Cracks at Top of CMU Wall 2

[DHKpeWI]

I have a question about cracks in a residential CMU basement wall in Wisconsin.

Most of the cracks are in the mortar joints at the top course. The top course of the wall is exposed with no soil bearing up against it. The wall is a non-load bearing, and the top of the wall is not tied into the floor diaphragm. There is a steel beam bearing on a pilaster on this same wall. The clear height of the wall is approximately 7’-0”. There is also a small step crack in this wall.

Since the top of the wall is not braced, I would expect the cracks to be located near the bottom of the wall. What would be causing the cracks in the top course?
Thanks

 

[hokie66]

Freezing of water which has penetrated the wall?

 

[PSlem]

sagging of joists rolling top course. See if crack larger on outside than inside.

 

[rowingengineer]

How long has the crack been present, how big is the crack and is the wall core filled? Did it happen at construction time, when the contactor walked the wall while the mortar joints were still green?

ANY FOOL CAN DESIGN A STRUCTURE. IT TAKES AN ENGINEER TO DESIGN A CONNECTION.”

 

[BAretired]

Could be expansion or contraction. The top course is exposed to the sun's rays whereas the second course is insulated by soil.

BA

 

[msquared48]

If there is any spalling of the CMU, what Hokie says is probably the cause. Otherwise, I tend to agree with BA here.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[concretemasonry]

A horizontal crack is usually a sign of a shear failure if there are enough lateral loads and not related to vertical loads.

Spalling in a new structure is not really "spalling" that is due to weathering cycles, but due to a loading in a new wall.

Not being structurally tied to the structure and being a part of the resisting of the lateral, but the lateral resistance may be applied to the top. More detailed information (age and season of construction, diaphragm construction, etc.) is required to provide the type of answer that is requested. It is difficult to imagine a properly constructed wall to be built with the code required anchor bolts between the foundation and sill plates.

Pilasters are always problematic with foundations because the inevitable assumptions that are made in the real load distribution. They are not practically effective since connection conditions and continuity are neglected for simplicity.

There is also the practical possibility that the top course was laid later and never had a chance to bond and cure before any loads were applied.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[dhengr]

DHK:
You mean it’s the found. wall, approx. 7' high with 6'+ soil loading, under the gable end of the bldg., with a pilaster for the stl. bm. which runs down the center +/- of the bldg.? A simple plan would help, and also an elev. sketch of the crack locations. Do the cracks exist through the mortar in the joint, or at the interface btwn. the mortar and the blk. shell, and mostly in the bed jnt. or also in the head jnt.? Tension, shear or bond failures? Does the crack go through the wall, any difference in width of the crack inside vs. outside, or only evident on the inside? Any similar cracking in the load brg. walls? While the wall isn’t a real load brg. wall, it is carrying the sill pl., rim jst. and gable wall. And, it is restrained by the sill pl., rim jst., anch. bolts, x-bridging, pilaster/bm., etc., at least to the extend that it will restrain the top course or two. Then, as what’s kinda a canti. retaining wall 7' high, deflects inward at its top (actually at the horiz. crack), due to soil pressure, it causes a tension or bond failure at one of the top few horiz. joints. The lighter loaded found. wall has less compression in the interior face shell in the top few courses, to counteract this inward movement and tension, so it tends to roll the top course or two inward at the first or second jnt. down.

 

[a2mfk]

a couple of sketches and pictures would save everyone a lot of time and guesswork and get you higher quality opinions...

 

[DHKpeWI]

Thanks to all for your responses and insights.
A little more info:
1)Attached are sketches of the plan and elevation of the wall and approximate locations of the cracks.
2)This wall is a basement wall, non-load bearing, with no visible reinforcing. This construction is common in Wisconsin, though I believe poured walls are becoming more popular.
3)There is not a sill plate nor is there visible anchorage of the top of the wall to the floor framing.
4)The cracks in the top course are only on the inside of the wall and are in the mortar joint and at the interface between the mortar and the block. In at least one location the top course of block is tipping away from the wall toward the exterior.
5)The step cracks are most at the block/mortar interface.
6)My understanding is that the house was built in the 1980’s.
7)No information on how long the cracks have been there.

Hokie66. There were some water stains on the floor, but the wall had recently been painted, so any stains on the wall were covered up. Wouldn't the water seep down to the lower portion of the wall before it could cause damage to the joint at the top of the wall?

