Lintel - Supporting 12" Masonry 3

[DJYork]

I have a school with a fine arts building. It has an opening for the stage of 45’, and I need to design the lintel. The CMU above is a 44ft and I feel I can not do triangle loading due to the need of control joints. Also, it is supporting a fly-loft and roof, because I can not do the triangle loading (DL = 4000plf, LL = 1200plf).

I have tried fixing column to help with deflection, but for a lintel of this length I am having very heavy members.

Is there any suggestions on a better approach for this long of a span?

 

[PMR06]

Fire the architect!! Just kidding, sort of.

Is there depth for a steel truss? Perhaps you could design a reinforced CMU beam?

 

[youngstructural]

My immediate thoughts mirror PMR perfectly. I would think you need beam action out of that masonry.

Regards,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[concretemasonry]

Take a hard look at whether you really need the "cookbook" guidlines for control joints. If you can justify eliminating the need for the control joints, it would solve many problems.

I assume you are in the interior, where you lose some of the factors creating the normal requirements, which usually are used for both interior and exterior applications in the interest of simplicity.

You would still have the curing and carcbonation shrinkage of the masonry units (at least 75% occurs before laying) and the mortar shrinkage. You may be able to eliminate the moisture shrinkage and the effects of temperature, which can be major items for an exterior wall, but may not exist for your specific situation.

 

[DJYork]

The lintel itself is interior, but half way up the wall it becomes exterior. While possible, I think I can get a deep masonry lintel work, I am hesitant with such a large span. I have never developed one for that type of span or loading, but I guess there is a first for everything.

I have plenty of room to develope what I need, and was indeed looking to do the truss. However, in general, we try to keep a 3/8" deflection on masonry, but for this long of a span I thing 3/4" to 1" should be adequate. I have been informed that as deflection occurs during loading masons adjust there grout spacing.

Thanks for everyones input.

 

[csd72]

For a span of this length the 3/8" is not practical and you really need to look at more like L/500 to L/600 as a limit.

If you have jointed masonry then the larger deflections will not be a problem as long as your joint widths are sufficient to take it.

The 3/8" limit is more for when you have arching and the masonry above a certain level will stay where it is and the masonry below this will deflect with the lintel. A crack would then develop at the interface between the 2. In the arching case the actual deflection is important and is not related to the span.

In your case, if you use jointed masonry, then all the wall will move as one. It is then the rotation of the panels and the curvature of the beam that is important. Both of these are related to span.

 

[UcfSE]

Have you tried a deep concrete beam or concrete-encased steel beam?

 

[Gordy2]

I had a project several years ago with the same scenario but a 50' span. I don't have my calcs anymore but a quick stab at it give me about 2.5 kips/ft and I had an 11' deep truss w/ W8x58 chords. The top chord was near the auditorium roof and was braced by the roof deck. The truss is firred out with light gauge framing and gyp, as is the rest of the opening below the truss.

I work for an A&E firm, we don't design them with the fly loft anymore. I guess there have been advances in rigging that have (thankfully) allowed us to get away from the fly loft.

 

[dik]

You should consider control joints 12 or 16 inches back from the support opening and design the wall as a deep beam. With the depth, you should be able to consider triangular loading, but check to see if you that you have sufficient resistance for arching action if you do... also problematic with control joint at 12-16". What do you have to provide lateral support for the top of the wall? Can you use an inverted 'T' beam with hangers to structure above?

Dik

 

[civilperson]

A steel truss with large depth, (L/8), will be cost effective using standard shapes. Rest the masonry on the bottom chord.

 

[DaveAtkins]

I am envisioning an auditorium in my mind, and I doubt there is enough masonry on each side of the 45' opening to resist the thrust generated by arch action over the opening. So, I would design for the full load, as you originally proposed. A steel truss, or maybe a steel plate girder, could be used, if a standard wide flange does not work. I doubt a CMU beam will work across this span, but as was mentioned earlier, perhaps a concrete beam (or precast?).

DaveAtkins

 

[DaveVikingPE]

big, deep precast, prestressed concrete beam.

I worked on three high school auditoriums in Brevard County, Flordia many years ago. These big beams spanned the stage and looked AWESOME.

 

[rholder98]

ACI 530 requires a maximum deflection of L/600, or 0.3" max, for a beam or lintel supporting masonry. csd72, I'm with you when the span makes that seem impractical, but how do you interpret the code requirement (assuming you are in the US)?

I designed one of these almost exactly as Gordy2 describes. To maintain the CMU look from inside the auditorium we supported a 4" veneer on the bottom chord. It seems to have satisfied the architect. It hasn't been built yet, though.

 

[miecz]

rholder98

Article 1.10.1 deflection limits don't apply to reinforced masonry designed according to section 2.3.

 

[rholder98]

That's what confuses me. Article 1.10.1 says it applies to masonry designed in accordance with section 2.2 or Chapter 5. However, section 2.3.3.4.5 says "Beams shall be designed to meet the deflection requirements of Section 1.10.1." Then in the commentary, it contradicts itself again. So what gives?

I just design all my lintels and beams to L/600, or 0.3" max, and figure I'm covered.

 

[miecz]

rholder98

I don't see these as contradictory. Article 2.3.3.4.5 says that beams must meet 1.10.1. Article 1.10.1 says to limit deflections when the beam is supporting unreinforced masonry.

So, if a beam designed to 2.3 is supporting a wall designed to 2.2, it must meet the deflection requirements of 1.10.1.

If a beam designed to 2.3 is supporting a wall designed to 2.3, it does not need to meet the deflection requirements of 1.10.1.

Many engineers meet those requirements anyway (see previous threads on this topic), but I don't believe it is required by the code.

 

[csd72]

This is the problem with the current situation in american codes - too many catch all statements that any good engineer knows are not always relevant, but the engineers are too scared of litigation to ignore them!

 

[MDukh]

I like the idea to fire an architect, but let’s try to work with them.
Recently we finished school project, which started w/ exactly same problem. We proposed to use metal stud wall over proscenium, which is much lighter, and not required such killing deflection restriction and brick veneer just for exposed portion of wall, above roof over audience seats. By our proposal, veneer should be supported by DLH joist.
Architect accepted our proposal without any further discussion.