Brick Arching Action

[ForestStructural]

I have an old school in Boston which has four stories of an 18" wide brick wall from the ground to the roof. At some point there was a full height, brick enclosure added to the outside face of the building (see photo). Naturally the school wants to remove only the first floor portion of the wall in order to gain the roughly 8' of exterior corridor to widen the kindergarten classroom from 25' to 33'.

The majority of the brick buildings in Boston have pockets to receive the floor joists in this case the joists appear to be extending through the entire 18" width of the wall. If i assume that there is an opening in the wall just above the proposed lintel so that arching action does not occur in regards to support of the 12"H x 18"W brick wall (dead load) and the second floor joists so that the full width of uniform dead and live load reactions would apply, would it make sense to assume that there is arching action at the level ABOVE the second floor so that I can design the lintel to support the second floor live and dead loads only and ignore the dead, live, and snow loads from the floors above?

 

[Ron247]

First, to answer your question, I would design the lintel for ALL the loads above it. That only answers the loading of the lintel question.

These are initial assumptions, thoughts, concerns.

• I assume:
  • The 18" wall is 3-wythes of brick, interlocked periodically with what I think they call "collar ties"
  • Your joist do not extend all the way through or they would have been exposed outside on the original construction.
  • The joist are not mechanically fastened to their supporting brick. That is the way most of the ones I have seen were built. In those cases, only friction holds the walls together unless someone as used tension rods. In my area, tension rods are present on less than 30% of the buildings. Generally, only present on free-standing buildings.
  • The brick wall is unreinforced. Your brick is like all brick, BRITTLE. The problem with brittle materials is that when they are about to break or fracture, they look JUST like ones that are nowhere close to being overstressed.
  • The brick walls are in good condition, assumed but possibly incorrect.
• Thoughts:
  • Arching cannot take place until your lintel sags to some degree. Till then, you are direct vertical bearing.
  • Lintel sag may crack the brick, thus reducing the chance to arch properly. L/600 is not stiff enough sometimes when the brick is already present. Works good for brick being laid new. A really strong lintel can potentially prevent arching.
  • Lintel sag may damage collar ties, thus no longer having the strength of an 18" wall, but the sum of three 6" walls worst case.
  • The opening will approximately double the soil bearing of the walls on each side of the opening. Will the soil bearing significantly increase settlement? How does settlement affect distribution of forces? If there is no arching, you will have a somewhat large point load at each jamb.
• Concerns:
  • Has someone added wall framing over the years that routes end reactions of the joists above to the joists below? Most original construction has each level of joists independent of the joists above. Any settlement or lowering of the brick could shift the floor weight above to the joists below.
  • What condition are the joist ends where they are pocketed? In my experience, most ceiling joist ends are rotted and some floor joist ends are to some degree.

Try talking them into smaller kids first.

 

[Nythius]

I typically rely on arching when designing lintels for masonry. One needs to be able to justify resisting the horizontal thrust force somehow though. Might be difficult with only 7ft of brick at the side. Perhaps could introduce some kind of tension tie though…

 

[bhiggins]

I would actually consider the wall segment above as a large deep beam spanning between the opening. I would check the allowable shear and bending stresses vs those allowed for unreinforced brick masonry (these are quite low, around 30 PSI). I would of course reinforce the span with a steel beam as well.

 

[Ron247]

As far as the lintel, I would design it for all the loads. If I am wrong, I am safely conservative. If I rely on arching or any kind of beam theory for the wall above, I still have one issue to evaluate. That issue is, what does it take for what is acting as a wall right now with direct downward bearing to change to an arch or a beam that distributes the load to the ends of the beam/arch? It requires the creation of an opening or clear span between supports. Right now, the clear span is 0".

What makes the opening below the beam/arch? It has to be sag of the lintel or outright locating the lintel some distance below the load. What weight makes it sag enough to create the arch or beam? What if the lintel is too stiff and does not allow this to happen?

Because I am dealing with unreinforced brick that is old, I am not taking the risk in terms of sizing the lintel. Also, this structure is not under construction, it has been this way for how many years? Also, the Floor Live Load test will be kids with 2 parents present on "Parent's Day" at the school or a PTA meeting. Add in the thrust as pointed out by Nythius and I have too many headaches to trust arching for sizing the lintel.

 

[CJLCivilStruct]

I tend to think once a span goes beyond around 1.5m to 2m, relying on arching behavior is a bit too risky for my liking. The span of around 4.8m (sorry my brain only works in metric) is quite large, and sizing the beam using arching behavior may result in a beam size which 'looks' a bit slender / wrong for the span.

 

[GeorgeTheCivilEngineer]

I had drafted a longish reply but the save draft function hasn't worked!

In short - yes arching action in the wall above is reasonable. If the joists are in good condition and mortared in then they shouldn't necessarily affect that, but prudent to allow for it. The load from the floor would be acting directly anyway. The rule of thumb for sufficient buttressing masonry is 20% of the span, but that is based on slightly smaller opening widths.

I do disagree that masonry walls are brittle - they show surprisingly high degree of flexibility. The bricks and mortar themselves are brittle, but the micro-cracks and dislocations in the bed joints throughout the depth of a wall, means that as an overall element, the wall panels can and will engage arching action without large cracking.

Ultimately design to what you're comfortable with although I suspect you will have issues getting suitable masonry bearing or foundations if you allow for the entire weight above.