Brick veneer at heights with high wind & high seismic 1

[NorthCivil]

I have been asked to design a brick veneer wall for a project that has:

high winds (~3-4kPa ULS)
high seismic area
high heights - the client wants to install the brick at heights up to 10 stories.

So I have read the prescriptive requirements of the ACI, and I have the design nailed down. The veneer will be broken at every story on a shelf angle, with vertical control joints around openings and at ~15ft intervals. The prescriptive wall tie spacings will be enforced.

Thoughts on specifying a steel reinforcement lattice laid in the mortar joints that the brick ties connect to? i have seen conflicting information about this. I also have reservations about this, as the reinforcement will corrode over time which will actually weaken the mortar joints.

I've been researching this for a few months now, scoured the internet for information, tossed and turned at night for a while, called a bunch of local and overseas brick-layers and engineers, exhausted all avenues. Most of the responses have been "I wouldnt touch it with a ten foot pole", or "She'll be right", but hard to get in touch with an engineer with real, practical experience on this topic.

Now I'm reaching out to this community hat in hand -

my gut still isn't settled when we are talking about such a heavy, non-ductile material built up to these heights, in a dense urban environment.

any tips or guidance?

 

[hokie66]

The high seismic area performance would concern me a lot, although I have no experience in that regard. As to wind, I don't view that as difficult to overcome.

In my opinion, the joint reinforcement is useless, and as you said, can weaken the veneer with time.

At the shelf angles, pay special attention, both in design and implementation, to the soft control joints under the angles. A major cause of veneer failure on multistory buildings is brick growth.

The other issue for serious consideration is the backing structure, which needs to be stiff. Reinforced concrete block?

 

[MotorCity]

Since you already have the design nailed down, I assume you got it to work without reinforcing. If that's the case, what is your reason for wanting to add the reinforcing?

In my opinion, if you add the reinforcing, you are simply providing a path for corrosion in every mortar bed that it is installed in.

If you have doubts about the prescriptive brick tie spacing, just add more brick ties. An additional brick tie every so often offers a lot less opportunity for corrosion than continuous reinforcement in the mortar bed.

Pay attention to the weep holes and flashing.

 

[hokie66]

As to corrosion of brick ties, use stainless steel ties. They are readily available now, and add peanuts to the project cost. Agree with MotorCity, more brick ties is good insurance.

 

[KootK]

my gut still isn't settled when we are talking about such a heavy, non-ductile material built up to these heights, in a dense urban environment.

As long as it's just veneer in front of stud/CMU, I wouldn't sweat the lack of ductility in the brick wythe itself. The brick ties should, of course, be spaced to suit a demand consistent with short period seismic forces per local code requirements. As other's have suggested, I'd not want to trade a ductility improvement against increased corrosion potential in this situation. Nobody wants a brick falling into a baby carriage during an earthquake but, then, that's probably also more likely to happen if corrosion has taken place.

HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[JLNJ]

Maybe a call to the BIA is in order to quiz them about your seismic anchorage questions.

If memory serves, the normal tie spacing is one per 4 square feet of brick. Maybe the BIA can give you the typical tie capacity. Once the ties are figured to be OK you can turn your attention to the mortar between the bricks. I imagine that rupture of the mortar (in flexural tension) between ties would be your failure mode.

If your architect wants a thermal break at the shelf angle attachment, please come back to this thread and let us know how you accomplished it. This is getting to be a bone of contention in masonry veneered walls.

 

[masonrygeek]

There is a lot to unpack in this thread. First, it is perfectly safe to design brick veneer in high seismic regions. There are many examples such as UCLA's Teraskai Life Sciences Building, First Congregational Church in SF, and Metropolitan State Hospital in Norwalk, CA. The key to a successful brick veneer project in high seismic areas is getting the right amount of veneer ties attached to the backing and to the veneer.

Veneer Tie requirements in TMS 402/ACI 530
Chapter 12 - Veneer (or Chapter 6 if you are using an older version) has requirements for wall ties (veneer ties). The general requirement is to place veneer ties at a maximum of 32" o.c. horizontally and 25" o.c. vertically. There is an additional requirement of 1 tie for every 2 2/3 sf for adjustable-type ties. In Seismic Design Cat D and above, TMS 402 says that the max area is 75% of the typical spacing so that makes it 1 tie for every 2.0 sf (Sec 12.2.2.11.2.2). That would be a spacing 16" x 18" although most designers place ties at 16" x 16" to meet the stud spacing or block joint spacing. this is a fairly close spacing so it doesn't make sense to "add more ties to be safe".

