Masonry Wall Problem 4

[UofAGrad]

Hi All,

I was called by a potential client about a project. There is an old building that is being remodeled, the roof structure replaced of which the trusses will be supported by a ~10' tall HCMU Wall, keep in mind the building has been in this condition for some time now. The client discovered that the wall cells are all ungrouted, no rebar was installed within the cells or extending from the strip footing into the cells. The permitting department wants an engineering review done on the wall and if needed a repair proposed.

Thinking out loud, the wall will likely pencil out axially once grout is added but fail in flexure. At a minimum we will need to grout the cells to allow proper fastening of the Sill Plate for attaching the trusses. Simply grouting the cells may not be enough, for the overturning moment where the cells attach to the footing, are their external fasteners that can be used to attach the concrete wall to the strip footing? I'll have more info on the project next week, what are your guys thoughts on how you would approach this?

TIA

 

[phamENG]

I'd start by checking it as unreinforced and see how far off you are. When you say HCMU, are you referring to hybrid masonry? Like brick and CMU? I've seen that and analyzed it before and it's actually pretty strong.

I agree adding a bond beam in the top of the wall is prudent. It may even be easier to just remove the top course of original block, lay in grout screen, and put in preformed bond beam blocks.

 

[JAE]

I think they mean Hollow Core Masonry Unit (HCMU).

 

[Celt83]

10' tall it may have originally been done using the Empirical Design, See NCMA TEK 14-8B

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[UofAGrad]

phamENG I'm referring to Hollow Core Masonry Units as JAE stated.

 

[XR250]

Why do you have overturning at the footing? Is there going to be a functional roof diaphragm and a method of getting the load into it? I would drop some bars down from above periodically and grout those cells. Could knock out the webs of the top blocks, cram some newspaper in the holes and make a bond beam.

 

[r13]

A few questions:

the building has been in this condition for some time now

What condition, trusses sitting on un-grouted CMU wall? Otherwise, what was the type of the old roof, and how was it attached to the wall?

Thinking out loud, the wall will likely pencil out axially once grout is added but fail in flexure.

I don't understand this sentence at all. Can you elaborate further?

Simply grouting the cells may not be enough, for the overturning moment where the cells attach to the footing

What is the change that causes the overturning moment which wasn't there before? Are you commissioned to bring this old building up to current code? If the effect exist, I suggest to remove and rebuild the wall.

 

[MegaStructures]

analyze the flexural strength required, flexural strength provided by grouted cell, and determine the difference needed to be taken by steel. Drop extra rebar in and grout, else add external steel framing to the side of the wall.

What is additional overturning from? Don’t see an easy way to resolve that with post installed rebar. An external steel frame as suggested could be used to fix that problem possibly?

Just thinking out loud.

 

[MegaStructures]

Can you put knee braces near the bottom of the wall to resolve overturning moments?

 

[kipfoot]

If you're in a seismic design category A or B, my first step would be to determine the capacity of the unreinforced masonry. 10' isn't very tall, though you don't mention the block thickness.

f[sub]t[/sub] = M/S - P/A (don't forget P/A including the wall weight)

It's not clear what moment you're talking about at the base. If you're considering out-of-plane loads and you don't have dowels, then the moment at the base will be zero.

 

[KootK]

Simply grouting the cells may not be enough, for the overturning moment where the cells attach to the footing, are their external fasteners that can be used to attach the concrete wall to the strip footing?

I'll assume that we're speaking of a wall that has lateral support at both the top and bottom of the wall. In that case:

1) For a wall of the proportions that you've described it is quite common to consider the base connection a pin in which case you only need to transfer shear at the joint (no moment). Obviously, the assumed absence of moments at the base is something of a convenient fantasy in most cases.

2) For unreinforced masonry, I believe that is quite common for there to be no positive connection to the foundation other than a full bed joint of mortar. Often, the base of the wall is somewhat retrained laterally be the presence of either an adjacent slab on grad or adjacent back-fill. You may want to investigate those possibilities.

3) If you were bound and determined to improve the shear connection at the base of the wall, I'm sure you could dream up something:

a) Intermittent steel clip angles fasten to the wall and either the footing or the slab on grade. Be mindful of corrosion concerns with this.

b) You might break out some cells intermittently, epoxy set rebar dowels into the foundation at an angle at the broken cells, bend the protruding dowels vertical, and grout the broken cells. This is labor intensive, of course, so pursue this strategy only as a last resort and, preferably, with contractor buy-in.

 

[phamENG]

Thanks - hadn't seen the H thrown into the acronym before. We usually call it un-grouted CMU.

