inspection, some plates were found bend as shown in photo attached. 

Building Clear height is 45'

Clear Span is 125'

Length is 455'

kindly advise the solution

Regards

 

[UmarMakhzumi]

Saqib, 

The solution would be to complete the erection asap and grout the base as-is. Load path will be complete once grouting is done. Meanwhile, you can provide shim stacks under column bases to avoid other base plates from bending. This is my opinion. You should contact the design office to get the solution from the Engineer of Record who has stamped the drawing as liability lies with him/her and follow what he/she says.

Thanks.

 

[Simple Structures]

1. Has overheating during welding (fabrication) bent the plate, before it arrived on site. Maybe the base plates was a little "thin" ?..or the weld too big...possibly the baseplate was bent at the outset? 

2. Looking at the "close spacing of the HD bolts", tells me the designer is trying to get a "pin" base and forgetting that the bolts need to be spaced out.... this shows lack of HD bolt design experience to me?

3. The bolts are usually put in a "tube" for bit of bolt flexibility, with such close spacing of bolts (sleeve) would mean the concrete pull out cone would have low value? ... the designer does not realise sometimes higher temporary erection wind moments are generated and can be the critical base design case.

4. The plate is welded to column all around...then the bending under the spirit level means that one needs to look elsewhere for the plate bending phenomena?

Normal Practice:

a. Make the baseplate thicker than column flange at least - then increase thickness if needed for temporary erection wind load?

b. The concrete is never at same level, so "shims" are placed on each column location by surveyor. 

c. Tight bolts and shims should be fine - if baseplate thickness and bolt design and spacing are correct. 

d. Complete WHOLE frame erection. Surveyor then checks "plumb (verticality) and align. If necessary adjust baseplate with steel wedges to get them plumbed and aligned.

e. Once everything is in correct place, grout under the baseplate with "non-shrink high strength grout".

Hope this helps.

 

[UmarMakhzumi]

@Simple Structureshas raised some good points. Just to add to it that shims have dual role. One is around levelling and /or filler plate and other is also to provide bearing before grouting is done. I have done some structures where we have used shims to level as well as to provide bearing (before structure installation) under columns and size of the shims was based on concrete bearing limit state using dead and construction wind (1 in 10 years).

Thanks.

[Syed Adnan Aqib]

hi everyone,

contact area for base plate to completely transfer compressive loads may be achieved by adding shims during erection and grout after installation is done.

Our PEB columns are welded with base plates which have undergone bending due to any reason, won't it make to alignment issue or introduce permanent stresses in structure.