BAretired & msuared48: if the problem is thermal, what would you recommend to prevent this? Would insulation on the inside of the wall mitigate this problem?

 

[a2mfk]

I see, non-axial load bearing but still acting as a retaining wall. What about reinforced grouted cells?

Where in Wisconsin? I have relatives in Menasha (near Appleton)that had lots of basement issues due to expansive clay. Many of the people in their neighborhood have had similar problems. Do you know if expansive clay is an issue in this area?

It can cause step cracks in CMU walls, see it all of the time with expansive clays... Think if expansive clays caused upward movement of the wall foundation and wall, acting like a deep beam the wall would have greatest tensile forces (in-plane) at the top of the wall, causing horizontal cracking. Its a possibility...

 

[a2mfk]

Also, I know you say in #3 there appears to be nothing restraining the top of the wall. Are you 100% sure? Because I like Dave's explanation for that horizontal crack only at the top of the wall as a possibility:

dhengr wrote:
"Then, as what's kinda a canti. retaining wall 7' high, deflects inward at its top (actually at the horiz. crack), due to soil pressure, it causes a tension or bond failure at one of the top few horiz. joints. The lighter loaded found. wall has less compression in the interior face shell in the top few courses, to counteract this inward movement and tension, so it tends to roll the top course or two inward at the first or second jnt. down. "

 

[BAretired]

Your sketch suggests that the diagonal cracks are caused either by earth pressure pushing in and causing a yield line or settlement of the central part of the wall relative to the ends.

Hard to say about the horizontal crack. Expansion or contraction is unlikely as the continuity of the top course is broken by two windows.

BA

 

[RHTPE]

Is the central steel carrier beam anchored to the top of the pilaster? If not, is there any evidence that the wall has slid inward under that beam?

I would tend to agree with dhengr - the wall has been pushed inward due to soil loads.


Ralph
Structures Consulting
Northeast USA

 

[msquared48]

The diagonal cracks do not pattern a lateral soil load from my experience, the cracking would have to go the other direction - from the top of the transverse wall to the bottom of the pilaster on either side.

Seems more like the central pilaster is more heavily loaded than the corners of the structure causing an excessive amount of settlement in the pilaster and diagonal tension in the wall causing cracking of the pattern seen. This could cause horizontal cracking at the top of the wall too.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[msquared48]

The second possibility for the diagonal cracking woiuld be a sliding failure at the pilaster location, possibly from over consolidation during backfill? The crack pattern would fit that scenario too.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[DHKpeWI]

Thanks for the additional input everyone.

 

[BAretired]

If the top of the wall is free, the diagonal cracks are consistent with a slab with three edges restrained and one edge free.

But is the top of wall free? If the pilaster is laterally braced at the top by the steel beam, then the wall is effectively braced by the steel beam and pilaster.

It is impossible to predict yield lines without knowing the boundary conditions.

I think dhengr is likely right. The horizontal crack at the bottom of the top course is likely due to the wall deflecting inward and the top of the upper course being restrained from moving in by the floor system, even if it is not tied in.

BA

 

[BAretired]

It might be a good idea to check whether or not the pilaster is plumb. If it leans inward at the top, it may not be adequately braced by the steel beam. This could be fixed by welding an angle to the underside of beam. The outstanding leg would be flush with the inside face of pilaster.

BA

 

[a2mfk]

These are all great structural insights, and I don't disagree with one of them as a possibility. However, it seems to me that you must identify the soil conditions prior to determining the actual cause of the cracking in the CMU. I would think a couple of 10 foot hand augers along the wall so you know the soil types adjacent to the wall and under the footing would be extremely helpful.

If expansive clay were found it may affect our opinions...

In #2 above you said no visible reinforcing. Did/can you hammer tap the walls to detect solid grout, followed by a test hole with a small masonry bit? If this is unreinforced, then any structural theory of why it is cracking is probably valid in lieu of no soil data.

And most importantly, the owner likely doesn't just want some theoretical opinions, they want a solution, so you will need to devise a repair, which I would not want to do without knowing the soil properties. You may end up doing a lot of saw-cutting and epoxy doweling to reinforce and solid grout several cells and probably a bond beam.

Of course there are lots of other options also, that may be the most economical (carbon fiber, separate reinforced wall in front of this one, etc. not very economical).