You didn't mention anything about additional loads - only remodeling. Is the new roof heavier? Are you adding another level on top? Are you in an area where unreinforced CMU is not permitted by code (high seismic, for instance)? I've seen lots of 10' CMU walls function just fine with no grout, no reinforcement, etc. I think there's a general prejudice against unreinforced block walls and, when people see them, they immediately think something's wrong. Not always the case.

Also, dowels are not always required to transfer shear at the base of the wall. The mortar that holds each course of block to the one above/below it will also hold the base course to the foundation. Just have to check the mortar's modulus of rupture. ACI 530 has tables for those capacities if you know/can safely guess at what's there. Or you can have a sample taken and have a testing lab give you some real numbers.

 

[XR250]

The mortar that holds each course of block to the one above/below it will also hold the base course to the foundation. Just have to check the mortar's modulus of rupture.

I rarely count on the base course getting glued to the footing other than for shear purposes. One would have to assume that the contractor has spotlessly cleaned the dirt off the top of the concrete.

 

[phamENG]

XR250 - that's fair, but then if I'm dealing with a situation that's sufficiently critical that I'd be worried about a little dirt (by which I assume you mean errant topsoil/organics/clay) at the interface, it's probably a reinforced wall anyway. For that matter, just about every CMU wall I design is reinforced.

When considering either a small/not especially critical unreinforced/ungrouted CMU wall that's new or an existing ungrouted CMU wall with a solid service history, I don't have a problem counting on it. We can second guess ourselves into oblivion believing that every contractor is just a dirty rotten thief that can't build a building. Sure, some probably deserve that opinion, but so do some engineers. At some point, we have to trust the craftsman to know his craft.

 

[UofAGrad]

Thanks for the reply's everyone, alot of good information to digest. To clarify a few things:

As KootK stated analyzing the wall base as a pinned connection is a convenient fantasy, but somewhat real since the bond from the mortar has not ruptured yet.I was originally thinking this connection would be a fixed one in which case I am sure the analysis would show the bond joints would rupture in theory. like I said the building has been here for some time.

The overturning loads on the wall will be from wind loads on the wall face and roof. The roof is being replaced with a higher pitched roof. The existing truss system is badly damaged and appears to be very much undersized. New trusses are long overdue I do not know the reason for a new pitch.

The base of the wall will be somewhat restrained by the new concrete SOG inside the building and same height back fill on the exterior.

A couple comments on fabricating various steel fasteners is where I was going with my original post, I was hoping someone knew of a fastening system already being manufactured for this use.

Thanks for the reply's everyone, this is why I love this forum it makes it easy to have an educated conversation on these subjects.

 

[phamENG]

analyzing the wall base as a pinned connection is a convenient fantasy, but somewhat real since the bond from the mortar has not ruptured yet.

I think I disagree with you here. It's a fantasy precisely because the mortar hasn't ruptured. As long as the mortar is intact, there is fixity between the CMU and the foundation. Pre-rupture, the pin behavior comes from lack of fixity of the footing in the soil.

Fortunately, if you design the wall to have a uniform flexural capacity based on rupture of the mortar equal to the max positive moment experienced assuming pinned ends (wl[sup]2[/sup]/8), the partially fixed moment at the base will never reach it - if it were perfectly fixed it would only get to wl[sup]2[/sup]/12.

 

[JAE]

Some appendages to KootK's response above:
1) Agree with this -
2) Yes it is quite common and I've done that in the past - but if I'm presented with a wall in a high seismic area I would go to KootK's item 3)
3) For a lot of old structures, the wall extends below an interior slab-on-grade or is on a ledge of a footing backed up by a slab of some sort. OK for shear in that direction but for shear outward, there needs to be some type of connection (again - for seismic situations....not that concerned with wind for typical story heights....sort of depends on the situation).

Basically - I don't (never) rely on mortar friction to resist seismic shears.

 

[KootK]

Basically - I don't (never) rely on mortar friction to resist seismic shears.

X2

It's a fantasy precisely because the mortar hasn't ruptured.

It's also a fantasy because the axial load on the wall, and the wall self weight, tend to pres-tress the joint such that said pres-tress must be overcome before tapping into the modulus of rupture.

 

[KootK]

Fortunately, if you design the wall to have a uniform flexural capacity based on rupture of the mortar equal to the max positive moment experienced assuming pinned ends (wl2/8), the partially fixed moment at the base will never reach it - if it were perfectly fixed it would only get to wl2/12.

Meh... not exactly. The most realistic case would be fixed at the bottom, pinned at the top. Still WL^2/8.

 

[phamENG]

Good point. Though it's still in line with the max calculated moment in the wall.

(And for the record - I agree with the not using mortar for seismic shear. If SDC C or greater, a retrofit would be necessary.)