BS 5628-3:2005 Clause 5.2.4 When to use?

[patswfc]

When is it appropriate to use Clause 5.2.4 and figure 6 from BS 5628-3:2005, which relates to internal masonry walls that dont support imposed load.

Depending on the size and tickness of the wall, figure 6 tells you what restraint is needed to make the wall stable. However Cl.5.2.4 states that factors affecting stability should be accounted for in the design. This includes wind load.

Buildings have internal wind pressures thus internal walls will be subject to some lateral wind loading. Therefore do you have to check the wall as a panel subject to wind loading.

I know that an internal wall is not going to see much lateral load but it could get a bit every now an then. Its very unlikely that it will see 0.5q, where 0.5 is the net pressure coefficient for internal walls as given in the wind load code.

What do other engineers do in this case? Do you just satisfy the requirements of this clause or do you design it as a panel with some nominal lateral load? If you do this what coefficient do you apply to the dynamic pressure q? Or do you design it as a panel for a wind load of 0.5q?

Thanks for any comments

 

[csd72]

The US codes have a minimum 5psf (about 0.25kN/sqm). Australian codes have a similar minimum value. Check that the british code doesnt also have a minimum.

If the room is not completely sealed off from the rest of the building then it will never have full internal pressure applied to it, I would say half of the internal pressure would be reasonable.

Also, is this really a structural issue? Ask your senior engineer if this is something you really should be involved in.

 

[pba]

This is what I do:

Determine by "reasoned" argument if internal wind forces need to be considered. - To be discussed later.
Determine if there are any other loads or conditions which need to be explicitly considered.

If I can argue that there is no particular load on the internal wall then fig 6 may be used. If there are loads then a design is needed and the max panel size is usually shown to be smaller than allowed by fig 6.

Now as I see it, there are various ways in which internal walls could see load.

Our usual assumption is that all faces of buildings have equal permeability giving internal pressures coeffs of 0.2 or 0.3. Now the whole internal space will tend towards one or the other. It is not reasonable to assume that there can be circumstances where the 0.2 and 0.3 values act simultaneously on opposite faces of the wall. Usually, if I've decided that wall premeability is so low that wind force is resisted by the wall I used the coeff 0.3 to determine the force on the wall.

Now if the building has variable premeability over its faces, the internal pressures from some wind directions can be increased and it seems reasonable to assume that internal walls do have differential pressures and a coeff of 0.5 becomes more resonable.

If there are dominant openings, then the internal walls are almost certain to have low relative permeability and this can give rise to very high internal pressures.

Also consider the type of building and its proposed use. The consequences of damage should also be considered so we would tend to be more conservative in places of public assembly than in domestic scale buildings...

Of course this is all subjective and it really does come down to engineering judgement - So judge correctly!

 

[patswfc]

Thanks for the replies.

I find this clause confusing as i dont see a case when it can ever be applied. I think that all internal walls can be subject to some lateral wind loading thus they should be treated as spanning elements and designed accordingly, not sized by rule of thumb guidance as given in this clause.

The reason why i ask is because i work in a small structural section with only one senior engineer and he uses this clause all the time without considering wind loads. I queried him about it and he told me that internal walls in buildings never see or see very little wind loading. He argued that these walls will definately not see the full dynamic pressure q, times the appropriate coefficient (may it be 0.3 or the max of 0.5) as doors and windows will be closed during storms, which is what q is based on. I dont agree with this as even when doors and windows are closed the external walls are not impermeable thus air can still pass into the inside of the building generating internal pressures.

I think that for my own piece of mind i will always design internal walls for either 0.3q or 0.5q depending on which is appropriate for the wall in question.

 

[pba]

Designing for 0.3 or 0.5 whan you don't need to is likely to be highly conservative and is certainly not what your client's want or expect.

Nowadays there are not that many buildings with internal masonry walls, but lets consider an office building with small offices either side of a central corridor. When the wind blows during a design storm you assume that all windows and external doors are closed. There is some permeability through facades and roof so the windward face pressure could give some internal pressure and the leeward and roof suctions could give some internal nagative pressure. Which condition you get depends on relative permeability and the wind code gives you alternate values. Now the permeability through a wall with all windows and doors closed has to be small, especially with current building regulations. The internal walls will have much higher permability. Mostly they will be non-load bearing and have a badly filled gap at the head. There will be internal doors, which are substantially more permeable than the external type so even with the doors closed the internal walls present little obstacle to equalisation of internal pressures.

This is covered by BS6399-2 Look at clause 2.6.1.1 - This is the 'normal' condition. All internal areas subjected to the same uniform pressure, so no force on the interanl walls. Note that you only need three times the external permeability for the interanl walls. Considering doors only,that might be a 3mm gap at threshold rather than a 1mm on the external door.
2.6.1.2 - This is the abnormal condition. You can look at Nick Cook's book for his method if you wish... Generally if this clause clearly relates to you use 0.5. As I said in my previous response, there are circumstances where I provide justification to use 0.3. Generally these are when I know 2.6.1.1 should apply but have difficulty accepting it.

 

[csd72]

patswfc,

Just to put things into perspective.

In Australia the winds are typically higher than the UK. Internal non-loadbearing walls are not usually documented by structural engineers and I have not heard of any problems as a result of it.

The line has to be drawn short of designing every single nail and screw, different engineers have different ideas on exactly where that line lies.

 

[patswfc]

Thanks for your help. I understand it better now thanks to yous.

I know that once your inside a building you never really fell any wind so why would the internal walls need to be designed for a high wind load of say 0.5q. Thus i have changed my mind and am now willing use this clause when appropriate.

 

[patswfc]

Forgot to say in the last post that i like the idea of a minimum lateral load on an internal wall, but this is not a requirement in the UK as far as i am aware. PBA correct me if thats wrong.
A minimum level of load will provide some extra robustness but thinking about it i suppose that providing restraints as required for fig. 6 probably does this anyway.

 

[pba]

To the best of my knowledge, there is no minimum lateral force requirement in UK. The provisions of table 6 should provide some degree of structural adequacy, but I've never checked to find out what force such restraint conditions could resist. If anyone who has the code has some free time, maybe they could enlighten us? I guess it depends on material but internal walls in UK are typically AAC blocks in mortar type iii, for domestic scale and medium dense concrete with similar mortar on larger projects.
patswfc - Does that square with your conditions?