Unreinforced Masonry Building Upgrade

[KayneR]

Hi there,

We have a client wanting to seismically upgrade their unreinforced masonry building.
The building is a shop so has long shear walls on 3 of the sides and is open along the front.
We propose that we put in a large stiff portal frame along the front of the building to give it lateral stiffness.
We designed the portal based on a 0.4s period. (based on AS/NZS1170.5 this will yield a conservative result as the loads are at a maximum until this point).
The deflection of the portal at the ultimate EQ load is such that it will not exceed half the thickness of the wall. (over half the wall thickness will likely lead to failure of the wall)
But using Rayleighs formula for period, T = (∑▒〖(Fidi)〗^2 )/(g∑▒(Widi)^2 ). F is the force at level I, d is the displacement at level I, W is the weight of the structure at level I and g is gravity.
We end up with periods of the structure exceeding 1.3 seconds (for a two storey URM building this seems excessive).

We can connect all the out of plane wall loads to the portal and the rear wall.

Does anyone know of any research about retrofitting steel inside URM buildings and the resultant period? or can anyone point out flaws in my logic so that I can learn from this?

Keen to hear what people have done in the past also.

 

[KayneR]

Additional reasons for me asking this question.
If the period of the structure is indeed 1.3 seconds then can we reduce the design loads on the portal and subsequently reduce it's size?
I am unsure if other countries codes are like ours but structures with a period greater than 0.4s the demand on the structure reduces exponentially in the code.

 

[dik]

I suspect the building will go into a torsional mode, and, am surprised the period is so slow high given the stiffness. Is it possible to grout portions of the wall?

Dik

 

[KayneR]

Thank you for your quick response Dik.
if we grout the walls that will increase their stiffness but would the resultant loads into the frame still be the same?
it's the deflection of the portal that seems to give the unrealistic period.
we could put the portal legs in concrete to increase the stiffness and reduce the displacement.

the torsion thing is something that is bothering me with this building. Because if we want the front frame to have an equivalent stiffness to the rear URM wall then we need very large steel portals that have a capacity/demand in the order of 8 - 9.

 

[dik]

You'll have great difficulty making a portal frame as stiff as a solid long URM wall... and will likely end up with a significant torsional component anyway. Is the area high seismic? What type of soils is it founded in and what is the foundation? I was thinking that you have lots of masonry area to resist shear and using the grouting to tie things together a bit.

Dik

 

[Toby43]

Tricky one. Would like to see where this goes..
Problem with Torsion? Does it cause failure of gravity system?
Single storey? what is the roof construction? If flexible it may control seismic response - See Fema P-1026 -- Anchorage provisions of masonry wall to flexible roof may be difficult, particularly for unreinforced masonry. Provisions of Fema P-1026 may not even be valid considering we don't have "typical diaphragms" here in Australia.
If concrete roof (carpark over maybe) then frictional transfer between concrete and walls?
No option for squat shear wall at front I guess?

Sorry KayneR, just posted a bunch more questions without been overly helpful. Maybe someone with more personal experience with retrofitting these structures may chime in and provide better guidance.
My opinion is that unreinforced masonry is likely to always pose a high seismic risk, despite well intended and thorough retrofit methods. Others may disagree

Toby

 

[dik]

That's why I was thinking of 'gluing' the parts together with grouted 'columns' every so often...

Dik

 

[KayneR]

Dik, we are in a "medium" seismic zone by New Zealand standards. Basically where the building is we take 1/6 of the weight of the building acting laterally in an EQ.
The building is founded on solid ground with effectively no chance of liquefaction.

Toby43, Presently the walls that will be subjected to the out of plane loads in the direction of the portal are supporting the gravity structure. The timber joists actually go right through the wall and even tie into the neighbouring building (but we can address that later, just looking at the idea of change in period)
This one is a two storey building with sufficient URM walls all around on the first floor level.
the roof is iron cladding and timber framing (not typical trusses).
The option for a squat shear wall is not a favoured one by building owners. I have seen in the past people specifying steel portals to strengthen these buildings. But they said they just designed for the lower period and therefore higher forces to be conservative and didn't really consider the period.

What we are thinking of doing is the portal frame at the front and put timber diaphragms at the two levels. We have an article in a SESoc journal here that allows for provisions for designing the diaphragm for a URM building. It even allows for provisions for a tounge and groove diaphragm (essentially what was put in buildings in the early - mid 20th century) which I don't know how comfortable I would feel relying on potentially 100 year old timber diaphragm and 100 year old fixings. I would rather put ply over top and use a new nailing schedule.