Deflection limits for masonry support members.

[Tomfh]

Hi,

What sort of incremental deflection limits do you guys use for members supporting masonry?

In our Australian codes the suggested limit is Span/1000, or Span/500 where provision is made to minimise the effect of the movement (whatever that means!).

The senior guys in the office just say to use Span/500. I am keen to follow this advice given how hard it can be to get things to within Span/1000.

What are your thoughts?

 

[MotorCity]

In the abscence of specific code requirements, I use L/600.

 

[UcfSE]

Our ACI 530 masonry code limits deflection to Span/600 or 0.3 inches.

 

[PSlem]

I just ordered ACI 530 as I knew L/600 from ICC. But how does .3" work? Say I have a span of 72". Do I have to meet L/600 or .3" which is only L/240?

The optimist sees the glass as half full. The pessimist sees the glass as half empty. The engineer see the glass as too big.

 

[haynewp]

You need to meet them both.

 

[UcfSE]

It's the lesser of the two. For reinforced masonry we usually omit the 0.3" requirement.

 

[haynewp]

Same thing.

 

[miecz]

Section 1.10.1 of my latest version of ACI 530 (1999), limits deflection to L/600 or 0.3 in when the masonry is designed to Section 2.2 or Chapter 5. Section 2.2 is for unreinforced masonry and Chapter 5 is for empirical design of masonry. I've always taken this to mean that masonry designed to Section 2.3, i.e. Reinforced Masonry, is not limited by Section 1.10.1.

 

[PSlem]

If you go back to 1950, MacDonald and Skempton recommended L/600 for brick load bearing walls only. Else L/240 OK.

The optimist sees the glass as half full. The pessimist sees the glass as half empty. The engineer see the glass as too big.

 

[UcfSE]

Reinforced masonry is not specifically subject to the L/600 limit because it is assumed that the reinforment will control the crack width when the wall deflects. As an engineer we do have the right to exercise judgment and decide to design to L/600 anyway where practical. That's standard procedure where I work, as well as with other engineers I know in my area. IMHO the benefit of small crack width and reduced cracking is worth it to help control water and pest intrusion. My opinion it is, that's all.

 

[rapt]

Tomfh,

"Provision is made to minimise the effect of movement" in AS3600 refers back to the masonary code and the definition of Articulated masonry. Basically, Joints must be placed at maximum 6m centres.

So, if the masonary walls are articulated, the L/500 limit can be used otherwise the L/100 rule must be applied.

 

[Tomfh]

Thanks for that explanation Rapt.

Tell me, do you know why the Australian deflection limit is L/1000 when other codes allow almost double that?

 

[rapt]

Tomfh,

The other codes do not differentiate depending on the type of walls. The Australian code committee considered that non-articulated masonary required more stringent limits which seems logical. The AS3600 figures for articulated masonary are similar to the standard values for other codes. If you think about the difference in movement and cracking in the two cases it is logical.

Basically, AS3600 is saying to always make sure your masonary is well jointed or pay the penalty with more expensive supporting structures.