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Q5CM (Structural) (OP)
8 Apr 12 9:11
I have a foundation with an Octagonal pedestal, the diameter of the pedestal is about 3meters and rises above the ground level by  about 1.5 meter.
the pedestal carries a process vessel which sits on a steel ring.
during commissioning and the vessel was filled we have noticed cracks at each corner of the octagon. the cracks are about 2 - 3 mm wide and they are moving on down the vertical face.

contractor is proposing to inject the crack with some repair material.
my question is; dosen't a crack mean that the section has failed and the design was not adequeate? is repairing by injection structurally sound? do the have to make some sort of calculations?
ferrarialberto (Structural)
8 Apr 12 9:26
What is the distance from center of anchor bolt to each cracked corner? And what is the diameter of anchor bolt?

Hello.

ing. FERRARI Alberto - www.ferrarialberto.it

msquared48 (Structural)
8 Apr 12 21:28
If this process vessel induces vibrations to the concrete pedistal and foundation, the foundation may not be designed properly related to the relative masses to control the vibrational response, causing the cracking you see.  

If this is the case, injecting the cracks with epoxy will not solve your problem.  

You need to engage a structural engineer to help you solve it.  The foundation may have to be redesigned and replaced.

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
 

Helpful Member!  BigInch (Petroleum)
9 Apr 12 0:37
Post a picture.  The cracks you show in your sketch could easily be shrinkage cracks.  The way you have drawn them, they do not appear to be related to load.  Why would a centrally located load applied to the pedestal cause enough tension (perpendicular across the crack) pointing in the direction to a corner of the pedestal that has 0 load there.  Can't draw any kind of a free body diagram for that corner that could attribute the crack either to an applied load, or to a resisting reaction being applied at that corner.   That said, epoxy grout might be just the fix you need, but!

A 2 to 3 mm is a lot to attribute to temperature shrinkage in a pedestal.  A 2 to 3 mm wide crack is not something to ignore.  Find out what is causing it (or what was responsible for causing it) before you decide what to do with it.  Perhaps the vessel was "bumped" on the corner before swinging over the center of the pedestal during erection.  Now, having no load there, all you might need is some minor cosmetic work; my point being, so far you have 0 actual information on which to base ANY decision you make, and the cracks are "moving".  In any case you can't do anything else until you have made those cracks stop moving.  

Don't inject grout to treat the symptoms unless you at least know what caused the disease.  Only medical doctors can get away with that.  

What would you be doing, if you knew that you could not fail?

saiid2000 (Structural)
9 Apr 12 1:00
You may want to check the reinforcement drawings to see how the corners are detailed. in the absence of adequate reinforcement the corners may crack/ spall.
hokie66 (Structural)
9 Apr 12 1:19
Agree with BigInch.  In order to make any more intelligent comments, a photo or two would be required.  In addition, information about the reinforcement and anchor bolt locations would be of assistance.  They are strange cracks, but don't necessarily indicate structural failure.  More information required.
Q5CM (Structural) (OP)
9 Apr 12 3:02
Thanks to all, I will try and get some photos posted,but getting a photo permit on this site is hard, but i will try
thanks again
Ron (Structural)
9 Apr 12 5:52
Agree with BigInch and hokie66...not necessarily structural failure.  More likely shrinkage.  Each angle change of the octagon creates restraint to shrinkage.  Further, the exterior face is more exposed to drying, thus higher shrinkage stresses.

Photos and rebar scheme needed.

If injection is needed, don't use epoxy grout, just epoxy.  Cracks too tight for grout.
asixth (Structural)
10 Apr 12 8:13
2-3mm is a massive crack. Normally when a builder or client wants you to investigate a crack it is 0.2 to 0.3mm.

I would be concerned because a shear model for concrete allows for aggregate interlock at the smaller crack widths, but I think with 2-3mm you are relying on shear friction of the reinforcement and not any concrete shear/tension capacity.

To me, it almost appeared as though a cone pull-out failure crack pattern was occurring at the corners. I would investigate whether the pedestal has reinforcement at 200mm each way and also investigate temperature. The thermal expansion between concrete and steel is roughly the same but steel absorbs it faster particularly if it has a dark paint.

I had a external FC clad steel frame which was poorly insulated and had dark colours on the external wall and first thing in the morning you could really feel the heat radiating through to the internal plasterboard when you touched it. It kept cracking the plasterboard along the panel joint.
ferrarialberto (Structural)
10 Apr 12 8:27
I totally agree with asixth.

ing. FERRARI Alberto - www.ferrarialberto.it

delagina (Structural)
10 Apr 12 13:17
how tall is this vertical vessel? how deep is the embedment of anchor bolt, how many rebars does the pedestal have. is the anchor fully developed?

is this a tall vessel and embedment of anchor is just 12d?

unless we have this info, we wont know if this is underdesigned.
Q5CM (Structural) (OP)
12 Apr 12 1:19
Hi Guys,
Sorry could not get a photo permit from the site manager, but here are some more details;

The foundation is octagonal
the size is 5m by 5m
the height is 1.75 m , 1m below ground and 0.75 above ground
reinforcement; 25mm mesh four layers - top, - bottom - upper intermediate,- lower intermediate
vertical bars , 25 mm every 25o mm

the vessel is 6 m heigh
the anchor is 300mm from edge
concrete is grade 50

I have requested the contractor to investigate the depth and extent of the cracks
ferrarialberto (Structural)
12 Apr 12 2:09
It's probable that the collapse occurred due to a differential thermal variation between steel and concrete.

ing. FERRARI Alberto - www.ferrarialberto.it

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