Joint reinforcement in brick veneer
The masonry code USED to have a requirement for joint reinforcement in the bed joints, but it was removed in the 2013 version of the code. While it seems like having a wire in the bed joints of the brick veneer would be a good thing structurally, shake table testing showed failure with bed joint cracking which would negate the bonding of the bed joint reinforcement. And as others have said, if its not stainless steel, there will be corrosion in the future. Besides the TMS 402 commentary on this, here is a paper that was used to justify the removal of the jt reinf requirement:

http://nheri.ucsd.edu/projects/2007...s/Okail_Shing_Klingner_McGinley_WCEE2008.pdf.

Shelf angles for brick veneer
Again, you can use TMS 402 (ACI 530) to see where shelf angles should be located. The code says each story of brick veneer shall be supported independently of other stories in SDC E & F. So having a shelf angle at every floor would be appropriate. As Hokie66 mentioned, the key is to make sure that there is actually a soft joint beneath the angle and not mortar is anything hard. As for thermally efficient shelf angles,those are becoming more commonplace. Check out Hohmann & Barnard, Halfen and Fero for stand-off angles and their respective details. Depending on which seismic zone this project is located in, it may not be necessary to consider thermal bridging due to shelf angles.

One last piece of technical information on brick veneer in seismic zones, here is a paper that condenses a lot of the discussion on this subject:

http://www.hms.civil.uminho.pt/events/sismica2004/209-226 Gregg Borchelt _18 p_.pdf.

 

[NorthCivil]

Thanks masonrygeek - great resources. If you have any other reading to pass on, it would be greatly appreciated.

evident now that joint reinforcement is out of the question.

Shelf angles and building movement definitely need to be considered. liaising with the building structural engineer on the particular deformations of the building in response to modes, creep (gravity), lateral drifts.

Also of note - though the building settles with gravity, brick is liable to expand with wetting (I read somewhere 0.1% vertical expansion).

I am feeling somewhat better about the system, still going to throw some numbers at it and keep on the research. Also looking now at a global solution rather than a local one - trying to talk the client into a design change to incorporate a large, beefy canopy over the sidewalk below as a failsafe in case some brick does come tumbling down.

 

[Agent666]

Northcivil, from some of your previous post I gather you are in NZ (hopefully I have this right?).

This is the product I've specified previously for the reinforcement. I disagree that it does nothing as one person noted, it increases the integrity of the veneer (edit - though have not read the links posted by masonrygeek so may change this view!). Though you need careful consideration of how the interstorey drifts are being accommodated between floors and at vertical joints at corners, similar thinking needs to be applied to that of using precast concrete cladding panels. For example consideration of the 'panel' of brick tilting over for out of plane loading, but no deformation in plane when the framing moves behind is required, you'll have similar incompatibilities you get at corners as using precast (i.e. might need a joint width vertically to accommodate the required drift to prevent the panels from impacting/pounding together). Although in the standards below you'll find the minimum deformations certain types of ties can sustain based on them being qualified to these standards, if your drift is less than these then I think you can probably relax the detailing associated with accommodating the drifts.

http://mpb.co.nz/product/bricklock/

or

https://www.eaglewire.co.nz/shop/cat/brick-ties-bricklock/bricklock

I'm sure you are already aware of the minimum tie requirements and minimum strength requirements in NZS4210 and AS/NZS2699.1. I'd be doing a specific check against these minimum capacities and a parts and components load for the face loading (chapter 8 of NZS1170.5) based on each ties tributary area of wall. Obviously at higher elevations in the building the accelerations could mean tighter spacing than the prescribed minimum requirements. This is because NZS4210 requirements are really intended for your low rise residential applications. Be careful to outline position of ties relative to the top course and end courses of brick in your documentation. Keep in mind that if you are using stack bond vs running bond you may need closer ties as well.

Also consider unlike most facade elements you'll likely need access to the outside via scaffold or similar to actually build it. Most other lighter weight panelised systems may not need the same access requirements on the outside of the building so this is definitely something to factor in.

There is also some information on brick veneers and wall ties in the NZCMA manuals here.

The brick and blocklayers federation also have some good guidance on veneers here.

Keep in mind there are limitations in on the height above ground of brick veneers in the NZBC, and as such using them in a high rise structure may require the use of a specialist facade engineer to design or review at the very least as it'll probably be an alternative solution to the NZBC.

If you're not in NZ then I've wasted my time I guess but provided another view on things and some further info in the links that might be of value.

 

[NorthCivil]

interesting link there regarding joint reinforcement at corners. the link

http://mpb.co.nz/product/bricklock/

few ideas to kick around:

1. the shake table testing mentioned above by masonrygeek resulted in the ACI standard removing the requirement for joint reinforcement. based on this, I can agree with removing joint reinforcement along a regular run of brick wall. But perhaps there is a case for limited amounts of joint reinforcement around corners in lieu of control joints?

