6.5m Goal post RSJ

[tony1851]

They mention wind loading on the side of the (enlarged) house. This is often the engineer's justification for specifying moment frames (aka goalposts); their reasoning is that the bending moment caused by the wind load is resisted by the strength of the frame.
But this omits one important source of resistance to the lateral wind load, which is the presence of those walls remaining in the house and which lie parallel to the wind.
They can be internal walls and external walls, particularly the front- and rear elevations. All these walls have what's called a 'moment of resistance' which, when added together, can often exceed the moment applied by the wind.
In many cases, and depending on the plan-form of the property, frames are not needed when removing the rear wall - often just a beam on its own, which is of course a lot cheaper.

(PS 'moment' seems an odd word - when i first heard of that in our secondary school physics lessons i though it was something to do with time! But in structural terms, its simply a force multiplied by a distance}.

 

[stevie888]

Thanks very much for the input. I challenged the SEs, to which they replied:
"I understand the big beam queries; this is quite an unusual section we use. We were surprised, as you were, but we have a big house here with the load area not only from the main house but also a big wind area to withstand from the first-floor extension (in most cases we're dealing with only one storey extension which gives us a much smaller wind area). Also, the wind load is quite big. With the current dimensions, we're not able to reduce the beam section. On the other hand, we can recheck the structure if you could tell us what the exact column height on site is. Having a smaller column, we would be able to reduce the beam section, I think. Another idea to make the connection stronger is creating haunches, but this will stick out in the room and probably not be a desired solution."

So, I'm going to measure up this weekend and revert back with a more accurate column height, although I'm not feeling confident.

If the column height is critical to their calc as they imply then why don't they know it???

The type of connection I showed scarcely sticks out very much. It would be interesting to see their connection detail.

 

[Boney25]

They had considered one internal wall which is directly in line with the goal post when calculating the wind load. Which reduced the load by 23%.

Should they have been able to take benefit from the other walls which are parallel to the beam too?

This is the connection design:

 

[stevie888]

That joint detail doesn't make a lot of sense to me: they say they need the beam depth but then put the bottom bolts way higher than they need be greatly reducing their effect.

 

[tony1851]

Re the wind loading: IMO their calculation, such as it is, is too simplistic. Its more realistic to consider the whole of the side of the building and add up all the resistance moments of all walls and wall panels, internal and external, throughout the ground floor.
The bedroom floors act as rigid plates and, in small-scale domestic buildings, serve to spread the moment applied by the wind over the whole of the structure. The same applies for the roof and 1st floor ceiling. The gravity load of the first floor structure also serves to increase the resistance moment of the ground floor walls.

On a separate issue, their calculation seems odd. They've worked out the wind force on their assumed wall area in line with the frame, without working out the moment it applies to the structure.
And for wall panel RP1, all they appear to have done is work out the area of that wall, and yet express that in kN? This doesn't give the moment of resistance of the panel.
Unless I'm missing something, I can't follow it.
At this stage, you're probably better off just going on with their frame, it certainly wont fall down.