Replacement of a shear-wall with a steel-frame

[As-Lag]

I want to remove the ground-floor section of a rear-elevation wall of a two-storey house and fit some bi-fold doors. Most of the wall is going so I need to prevent the building from swaying by using a moment-resisting steel-frame or goal-posts. The problem I am having is on how to assess the forces.

My initial assumption is to assess the structural stiffness of the masonry wall as it is now and to size the steel-frame with a similar stiffness. For this, I would need to be able to calculate the racking-resistance of the wall. Or am I wasting my time.

I would be grateful for any advice please.

Regards

 

[phamENG]

Wasting your time. No way is a steel frame ever going to come near the lateral stiffness of a masonry wall.

You have to analyze it like you would any structure - figure out the loads (dead, live, snow, wind, seismic, etc.), apply them to the building as you want it to be, determine the load effects in the members and connections, and size them accordingly.

 

[As-Lag]

I am fine calculating the moments and deflections, its the lateral force that is confusing me. If I apply wind to the end of the building, say a row of terraced houses and the wall being removed is in the middle terrace, I am unable to picture the sway.

 

[phamENG]

It's all about the load path. Apply the lateral load to the walls. They have reactions top and bottom at the floor diaphragms. The floor diaphragms then have in plane shear and moment. The shear is resisted at their "supports" which are the shear walls - or moment frames in your case. You have to decide if the diaphragm can be idealized as flexible or rigid, or if you need to consider diaphragm stiffness in the load distribution to the LFRS (this is often driven by the building code). That will let you apply the lateral load to your frame, determine moments, shears, and axial load in the members, and design accordingly.

If you're still not sure what to do, you should sit down with an engineer in your office with experience in this kind of design and work through it. If you have specific questions about a particular step of the process, post some pictures/sketches.

 

[patswfc]

For a mid-terrace house you are correct that it wont sway as the houses at each end will take the lateral wind loads. However you shouldnt rely on these other houses, thus should treat the mid-terrace as a separate building that is able to look after its self.

 

[As-Lag]

Dear All

I am doing a Masonry Seminar on Thursday and I will get back to you if I am able to answer my own question.

phamENG. Wish I did have an Engineer to talk to. That is why I am on this forum. Although I have an honours degree and Masters in Civil Engineering, I have never worked in an office or had a Mentor. For 35 years, I have either enroled on courses for education and in later years, by self-directed learning. I am grateful to all the checking Engineers that have given me advise over the years. Wish I had had some office experience though. Always feel that I am 'winging' it but in 35 years none of my work has been rejected or fallen down. Never know though.

Regards

 

[Enable]

That's alright As-Lag. I have been surrounded by engineers throughout my entire life and also no mentors so you are in good company (or at least mine)!

What phamENG is trying to get at (I think) is that your lateral resistance issue is a one very similar to designing the wall anew. Like his comment above: you have lateral loads applied at the floor elevation (either from wind or seismic) and those get transferred to your wall on the basis of tributary area (in the case of a flexible diaphragm) or by relative stiffness (in the case of a rigid diaphragm). You need to assume which one you have. See this thread as an example.

However, in your case you also have an interesting quandary which is that you're introducing a new system that will not posses identical stiffness in all planes as the original (actually pretty much guaranteed to be less), and while you may be able to proportion it for the loads of the original masonry wall, the adjoining masonry may take more of the load than previous due to its increased relative stiffness. You need to consider this to make sure you don't cause an overload of the existing!

Moreover, your connections from the steel moment frame to the adjoining masonry are probably the tallest task and need a careful consideration of how you intend to engage the masonry wall beyond the connection location (i.e. want to make sure that the moment frame and adjoining masonry acts together(ish)).

Put some numbers and sketches together and come back if you need help. Good luck!