REPOST: Steel Lintel Bearing on Masonry[Subject Prev][Subject Next][Thread Prev][Thread Next]
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- Subject: REPOST: Steel Lintel Bearing on Masonry
- From: "Michael L. Hemstad" <hemstad.ml(--nospam--at)tkda.com>
- Date: Mon, 25 Feb 2002 16:13:02 -0600
Most of what I have to say about lintels is pretty basic, but, for what it's worth:
1. For CMU, the beams need to be in multiples of 8 inches. I suppose you could make a 12 inch beam work with a half height (4 inch) course, but it's usually not worth it. (Like I said, pretty basic.)
2. For openings of maybe 8 feet or less, we usually have a choice between CMU bond beam and steel beam with plate. The bond beam is fireproof and rustproof, but you can look up and know the steel beam has been placed. We've also had problems with masons substituting block with knockout tabs for the typical U-shaped bond beam block. The reinforcing ends up in the middle of the block instead of at the bottom. Our Structural Notes now forbid this for lintels, although we permit the knockout block for continuity bondbeams (e.g. at the top of a wall). "Forbid" is a relative term, of course; it assumes someone reads the notes, and then follows them.
3. Any time we have brick to support over more than about 4 feet, we use an HSS section with an eccentric plate. For 4 feet or less, we usually end up talking ourselves into a galvanized angle for the brick. I'm not aware of any problems with this, but I won't claim it's the greatest because we usually end up bearing it on the brick. I'll wait for the flamers to nail me for this. The bigger openings, or unusual brick overhangs (e.g. fancy corbeled cornices) cause concern for overturning of the beam at the reactions. We bury a steel column in the wall rather than try to carry this into the CMU.
4. We try very hard to hold deflection to less than L/600. This helps a host of problems, edge-bearing overstress being high on the list.
5. We don't assume arching of the carried masonry. Too many times, the architects have thrown an extra door or window in at the end of a wall, believing our details will still work and invalidating the arching assumptions.
6. As previous posters have said, lateral loads are an issue. An HSS usually performs a lot better than a wideflange. A related issue is unbraced length; we consider the beam unbraced. If the lintel is carrying an interior wall, unbraced length may matter more than the nominal lateral load required by code. If you have something to dump lateral load into, of course, it can also brace your beam.
7. For spans longer than about 8 feet, we deal in steel or reinforced concrete beams. A few years back, one of my supervisors, now retired, put a concrete beam over a sixteen foot opening in an otherwise steel framed building. The contractor went nuts, because he had to bring a trade back for just one item. On the other hand, the concrete beam has the same advantages listed above for the CMU bond beam: it doesn't need fireproofing and it won't rust if exposed.
8. You mentioned openings up to 30 feet. This, in my experience, is pretty big. Any span close to this will require columns at the ends of a large steel beam. You still need to try hold deflection down to L/600.
9. Some engineers weld rebar dowels on top of their lintel beams. I've never decided whether this is worthwhile or not. I suppose it is if the wall above takes lateral load, and the beam has to resist it (as opposed to dumping it into a floor slab). It does make shipping difficult.
10. Our bearing plates are usually just a continuation of the 5/16 inch plate shelf, primarily because it's simple. This has the disadvantage of potentially causing stress concentration at the edge of the opening. It hasn't proven a problem for us, primarily because of the stiffness we design into our beams and the use of columns for longer spans.
This is all I can think of in one sitting (...and 15,000 engineers breathe a collective sigh of relief...), and I really haven't addressed your question. No, we don't get too excited about therrmal expansion gradients. We're more concerned with holding the lintel in place.
Mike Hemstad, P.E.
St. Paul, Minnesota
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