Deflection requirements for wind load, design to British standard 2

[LR11]

Hi, it's not clear to me how lateral deflection from wind load is checked when designing to British standard.

I can't seem to find guidance on what CRITERIA applies for WHICH LOAD COMBINATION, and if wind is considered a leading variable, and does the Characteristic combination apply, and when to use combination factors of ψ0=0.5 and ψ1=0.2.

What's the best source of information for this?
Let's say you have a building subject to lateral wind, what’s the criteria for deflection, and for which load combination (is it a leading variable, or which value of ψ to use, and does the Characteristic combination apply).

 

[HTURKAK]

-This statement implies your question is for Eurocodes rather than BS , right ?
-Lateral deflection check should be for SLS loading . Recommended horizontal deflection limit is L/300.
-Values of factors ψ0, ψ1, .. for buildings changes acc to Category of bldg ( A, to H )
- Pls provide more info. for the bldg to get better responds.



He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[LR11]

Thanks for explaining HTURKAK.
It’s for design to BS EN, apologies if this confused. I don’t have an example as this is a general question, consider it in relation to a braced frame or portal frame. Question is regarding lateral displacement, from wind load.

The issue I’m having difficulty working through is that deflection limit seems to be for the Characteristic load combination where wind is taken as the unfactored load, without any combination factor ψ.

The limit of H/300 you mentioned, I saw in Annex to BS EN 1993-1-1 (Design of steel structures, Part 1-1: General rules and rules for buildings), see below. This is very stringent, almost the same as the H/400 for crane supporting structures, for which the total load on the building is typically much less.

Annex to BS EN 1990:2002 (Basis of structural design), in NA.2.2.6 Clause A1.4.2 says:
• For function and damage to structural and non-structural elements (e.g. partition walls etc.) the characteristic combination (i.e. expression 6.14b of EN 1990).
• For comfort to user, use of machinery, avoiding ponding of water etc. the frequent combination (i.e. expression 6.15b of EN 1990).
• For appearance of the structure the quasi-permanent combination (i.e. expression 6.15c of EN 1990.
For the second instance (frequent combination), ψ1=0.2 applies.

Additional information from SCI:
SCI P399 (2015) Design of Steel Portal Frame Buildings to Eurocode 3
12.1.2 Existing guidance on deflection limits
The recommendations presented in Table 12.1 are taken from Reference 16. That publication noted (in 1991) that “Early feedback on this table has suggested that some of the values may be more stringent than is necessary. Pending outcome of a wider consultation on this subject the indicative numerical values given in this table should be regarded as provisional." Despite this note, the deflection limits have been accepted by many designers as the de facto standard and are unchanged in Table 12.1.

And the above is in relation to a limit of H/100 for profiled metal sheeting which is much more forgiving than H/300.


FROM: BS EN 1993-1-1 (Design of steel structures, Part 1-1: General rules and rules for buildings)

Is it the case then that the H/300 applies for the full wind load?

 

[GeorgeTheCivilEngineer]

The issue I’m having difficulty working through is that deflection limit seems to be for the Characteristic load combination where wind is taken as the unfactored load, without any combination factor ψ.

The deflection limit is for SLS when wind is the leading action, so it would not have a phi (or gamma) factor. I would also check for when wind is in combination but it is unlikely to be controlling.

The limit is stringent because walls are often constructed with brittle finishes and the dead load will not cause any significant lateral displacement. So you should be thinking this is equivalent to L/360 for live load deflection in a beam supporting a brittle finish.

Because the H/300 isn't a requirement, you can vary this significantly if there aren't brittle finishes. As you have noted, in portal framed building with purlins + cladding, this could comfortably rise to H/100. Indeed, keeping to H/300 for a normal portal frame is not economically easy! You can also utilise limited base fixity when considered SLS loads (see SCI P399 7.4.1) which dramatically decreases lateral displacement.

The IStructE guidance gives a little more background but nothing substantive.

I've never found H/300 an issue for braced frames and I once used this in a presentation for architects by demonstrating how much heavier a steel portal frame needs to be to have the same lateral stiffness as a braced frame. I don't remember the details but it was a lot.

The other thing to do is make use of the various factors in BS EN 1994-1-1 to get a more accurate (and lower) wind load, and not to assume a conservative value. Because the wind loads peak near the end of the building, but the last frame is under 50% less load than the adjacent bay (or can be braced using the gable posts), this can give savings for the rest of the structure.

 

[HTURKAK]

Deflection is checked for SLS , persistent loading case.
The combinations are as follows ;

C1 = 1.0Gk + 1.0W + 1.0ψ0Qk ( if wind leading action )

C2= 1.0Gk + 1.0QK + 1.0ψ0W ( if imposed loads leading action , and wind accompanying action )

I will suggest you to look SCI P365 the worked example .
.........


He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[LR11]

Thanks to both for the input, this is clear now.

Also the term "suggested" gives some flexibility as noted.