deflection limit for spandrel beams supporting brick? 3

[hippo11]

I've read on the Brick Industry Association website that L/600 or 0.3" should be used to avoid cracking mortar.


My question is, what loading would you apply this limit to? Of course it depends on construction sequence...

But I've heard from one person that it should be for superimposed dead + live, but then from another person that it should be for live only, with the rationale that the brick mortar is still wet when the brick "sees" its self-weight and therefore won't crack from its self-weight...

 

[lkjh345]

Hippo:

I generally use full dead and live load for designing lintels. Use L/600 criteria.

May be a little conservative, but if you have more than a few courses of brick on the beam, and the masons but up a few course, then get stopped for a few days (3 day holiday then a couple of rain days following) the mortar in these courses will be reasonably set before the rest of the masonry load is appiled. I would rather be a little conservative in my thinking and design, than getting a phone call from the owner wanting to know why his brick wall is cracked and leaking water.

 

[jike]

I agree with the approach that lkjh345 suggests. This is exactly what I do and it will account for all the variation in construction sequencing that I see. Don't try to guess at the sequence of how it will be built because as soon as you do, you will be wrong.

 

[JAE]

Yes..agree essentially..but you can delete from the loads the self weight of the beam and any supplemental steel used to hang the brick. Also, if you mandate in your plans that the adjoining steel, deck, and/or concrete slab must be in place prior to setting any brick veneer, you can delete them as well as they will all be in place prior to the brick being installed. But the weight of the brick and windows, wall framing, etc., should all be included as these may occur after the brick is begun.

 

[hippo11]

So is everyone generally in agreement that it may be a bit unconservative to negate the self-weight of the brick in its own deflection calcs due to mortar wetness...because there may be delays in bricklaying, etc., and the mortar in a course below very likely could actually be hard when the course above is laid?

 

[haynewp]

It should be considered unconservative unless you are specifying the process of construction, but I have known lots of engineers (including myself) that have used L/600 or 0.3" for live only and have not had complaints. Hey weren't we discussing providing the construction process to the contractor just the other day?


When things are steep, remember to stay level-headed.

 

[JAE]

haynewp - what you say is what I most usually did - until I was hired to review a design where the engineer provided L/600 for the live load - and the horizontal strips of brick spandrel cracked. The spans varied from 35 to 44 feet and the brick strips were about 6 feet in vertical dimension. The 0.3 factor was not considered so that may ultimately be the cause.

 

[haynewp]

JAE, did you get an actual deflection measurement on that beam? Did you notice how much superimposed loads were present (partitions etc.)?


When things are steep, remember to stay level-headed.

 

[haynewp]

Also, was there a control joint in those spans? I have got a 43ft composite girder going up now in a similar situation carrying 15 ft of brick the entire way. The live deflection (50 psf floor live) is at about 0.5" so I am a little concerned about the brick construction process on this girder, as it is already not looking great for cracking as it is. But there is supposed to be a control joint at midspan which will help some.


When things are steep, remember to stay level-headed.

 

[mrengineer]

See also threads 507-159624 and 507-169291.

 

[JAE]

Hello haynewp....no control joints at all (part of the problem eh?) - I never had a chance to measure anything - they got me involved way after they came in and added intermediate columns. Interior partitions were typical office layout - some walls but mostly cubicles.

The funny thing was that right after I did the study of the building with all the problems, we had a similar project in house that involved banded brick spandrels. I took greater care in checking the superimposed DL + LL against the L/600 and 0.3" deflections - even asking and getting some additional control joints in the brick on some longer spans.