Domestic Calculations

[DTC87]

Hi,

Can anyone offer an explanation why the end bearing support for a beam cannot be positioned over a window or opening in accordance with NHBC even if a lintel or another steel beam is designed specifically to take the point load transferred and span over the opening?

Thanks

 

[Ron]

NHBC sells warranties. They want to control the possibilities of loss, not the probabilities of loss.

Besides....not really a good idea considering long term creep deflection and the additional load that would put on a window that's not designed to take load in that plane.

 

[DTC87]

Ron,

Thanks for the reply - I appreciate and understand your feedback

However, if the steel beam was well over-engineered (particularly in terms of stiffness) to control deflection and clearance provided to the window to avoid loading it, would there be enough justification to have such a scenario? Or is it a hard and fast rule that cannot be designed out?

Thanks

 

[MIStructE_IRE]

Since every “beam” by definition spans an opening, does that mean that no one beam is allowed to carry another?!

Seems like a ridiculous requirement to me... Then again, I’m in Ireland and we don’t have any such restriction.

 

[XR250]

Sounds like a bunch of nonsense although probably precipitated by tract builders forcing their engineers to meet minimum code - and the fact that LVL's notoriously sag more than the numbers would indicate.
I try to keep every beam and header to at least triple the minimum code when practical.

 

[BridgeSmith]

If there is an engineering reason for the requirement, I would guess that it has to do with deflection. A header across a window frame will deflect more the a column, even if designed adequate for the load.

I'm not familiar with the NHBC, or even what it is, but many minimum code requirements are set for details that are not typically designed or engineered. My understanding is that, especially in residential construction, much of the framing is fairly standardized and based on typical configurations. As long as the configuration stays within the boundaries of typical construction, little, if any, engineering design is required. All that to say, are you sure the restriction applies to a situation where an engineered design is performed, or only to those where a design is not performed?

 

[raligt]

Never heard of this NHBC requirement and I have done this detail a few times. Maybe design to span/500 for your supporting beam if you are worried?

 

[DTC87]

All,

Thanks for the responses. The NHBC requirement is in the following link - please also see the attached screen shot.

http://nhbccampaigns.co.uk/landingpages/techzone/previous_versions/2011/Part6/section5/appendix.htm

Totally agree that I think it must be to do with deflection and probably because at these locations (windows / openings) typically only lintels are provided - therefore the NHBC prefers to avoid these scenarios. However, I think if a beam is suitably designed, as per any beam that spans across an opening, then it should be ok to support over an opening - particularly designing in extra margin on the deflection.

 

[CANPRO]

Agree with others that this shouldn't be a problem, I've done it many times myself. But to play devil's advocate here: you can design the lintel for a tight deflection limit, but you don't have any control over how the window is installed - if they install the window tight between the header and the sill, you're going to have a load bearing window.

 

[BAretired]

NHBC is an acronym for National House Building Council in the UK.

BA

 

[charliealphabravo]

It looks to me that the restriction applies to the prescriptive beam design tables in the standard, so in that case it is what it is.
I don't see it as limiting engineered design since there is language allowing alternative methods in the beginning of the standard.
Just fyi though, I'm not familiar with the standard.

Edit: Also there is no similar restriction for wood frame walls. I only see it under an appendix in reference to prescriptive steel beams supporting masonry partitions and bearing on exterior masonry walls.

It just looks like a prescriptive provision to me. Not a restriction on engineered design.

 

[Ron]

DTC87....you are dealing with a contractor and warranty organization...they have little, if any, knowledge of engineering. If they are not codified and mandated, ignore them. Most building codes allow the rational analysis of conditions that deviate from prescriptive requirements of the code (at least in the US, that is allowed). Understanding that you are in the UK, it might be a bit different; however, logical engineering proof should prevail.

One reason for limitations such as this is the variation in what different engineers would find acceptable. Some engineers would allow L/300 deflection for such applications, while others might require L/500.