Structural cracks while side extension is being built

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Hello, I would really appreciate some advice from an Structural engineer.

We are having a single story side return extension being built and large cracks (3mm-6mm) have appeared in all room above where the load bearing walls were removed.

The cracks inicial appeared when the house was propped with acrows and got worse when the RSJ went in and acrows removed.

The door opening RSJ (around 5m long) is sagging a little (3mm) in the middle.

RSJ were calculated by SE. The builder is well experienced and cames recommended. Are these cracks and deflection expected?
SE is doing a site visit but would really appreciate some advise.

Thanks!
 

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Do you know the size of the steel used?

I would guess that they perhaps overlifted when they propped it, and then its sagged back down when the props were removed, plus deflection of the beam.

However, I'm not that experienced, but I think it looks worse than it is, and I'm sure I've heard that cracks above are usually OK, and to be somewhat expected, it's cracks underneath the beam that are to be worried about
 
3mm deflection over 5m isn't significant. The cracks in the photos, however, are...

Where is/are the new opening(s) in relation to the cracks?

If the cracking happened when the acrows went in, then there are a couple of reasons that could have happened.
1: What needles were used? Were they undersized? Did they deflect?
2: What were the acrows supported on? Did the supporting structure deflect / settle

If the cracking got worse when the acrows were removed, there are a couple of reasons this could have happened:
1: Pad stones or grout under them not fully cured
2: Grout between beam and wall above not fully cured
3: Beam undersized (3mm deflection suggests this isn't the case)

How bad were the cracks prior to removal of the acrows and how much worse did they get?

Hard to say much more without more info (how the cracks relate to the new structure / openings, and a plan drawing of the alterations and what structure has been removed would help.

Let us know what your SE says.
 
Is this deja-vu? I seem to recall the two windows and cracks.

Anyway, it's quite simple - there should be no cracking when installing a beam. "Within tolerance" or not.
 
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Is this deja-vu? I seem to recall the two windows and cracks.

Anyway, it's quite simple - there should be no cracking when installing a beam. "Within tolerance" or not.
Definitely never posted before.

What are your views on what could be going on? Thanks!
 
Either
  • the builder has jacked the building up too high and it cracked on the way down
  • or the builder has not packed the wall properly and it dropped
  • or the beam has not been designed properly
With calculations, crap in crap out applies and calculations may be correct even when the assumptions and figures are wrong.

You need that assessing and the cause determining and not just letting the builder make good the plaster. It may need an independent person to assess it, not the builder or SE for obvious reasons.
 

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