Drift Limits for Brick Cladding 1

[profplan]

As background, a two story, 3 wythe brick exterior clad rectangular building built in the 1930's is to be gutted and replaced with a new 2 story steel structure. The brick exterior will be temporarily braced and preserved in the new building.

The Construction manager wants to use a pre-engineered metal building as replacement. My experience is that PEMB designs are typically flexible, H/150 or even less.

The brick walls have multiple windows, 1/3 the area of the wall faces. The brick condition appears good.

What would be an acceptable drift for the PEMB frame to control the potential for brick distress?

 

[WillisV]

H/500 under ten yr wind

 

[profplan]

WillisV - Thanks, can you explain why a 10 year wind is used?

 

[Lion06]

The 50-yr winds are used for strength, while the 10-yr winds typically used for serviceability.

 

[dhengr]

Profplan:
I think you kinda have your question bass-ackwards. It should be how does my new structural system support the existing brick walls so as to maintain their integrity and use them to their full advantage? I don’t see how else it will work, the brick won’t allow drift, that isn’t a word that has meaning in masonry design like yours.

If you’re the EOR (engineer of record) on this project, you get some say on the matter, then the project manage gets to go shopping for price as long as his structure meets your design criterial. PEMB’s are pretty darn flexible and work best when covered with a metal skin on purlins and girts, allowing lots of bldg. flexibility. They don’t work worth a damn when they are used to support a brick veneer. And, your structure is kinda a third case; 3 wythe brick exterior bearing walls and interior steel framing. You get to spec. all of the loadings on the steel, and you get to spec. both vert. & horiz. (lateral) deflections of the steel. The floor and roof framing must act as shear diaphragms, and to carry gravity loads to your satisfaction. If you look at the relative stiffness of the two systems; the steel framing should support gravity loads and the brick walls against lateral loads perpendicular to the walls, and the floor and roof diaphragms must take these lateral loads to shear walls parallel to the lateral loads (wind & EQ). And, don’t forget the lateral loads the typical PEMB frame causes at the foundations, they won’t be easy to accommodate within the existing bldg. envelope.

You have to have a fair understanding of the make-up and detailing of the existing walls, their condition and how they are loaded now. What is the existing interior structure and how does it load and support the walls now? Consider the condition of the walls and potential maintenance (tuck pointing), crack repair, etc. If you don’t change the wall loading and support appreciably, with your new structural system, your home free. New loadings, and openings, and the like require special attention, of course. And there, the new steel may pick up the load. The PM wants cheap structure so he can spend more on carpet, etc; you want good sound structure that preserves the walls. When you provide that, with a good clean structural steel system, and/or insist that the PEMB perform to the same level, he won’t save any money with the PEMB. The way to save money on this job is to change the wall loading, lateral support, function and openings as little as possible, or there isn’t much sense in trying to save the walls.

 

[JAE]

dhengr - it isn't brick veneer. It is a 3 wythe brick wall.

 

[dhengr]

JAE... Sorry I didn’t imply that strongly enough for you, when in the very next sentence I said “And, your structure is kinda a third case; 3 wythe brick exterior bearing walls and interior steel framing.” Otherwise, thanks for the critical (I mean constructive) addition to the discussion.

 

[JAE]

Sorry - saw the "veneer" word and didn't read on. My bad.

 

[hokie66]

Agree with dhengr. A typical PEMB rigid fram won't work for this application. Floor, roof, and wall diaphragms or cross bracing are required for adequate stiffness.

 

[JAE]

Well...you can TRY to specify and get a PEMB company to design to a particular drift. There will be push-back of course, but you could discuss this with a few local companies and see what they say.

I would think it is possible for them to create a PEMB that creates the required drift limits. They're just not used to it.

And by the time they provide one, their supposed savings might be gone relative to you just engineering a frame that works.

 

[profplan]

Thanks all for the valuable comments.

dhengr, I see your point regarding the question.

I work for the EOR. It appears the CM will likely convince the owner to use the PEMB. Cross bracing has been discussed, to increase frame stiffness and reduce drift to limits suitable for a brick exterior. The existing brick will be self-supporting only, with no floor loads. The new floor and roof beams will cantilever past outer columns to laterally brace the brick exterior, with connections to allow vertical slip.

