Slab supporting precast veneer panels has cracked. 2

[chrisrosebud2001]

I have a situation where a slab supporting precast concrete panel veneer has cracked and I am having trouble determining the cause for the cracks. The foundation system is grade beam on auger cast pile. The structural system is steel framed with brick veneer. However, in this particular side of the building, precast panels with thin brick were used due to limited space. At this location, the slab is structural and spans from grade beam to grade beam, so the slab is sitting directly on the grade beam. The precast panels are then supported on bearing pads sitting directly on the slab. The cracks do not appear to be bearing related, but appear to be bending and shear related. Finally, I have also noticed that bearing pad locations do not match those indicated on the shop drawings. The vertical supports were supposed to be in the same location as the lateral supports, but they are not.

 

[chrisrosebud2001]

Additional picture

 

[chrisrosebud2001]

Additional Picture 2

 

[chrisrosebud2001]

Additional Picture 3 Also note that in some locations there is a slight gap between the grade beam and slab.

 

[ztengguy]

can you do a wide shot so we can see the whole situation?

 

[chrisrosebud2001]

There wasn't enough room between the construction and an adjacent building to get a wide picture, but here is an elevation with the pictures and locations included.

 

[Once20036]

Can you provide a cross section?
Is there any relationship between the cracks and the vertical support points or between the cracks and the lateral support points?

 

[mijowe]

Have you checked the grade beam design? Does it have negative steel? Have the cracks propagated into the grade beams or are they only in slab? It looks like the cracks are at locations of negative moment and deflections in the grade beam may have caused the slab to crack.

 

[chrisrosebud2001]

The cracks do not propagate into the grade beam and the grade beam does have negative moment steel. Also note that the grade beam is cantilevered. However, the calculated maximum deflection of the grade beam was only 1/16". Attached is a section detail and I will also provide a provide a partial foundation plan in the next post.

 

[chrisrosebud2001]

Foundation plan

 

[mijowe]

do you know how the grade beam rebar was detailed? the detail is sort of generic. Were the top bars continuous over the the supports? without direction I could see all the bars being cut to the same length with the top bars being stooped and started at the same place as the bottom. Are the top bars on the cantilever grade beams hooked?

the column, pile, cantilever grade beam is curious to me as well. The columns and not the precast could be the source of the larger loads? Getting the beam in line with the column like C line makes sense, but the roundabout load path at B and D seems to be made a little more difficult than it needed to be.

is there a reason why there are 4 piles at b and D and only 2 at C, the trib areas seem to be the same?


not sure i am helping any, but you have got me curious

 

[PMR06]

Could these cracks somehow be related to the column blockouts? Perhaps they were not formed and poured as cleanly as shown on plan, or the infill itself is cracking?

 

[mijowe]

^^^ that is a good point, I like it, can you see the extent of the cracks inside the building?

 

[PMR06]

And I'm not sure if related to slab cracking, but your ties 16" on center do not appear to be spaced at least d/2?

 

[hokie66]

My best guess is that the slab cracks are restraint shrinkage cracks. The grade beam was cast first, thus restrained the lightly reinforced slab (no reinforcing at the edge), and the slab cracked in direct tension. Do the cracks continue inside?

 

[chrisrosebud2001]

The grade beams were detailed to be continuous. I will pull the shop drawings tomorrow to verify that they are indeed detailed as continuous and that the bars are hooked. The additional piles at lines B and D are due to heavier loads from the columns being interior as well as being part of the LFRS. The cracks do not continue to the inside and do not appear to be linked to the column blockouts. Previously, there was a crack noticed along the length of the exterior grade beam in the slab shortly after the precast was placed. The location of this crack is approximately near the location of where the vertical hook bar comes out of the grade beam. At the time it was judged that the exterior portion of the slab was still adequate as the bearing stresses were still within allowables.

 

[hokie66]

Nothing to do with moment or shear or how the bars are placed in the grade beam. The restrained slab had to shrink and crack, and because there was no reinforcement close to the outer edge, the cracks were not controlled.

 

[dik]

Did anyone check the elevations of the various bearing points?

Dik

 

[chrisrosebud2001]

I did verify that the grade beams were detailed as continuous. I do not think the actual elevations of the bearing pads were checked. However, the heights of the shims/bearing pads to vary over the length of the slab as there was some variation in the levelness of the slab. This is mainly due to the tolerance in the cast in place concrete. The precast panels were checked and they are plumb. Hokie, do you think the shrinkage could also explain a slight gap between panel bearing points between the slab and grade beam? In moving forward, I plan to recommend that the cracks, panel elevation, and possible panel rotation at the base be monitored. And for now, sealing all cracks to prevent water infiltration. Does anyone have further suggestions?

 

[ron9876]

I think hokie66 is correct that the cracks don't have anything to do with the grade beam reinforcement. Normally the slab shrinkage cracks wouldn't be visible because the slab couldn't move far enough to show a crack of the size in the photos. In some of the photos it looks like there is an open joint between the beam and slab. Shrinkage across these open areas is likely causing the problem.