Brick Deflection Limits 1

[mattmlm]

I am aware of the commonly accepted limits of 0.30" or L/600 for brick lintels, whichever is less.

For very long spans the 0.30 requirement can be unreasonable. For example, a 40 ft. span truss that deflects 0.50 inches (L/960) has less angular rotation than a 15 ft. beam that deflects 0.30 inches (L/600). In my opinion, the angular rotation is the issue, not the absolute deflection.

Is there any relief to the 0.30" requirement for very long spans?

 

[haynewp]

I haven't done any lintels that long before but that makes sense. I would add a full height vertical movement joint at midspan in the brick to help out.

 

[rapt]

mattmlm,

Australian code has not total deflection limit but limits deflection to L/500 for articulated brickwork (joints ar 18' or less) and L/1000 if not jointed.

.3" at L/600 is equivalent to about 15' so similar to the aticulated brickwork span in the Australian code.

This agrees with haynewp's comment suggesting jointing the brickwork.

 

[PT999]

Question??
Does L/600 deflection also apply to veneer brick, such as walls made of 1 whythe brick with cold formed metal framing and wall board?

 

[jike]

On long spans, it is extremely important that you provide a vertical control joint at each end of the lintel.

 

[miecz]

If you're conforming to ACI 530, the deflection limits are for unreinforced (or empirically designed) masonry. Suppose you called for a wire in the joints to qualify the wall as reinforced. Then, any wall crack would be contained by the reinforcing and the deflection limits wouldn't apply.

 

[apsix]

"Does L/600 deflection also apply to veneer brick, such as walls made of 1 whythe brick with cold formed metal framing and wall board?"
I would say yes; the wall doesn't know if it is a veneer or load bearing in this context.

 

[miecz]

ACI 530 Article 6.2.2.3.1.5 holds the deflection of wood beams supporting masonry veneer to l/600 or .3 in. The way this section is written, it almost seems that this limit is not meant to apply to steel lintels.

 

[mattmlm]

After reviewing your answers, this is what I have decided is an appropriate approach to the problem of supporting brick on long span elements:

"The curvature of the support should be limited to L/600 but it can exceed 0.30" if control joints are provided at 15 ft. o.c."

With control joints at 15 ft., if the supporting beam is limited to L/600, the deflection of the independent brick panels between control joints would be (15)(12)/600=0.30. The independent brick panels will not know that the overall deflection of the beam is more than 0.30.

 

[ArneG]

You can detail yourself out of the .3" deflection requirement with reinforcing or by jointing.

I wouldn't though.

Brick is a "real" load. That 40 psf is there all day long, all week long, all year long, and usually masonry buildings arn't the type that get ripped down in 25 years.

I've used L/600 for 30' spans on spandrel buildings before (pre-IBC), and that 0.6" of deflection is noticable.

And what do you say in 30 years when you come back and 20% of the section is corroded away?

Anyways, I'm a proponent of building with brick conservatively, use the 0.3" maximum as a tool to educate architects, owners, and contractors that it really does make sense to limit masonry deflections.

 

[jike]

It is also a buildability issue, as well as a visual issue.

The masons will have difficulty if the beam deflection is too much! As they build the wall they can increase the masonry joints slightly. Keep in mind a masonry joint is 3/8" and they want to build those joints level. As the beam deflects, they try to widen the joints to keep the top course level. As the lower courses cure, they add to the overall stiffness. Intermediate control joints will prevent the masonry from arching.

 

[mattmlm]

Those were good replies. I can see the buildability issue as a big deal. The mason has a string that stays level, which would cause the joints to grow at midspan. If the brick were able to be built following the L/600 curve, I don't think it would be very noticeable, but that won't happen. So, I take back my previous conclusion and plan to stick to the 0.30" limit, regardless of span.

 

[rapt]

If it archs, it leaves a gap at the bottom as the beam deflects and allows water to leak in!

If it does not arch it will not crack!

The deflection when they are building the wall, before a lot of the load is added and before long term deflection, will be very small if the total long term deflection is going to be .3 or .4".

We can come up with lots of arguements like this.

 

[frogit22]

My experience in doing some peer reviews is that it is common to apply some judgement to this situation.

 

[miecz]

A related issue is the unsupported length of the beam compression flange. I take it as the clear span of the beam. Seems to me this usually results in a beam that meets the deflection criteria, but I'm usually dealing with much shorter spans.

 

[rapt]

jmiec,

Are you saying that you use a flange width for the beam equal to the span length of the beam? Your deflections would always work according to this. Too bad it is not what really happens.

Why don't you use the code defined flange width which tries to estimate the width of flange that will actually contribute. It is maximum 25% of the span length but related to slab depth as well.

 

[WillisV]

rapt: I believe jmiec is saying he takes the unbraced length of the beam equal to the entire clear span and therefore LTB controls his beam sizes and typically requires a large enough size for strength that the deflection is ok (as opposed to assuming a fully braced top flange and choosing the beam size based on the full plastic moment for strength).

 

[miecz]

Thank you WillisV. That is exactly what I mean to say. I believe some designers treat lintels as fully or partially braced by the masonry. It's a topic I've never seen discussed in a text or code, so I go with the entire clear span. I would be interested to here how other designers treat this.

 

[DaveAtkins]

I always treat a steel lintel as unbraced.

DaveAtkins