The existing building was framed in wood, with knee braced frames in the long direction. In the short direction there are wood joists infill between the longitudinal frames. It looks like the brick may have provided lateral resistance in the short direction. I don't see any interior masonry/brick (shear) walls, and there are no existing drawings.

Other than specifying the drift limit, H/500 (10 yr wind), for the PEMB, rigid diaphragms, and vertical slip connections at the brick walls, are there any other structural concerns using a PEMB?

A careful visual inspection of the existing 3 wythe brick wall for cracks, deterioration will be done. Are there any other techniques/tests you suggest to document/confirm the condition of the brick prior to the construction?

 

[dhengr]

A waver from the owner stating that he won’t mind that stiff trampoline surfaces are substituted for what should be floor and roof diaphragms. Some special design to transfer concentrated shear diaphragm loads to old brick at concentrated slip joints. You might want to add 25% to your fee for all the extra screwin around you will have to do, load and detail wise, so the CM’s brother can sell the owner a less than desirable PEMB. Compare the cost of the two.

 

[concretemasonry]

Keep in mind the stiff brick masonry will see the wind loads and be expected to transfer those load to the steel structure until the steel structure moves and the loads are on the masonry, since it may be stiffer. This in refernce to lateral load resistance.

This is a situation that the "cladding" may be a structural element because it sees the loading before the interior does. - Sort of like the classic problem of brick being more resistant to perpendicular wind loads and the steel stud "back-up".

Seismic could be a different situation since the exterior is very old, massive and probably not intended to be a structural system in the new combined structure, but it does receive and transmit loads. - Not a computer program job since it requires radically different materials and connections and is a real engineering challenge.

You have two different structural systems dividing the loads depending on the magnitude and direction. At only 2 stories, it is more qualified and simpler, but the brick construction is probably continuous.

Dick


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

 

[abusementpark]

So the existing brick wall is essentially only a curtain wall now?

Have you verified that it can act as a curtain wall based on modern codes?

 

[dik]

I typically use L/720 for deflection with brick walls...

Also, take care of the building envelope. Changes to this can cause serious problems. Old brick used to 'wick' moisture through the mortar joints; there wasn't a vapour barrier. New buildings can be vapour retardant... also, changes in lifestyle cause much greater humidities within the building.

Dik

 

[profplan]

Thanks for the posts.

concretemasonry, Fortunately, just wind will be a consideration based on the location. I realize that the brick has to structurally span between floors (and roof) to resist wind unless I provide a structural backup stiff enough to relieve the brick.

abusement park, The situation is similar to a curtain wall but not exactly as the brick wall stacks down to the foundation, laterally braced at 2nd floor and roof. Looks like I'll need to determine a flexural capacity of the wall to span between floors by sampling (cores or other Destructive Testing)?

dik, Do you check L/720 for a displacement at floors (drift) or mid-height between floors(say, for design of backup)? I'll probably have a hard time convincing PEMB manufacturers to provide L/720 for their frames. I'll ask the architect about the moisture issues.

 

[a2mfk]

dhengr- Sounds like you feel about the same about PEMB as I do. And it seems like they get crowbarred into projects they don't belong by people who don't know better. Or worse, do know better, and are trying to sell a PEMB to an ignorant owner who doesn't understand the full picture and the hidden costs. I have seen several buildings, and even worked on a few, that use 8" CMU as a "cladding" on a PEMB, when they make perfectly good bearing and shear walls and are just sitting there doing nothing...

I would say from this description, this is definitely NOT the place for a PEMB, it should be a steel-frame with maybe some very stiff CMU shear walls (stairs or elevator cores), and one EOR should be driving this train with all of the little details and responsibility on one set of drawings. There is a wide variety between the quality, knowledge, and detailing expertise of PEMB mfrs. If you can get the architect or owner's ear and describe that any price the PEMB initially gives them will be based on very inaccurrate standard models of buildings that do not meet all of your requirements.

Brick support- you can always treat the brick like a cladding if you don't know enough about its existing strength, and using light-gauge studs or steel angles and HSS sections laterally or vertically to support it...