Masonry Stair Walls and Connection to Primary Structure 1

[McCalf]

There are many reasons engineers are interested in connecting masonry stair walls to the primary structure. For shear wall applications a connection is needed to transfer forces out of the floor diaphragm and into the masonry. Sometimes it would be nice to brace masonry walls to the floor to reduce spans for out of plane bending. Unfortunately, every client seems to have a different interpretation for what connection are permitted to rated walls.

Some architects allow vertical slip connections as long as no gravity load is transferred into the wall.

Some architects insist on a completely independent stair structure. No connections of any kind at the floors; independent roof structure with joints in the roof deck.

Some architects do not allow connections at the floors, but will allow a shared roof structure.

Some architects will allow "break-away" connections that fail at low temperatures in the event of a fire. Epoxy anchors?

Has anyone else experienced these limitations? Does anyone have some code references that describe what connections are permitted to rated wall? I can never get a straight answer from our clients regarding attachment to rated walls.

 

[JAE]

You have very bossy architects.

 

[kylesito]

Some architects allow vertical slip connections as long as no gravity load is transferred into the wall.

It's the engineer's responsibility to design the walls.

Some architects insist on a completely independent stair structure. No connections of any kind at the floors; independent roof structure with joints in the roof deck.

It's the engineer's responsibility to design the floors and roof.

Some architects do not allow connections at the floors, but will allow a shared roof structure.

It's the engineer's responsibility to design the connections.

Some architects will allow "break-away" connections that fail at low temperatures in the event of a fire. Epoxy anchors?

It's the engineer's responsibility to design the anchors.


JAE was right, your architects are too pushy! I'm being comical but all the things you mentioned are things WE tell the architects. They give us the location of walls, floors, and roofs and we design them, connect them, and anchor them to make the building stand up. If the architect wants to make the structural decisions, he is more than willing to stamp the structural drawings!

On a more practical note, we have had a lot of success in design build projects (so the contractors like it) in basically framing around a stair tower with beams/girders and then attaching a long leg angle to act a pour stop on the top flange of the beam butted up against the masonry. We then anchor the vertical leg of the angle to the wall and set the deck on top of it. The guys in the field like it because it is easily adjustable and quick to install and the architects don't have an opinion about it.



PE, SE
Eastern United States

"If a builder builds a house for someone, and does not construct it properly, and the house which he built falls in and kills its owner, then that builder shall be put to death!"
~Code of Hammurabi

 

[McCalf]

I'm not sure that the architects are being pushy. I think there are some legitimate fire safety concerns when you are dealing with stair walls as a means of egress. Unfortnately, I think that each architect interprets the code differently.

On a more practical note, we have had a lot of success in design build projects (so the contractors like it) in basically framing around a stair tower with beams/girders and then attaching a long leg angle to act a pour stop on the top flange of the beam butted up against the masonry. We then anchor the vertical leg of the angle to the wall and set the deck on top of it. The guys in the field like it because it is easily adjustable and quick to install and the architects don't have an opinion about it.

This is exactly what I would like to do. Have you ever received any comments from the architect, fire marshall, or permit office regarding this connection detail?

 

[JAE]

McCalf - I've never heard of anything like this regarding stair walls (>30 years) such as putting roof joints in the roof around the shaft walls, no direct connections, vertical slip joints.

That sounds all very ridiculous. Stair shafts are used all the time for shearwalls with direct connections to the diaphragms.

 

[kylesito]

Never any comments by any party. I see the point about the importance of fire codes in how they relate to stairwells but fail to see the significance of how this detail plays into that discussion. I am just not seeing a situation where the connection of a floor/roof to a stair tower affects anything to do with the fire rating. All it requires is a grouted bond beam (or not even that if the right anchors/spacing are used). I just can't see how this affects anything from a fire rating perspective as the wall would have all of these elements in them anyway.

You can have the discussion with the architect too that either 1) you use the masonry walls you are already building to act as the LFRS, or 2) you build extra LRFS into the structure thereby increasing the construction costs. Seems like a straightforward decision to me.

PE, SE
Eastern United States

"If a builder builds a house for someone, and does not construct it properly, and the house which he built falls in and kills its owner, then that builder shall be put to death!"
~Code of Hammurabi

 

[McCalf]

I always thought that the limitations set by some of our clients are a little extreme. In some cases we are able to convince them that connections are required; but sometimes the architect is adament in limiting connections to the stairs.