2. perhaps there is a case for limited joint reinforcement in lieu of control joints at openings? as mentioned in page 10 of this manual?

http://www.nzcma.org.nz/document/279-28/NZCMA_MM_-_5.1_-_Veneer_Walls_-_17-04.pdf

3. Also wondering weather "rigid" ties are actually better than corrugated ties. as per section 5 of the paper put forth by masonrygeek - it noted that rigid ties did have a higher capacity (1.2kN/tie) than corrugated ties (0.85kN/tie), though the rigid ties failed more "suddenly" ie, maybe corrugated ties are more ductile?

Agent666 - I do practice in NZ as well as other jurisdictions - this project not in NZ, though do appreciate the input, has raised a few more ideas.

 

[masonrygeek]

Some more information to consider:

Be aware of the documents you are reading and to what materials they apply to. The NZ manual is discussing concrete masonry and not clay masonry. So some of the recommendations are specifically for a concrete masonry veneer (which shrinks)and not a clay masonry veneer (which expands). The discussion about joint reinforcement on page 9 is related to shrinkage of concrete masonry and not to seismic issues. Also note that the NZ manuals discuss mostly low-rise structures and some of the recommendations don't relate to taller buildings.

As to whether you put vertical movement joints near the corner as Agent666 mentions or joint reinforcement at the corners to strengthen them, you have to go back to what you are trying to accomplish. Are you trying to let the veneer and structure move during a seismic event or are you trying to stop the movement from happening? Ductility is what is desirable in earthquakes so you probably want to allow the building to drift (assuming its a 10 story building) and not impact the veneer. That means you are creating panels of brickwork with shelf angles at every floor and vertical movement joints (called isolation joints in this case) maybe at corners to allow the movement to occur. When you place joint reinforcement in the veneer at the corners you are stiffening up the veneer which will attract load. So that may not be the best choice.

IMHO, corrugated ties are a piece of crap (at least when used in non-residential construction). While they do provide more ductility, their installation is fraught with problems (wrong nail or screw, not attached to stud, fastener not attached close to bend). Using an adjustable veneer tie provides a better connection and while it does increase the force on the tie, it is able to handle the higher loads.

And for a totally new idea, how about using a reinforced brick veneer. This is really a prefabricated brick panel which is anchored to the structure similar to a precast panel (much better looking of course ). Here is a guide that discusses this type of system:

http://www.brick-wscpa.org/cms/download.php?action=publications&id=7.

Maybe the reinforcement in this veneer will help you sleep at night...

 

[Agent666]

I'd only put the corner reinforcement in a residential low rise (1-2 storeys) where the drifts are typically very low (less than 1% typically). Tying it together around the corner otherwise should be avoided. What I was trying to say regarding the vertical 'isolation' joint is nicely summed up by masonrygeeks 2nd paragraph above.

See sketch below to reiterate this issues with brick accommodating the full drift at the corners, note this can mean quite wide isolation joints, if you have 50mm interstorey drift, then the separation needs to be 50mm, and the ties might need to accommodate 50mm of relative lateral movement (make sure the architect understands what these joints might look like). So the overall magnitude of the drift is quite important (you may need a relatively stiff structural system to get the cladding and ties to work as a complete system). It's no different to precast cladding and isolating the panels form one another to be able to accommodate the drift, you just have the additional complication of the ties needing to accommodate the in-plane drift:-

 

[NorthCivil]

I am with you Agent666 on your detail above being the preferred detail for a corner. though architecturally, this is a tall ask - foresee a real battle with the architect.

important to note that the movement required must consider the allowable movement of the silicone used at the joint. The most flexible sealant I am aware of - Sika Hiflex - only allows for +/- 25 % movement on the spec sheet.

So in theory, if you are required to have 50mm of movement, you would need a 200mm sealant joint. Problem is, the largest allowable sealant joint that Sika allows is 1.5", thus, the maximum movement allowed would be about 10mm. conveniently this is about the most movement I would be comfortable with anyway on a brick veneer

 

[Agent666]

You can get other types of 'prefabricated' isolation joints that may work for wider gaps, though I'm struggling to find anything to show an example of what I mean for a wall. But they are perhaps more appropriate for wider seismic gaps between separated structures rather than single isolation joints in brick.

I don't think anyone would be keen on even a 50mm sealant joint in terms of how it looks!

If it doesn't work structurally then sometimes the architects just need to join reality rather than live in a dream world where impractical things work because its what they want. If they have a problem with it see if the architect would like to be hit on the head by a brick because thats what it comes down to potentially. Either create sufficient separation or risk it all going sour when stuff starts moving and locking up.

 

[Agent666]

This is the type of thing I was thinking of.

https://www.c-sgroup.me/products/allway-expansion-joint-covers/exterior-joint-covers/asm-x-series/