Vertical and horizontal bending of brick walls 1

[Sparkes1992]

Hi guys,

Was hoping someone could shed some light on horizontal vs vertical bending of brick walls. I'm guessing if you had a wall that's restrained too and bottom then you would probably only need to design for horizontal bending of the brickwork. When would you look at both horizontal and vertical?

 

[PengStruct]

If the wall had buttresses spaced horizontally a distance that was less than the vertical span between supports, it may be better to design the wall to span horizontally. Technically, if the ratio of vertical to horizontal span was not too high you might get some two-way action but I've never seen this done in practice or addressed in codes for un-reinforced masonry. In either case, for un-reinforced masonry flexural tension is not normally allowed and the actual allowable stresses are very low per ACI 530.

 

[hokie66]

Most codes do allow unreinforced masonry to be designed for horizontal flexure, but not for vertical. But then, some specifically residential codes, for some reason, allow unreinforced basement walls.

 

[dhengr]

Sparkes1992:
As PengStruct suggests, the wall panel will tend to span in the direction of the shorter span length, and you might design it that way. There is no doubt that there is some two-way spanning action of the wall panels too. The horiz. (bed) joints are fairly weak because they are continuous, and because bond and tensile strengths are so low. But, they have the benefit of gravity loads (DL only?) cancelling out some of the tensile stress which you are designing to prevent. Spanning from column (or pilaster) to column the joint weakness is less significant because the joints are not continuous, except in stacked bond.

 

[XR250]

When did vertical flexure become a no no? I thought it was direct tension that was not allowed but flexural tension was OK.

 

[hokie66]

XR250,

The answer is code specific. We have a lot of questions on this site about horizontal cracking in basement walls, I think mostly in the US, where that residential code must allow unreinforced basement walls.

 

[XR250]

The answer is code specific. We have a lot of questions on this site about horizontal cracking in basement walls, I think mostly in the US, where that residential code must allow unreinforced basement walls.

The residential code does allow un-reinforced masonry basement walls. But I believe ACI-530 also allows flexural tension regardless of orientation (at least it used to).

 

[jayrod12]

I'm pretty certain the CSA code allows it, and it fairly closely mimics ACI. It's just a different reduction factor applied when it's bending the other direction. If I remember correctly.

 

[PengStruct]

Yes I think ACI allows it but I'm just cautious about using it for sustained lateral loads such as at basement walls. If the stress is really low then fine but I also see a lot of bad CMU work done by small contractors.

 

[concretemasonry]

PengStructural -

The problem with some small contractors building basements is quite common, maybe because of loose specifications and/or controls. Also, the ignorance of the possible availability of CMU units. - There are many CMUs that could be used for higher loads, while the range of availability of clay brick configurations is limited.

Some smaller contractors are certainly a questionable labor source, but the good ones do superior work (in comparison to some larger contractors/builders and do a great deal of the basements because of the creativity and ability to handle small basement efficiently from a logistic and construction/quality basis. - I have seen this based on about 25 years experience as an engineer and material supplier on codes and material standards (TMS, ACI, ASTM and many local and state codes).

There are many unique units that have been used domestically and internationally for CMU construction. Unfortunately, the domestic (U.S.) engineers have not used the resources and units available in comparison to many foreign engineers/contractors/builders using ACI 530. The ACI 530 document is the basis for most international masonry designs.

Even for the lowly basement, some builders have created reliable construction methods and the ability to use the same systems and CMU for both reinforced an unreinforced. As a supplier, we had a larger home builder (200 to 400 homes per year) that had unique CMU shapes made for him. He could afford to pay a higher price for specific units that were more economical and provided a higher quality home.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[XR250]

The problem with some small contractors building basements is quite common, maybe because of loose specifications and/or controls. Also, the ignorance of the possible availability of CMU units. - There are many CMUs that could be used for higher loads, while the range of availability of clay brick configurations is limited.

Most basements in my area are unreinforced CMU if the backfill heights fall within those listed in the IRC. I usually do not see any issues with them as long as they use decent backfill and drainage. Usually, the cracks develop at the mortar joints of the CMU so I do not see how using higher strength CMU is necessary.

 

[Sparkes1992]

The Australian code AS3700 gives design guides for both verticle and horizontal bending i believe