Domestic Portal 1

[Contraflexure74]

Hi,

See attached sketch.

I have been forced to adopt a portal design for a house project as the Architect has specified glazing in places which obstruct shear walls at the end gables and he doesn't want to see any cross bracing.

My solution involves introducing a series of portals at 4.95m centres including the gable ends.

I'm relying on the diaphragm action of the roof build up in the other direction to act as a deep wind girder in the opposite direction.

I have a moment frame along grid A as this is a fully glazed façade and again no cross bracing permitted.

Along grid B I have masonry walls built into the column webs which act as shear walls.

The loads on the frame result as per the bending moment diagram, (gravity & lateral wind loads). Dead = 0.8kN/m2, live = 0.6kN.m2 and wind = -0.5kN/m2 suction on the rafters. Lateral wind load is 1kN/m2 with 0.3kN/m2 suction acting at the same time on the opposite face.

The UB rafter is fully restrained by the roof makeup detail and deflection governs the design at 6.4m span.

The column along grid A is unrestrained for the full height but my check of combined moment & axial for the full height is less than 1.0.

My column to beam connection has been designed to withstand the moment and shear at that junction.

When I model the frame the lateral deflection at the top of the column is circa 6mm as I was able to model a nominally fixed base connection, so h/300 is satisfied.....even though the built in masonry walls along grid B should help things even further in this regard.

My question guys is do the column and beam sizes appear okay at a glance as I haven't done any portals of this scale before?

Any thoughts welcome. Apologies in advance for the long winded post and sketch.

John.

 

[KootK]

1) I think that your hand sketches and calcs are glorious.

2) I don't have a good sense for the appropriateness of your member sizes. I'm afraid that I can't help you there.

3) Personally, I would consider omitting the moment frames on grid A and designing the structure as three sided for the following reasons:

a) it'll save some money.

b) given the stiffness of the masonry shear walls and rigidity of the diaphragm, I don't see the moment frames ever seeing any appreciable load.

c) with the multiple portal frames, you've got all kinds of capacity and redundancy in your ability to deal with lateral load eccentricity.

d) I know that you practice in a low seismic zone.

You should know that there is substantial diversity of opinion among our kind when it comes to the use of three sided building lateral systems: Link.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Contraflexure74]

Thanks Kootk,

Very helpful, never thought of modelling it as a 3 sided building...brilliant.

John.