The architect's argument has always been:
If a beam or floor structure with a posistive connection to the stairs fails during a fire event, the stairs could be damaged or destroyed during the collapse. Therefore, the connected elements would require a fire rating that meets or exceeds the rating of the stair enclosure. Fireproofing is sometimes ugly, costly, and generally something the architects like to avoid (especially spray fireproofing).

kylesito - since the angle is used as a pour stop, I assume that the anchors are encased in concrete. Are you concerned about the vertical deflection of the beam so close to a rigid wall? I think that the anchors will transfer gravity load to the wall as the beam begins to defelct.

 

[PicoStruc]

I think there are some legitimate fire safety concerns when you are dealing with stair walls as a means of egress.

personnaly, I would have concern about seismic too. In Seismic Point of view, you don't want your block wall to take any force (vertical & lateral). The walls should be isolated from the rest of the structure to not add stiffess (that will change dynamic properties... stiffer building attrack more forces) nor resist seismic forces !

But i Know that the construction practice not always respect that because of fire safety...

It's a contracdiction between architecture and engineering requirements

 

[structSU10]

If it is to be seperated from the structure, dynamically i would think the stair shaft would vibrate at a different frequency than the building, and thus banging could occur if improper seperation was provided. Whether it's much of a concern, I'm not sure. Most details I deal with would certainly transfer some load to the shaft, by bearing of the face of the shaft onto the diaphragm and such. I've never been certain on the best way to deal with a shaft that is not part of the LFRS.

 

[kylesito]

Maybe that is why it's so universally acceptable where I am located....90% of our structures are SDC C or lower. And going along with this, the wind we design for also does not create significant shear forces enough that we are ever close to exceeding the capcity of the wall.

I could see the argument about connections failing in a fire event. Although I am not a fire protection engineer in the least degree, my limited understanding tells me that it would have to be a serious fire event to reach the anchors which are encased in the floor slab. Nevertheless, we have never had a situation where anyone has questioned this detail.

I suppose it's an interesting point that you want to keep the stairs isolated for both egress and stiffness considerations. I must admit I don't know how to argue against that logic. All I can say though is that it's tough for us to justify the cost of moment frames when a perfectly good masonry that already needs to be built can do the same job. But, I think a lot of this is the flavor of the building industry in the area it's being built in.

McCalf...we have considered the deflection of the beam and camber the beam itself. We have not had a problem, however, in the installations we have done with this detail about overstressing the wall due to forces being transferred. If we ever had a beam long enough we were worried about it...we would probably provide vertical slots in areas of high deflection.

PE, SE
Eastern United States

"If a builder builds a house for someone, and does not construct it properly, and the house which he built falls in and kills its owner, then that builder shall be put to death!"
~Code of Hammurabi

 

[grantstructure]

We've had the interpretation of stair separation that you reference imposed on us by the city in a few instances. It's crazy.

 

[a2mfk]

I'm with the majority of the other posters. Unless there is some very specific fire egress requirement, which I have never dealt with, those are never things I have heard from an architect.

 

[PicoStruc]

those are never things I have heard from an architect.

This is the problem in seismic design. We suppose that the wall don't participate or stiffen the building...and are not bearing

The architect and contractor build bearing walls !

 

[matth35]

I wander if the architect is applying the 2 hour fire SEPARATION rules to a 2 hour fire rated wall. The 2 hour fire separation intends for the structure to be separated from the fire wall, but I do not believe this should be applied to a fire rated stair well.

 

[MJB315]

I see the concern on moment frame buildings. How do you justify attaching a masonry shaft to a moment framed structure? Wouldn't the shaft attract in a lot of load while the building drifts? I'd be interested to hear how people handle that condition.

"We shape our buildings, thereafter they shape us." -WSC

 

[PUEngineer]

We had a project recently where one of these conditions occured. All egress areas needed to be isolated from the main structure. Masonry freestanding walls had to be provided around stair towers and elevators. I think the structural steel could be fireproofed in the areas of the egress elements, but in my case, it was exposed steel in an office building. So, we were pushed to using the freestanding walls. This was a low seismic area, SDC B.