Residential Portal Frame Deflection Limit

[Arun238]

Hi there,

Just trying to understand the deflection limit for residential portal frames designed in New Zealand. There is a 8mm or H/300 limit specified for specific bracing elements as per the P21 test paper by Branz.

Referring to 1170.0 serviceability limits, The in plane limit for plaster or gypsum walls in Height / 300 against SLS wind loads with a parameter specified as mid-height deflection.

Does this mean that the SLS seismic limit is 8mm or H/300 at knee and SLS wind limit is Height / 300 measured at mid height for a portal frame.

Thanks in advance.

Arun

 

[Tomfh]

What sort of absolute limit for these industrial beams? Eg if you had a 15m girder supporting some floor beams?

 

[human909]

What sort of absolute limit for these industrial beams? Eg if you had a 15m girder supporting some floor beams?

No absolute dimensional limit as generally there isn't partitions etc that excessive deflection will affect.

I've been running on the same 1/350 limit that I've used elsewhere as rightly or wrongly that seems to be suitable. For cantilevers with low dead loads I generally also consider a 1kN load and keep deflections below 1.5mm as for such situations you can readily get something to pass but it will still feel excessively bouncy underfoot.

 

[GeorgeTheCivilEngineer]

blush What deflection criteria are most people using?

UK practise is to assume some level of base fixity. Either 20% (for serviceability deflection) or 10% (for stability).

Eurocodes has never set out any deflection criteria because they couldn't get agreement so it's all done on secondary design guides. THat might change in the later editions but IStructE or SCI guides is what I use.

 

[Greenalleycat]

The basis for 12mm - our code has L/240 but max 12mm for lintels
I've understand this to have two functions
1/ Prevent binding of doors or windows below a door as these are sensitive to the absolute deflection
2/ Limit unsightly deflections to the viewer - if we look at a lintel or whatever that has a straight line (e.g. door head) below it then our eyes can see the absolute deflection from one end to the other

A possible 3rd reason
3/ In residential you get lots of funny junctions that are not explicitly considered. The classic is the 1st joist in beside the wall...
The joist on the parallel wall has 0mm deflection, the 1st joist in (400mm away) has say 12mm...that 12mm deflection will show up in the ceilings and floors as a sharp change
Limiting the absolute deflection is crucial to preventing this being too awful

A possible 4th reason
4/ It's a bit of a fudge for people forgetting to consider effects such as stacked beams
If you have 12mm in a lintel and 12mm in a joist then you can easily get 15-20mm midspan deflection cumulative
Many times designs are done in isolation (design joist then design lintel) without considering stacked effects

@human RE the industrial girder:
If I was designing an open industrial girder I would probably not consider an absolute limit as I don't see that as important in that use
L/? criteria will be important though to prevent excessive movement/noise under wind, or ponding under snow

@George re base fixity
We have no Codified guidance on base fixity, but a professor here has done a lot of work recently and has posted similar guidance
In practice I only use this occasionally, usually if I want a specific size to work
It leads to more complexity in baseplate/hold down & foundation design, and can require a fair bit more design time
To fudge this I usually just design to 10-12mm for a 2.4m frame (L/200-240) instead of L/300 and assume that the last bit of stiffness is provided by the baseplate
A bit unscientific but I feel it's appropriate for typical residential frames

 

[GeorgeTheCivilEngineer]

@George re base fixity
We have no Codified guidance on base fixity, but a professor here has done a lot of work recently and has posted similar guidance
In practice I only use this occasionally, usually if I want a specific size to work
It leads to more complexity in baseplate/hold down & foundation design, and can require a fair bit more design time
To fudge this I usually just design to 10-12mm for a 2.4m frame (L/200-240) instead of L/300 and assume that the last bit of stiffness is provided by the baseplate
A bit unscientific but I feel it's appropriate for typical residential frames

It's a bit of fudge. SCI guidance allows normal, nominally pinned baseplates and apply base fixity for SLS cases - so no additional design required. It's worth having a squint at the particular guidance on it:

https://steelconstruction.info/images/4/45/SCI_P399.pdf

Chapter 7.4

 

[Tomfh]

I've been running on the same 1/350 limit that I've used elsewhere as rightly or wrongly that seems to be suitable

If you’re including 60% live load I suspect you’d be more than covered.

We commonly check dead plus live at L/250, and dead alone to L/360, although we tighten these up for longer spans.

I’ve had phone calls over as little as L/440 dead load deflection (a 14.5m beam). Once someone feels it’s a problem, it’s a problem. Because “beams shouldn’t sag”.