Masonry Pilaster

[jtwright4216]

I was wondering if someone could help me out or point me in the right direction. I am designing a 30'-0" tall cmu masonry wall with 32"x16" pilasters at 16'-0" o.c. The code that I am using is the IBC 2009. The building is a Rec Center with dimension of 80'-0"x50'-0". The glulam girders are spaced at 16'-0" and rest directly on the pilasters. There is only 2'-8" of wall solid wall on each side of the pilasters until openings are present (Windows and doors). The wind speed is not very high, only around 20 psf on the wall. Seismic forces are very low also where this building is going.

I read the NCMA TEK manual (wind loading) and you check the wall to span horizontally between the pilasters then design the pilasters for the plf. But what about fixity of the pilaster and wall? The Tek manual goes into a combination of different ways to resit the load. In my case I have a flexible diaphragm typical continuous footing and would consider myself pin-pin in the out-of-plane direction.

Your looking at 20psf *16'-0" = 320 plf (vertical) on the pilasters. Then of course you have like 35k axial load. That will give us final reactions of 40 downward and 5.3k out of plane (pin-pin). So, then the wall will have horizontal joint reinforcing to transfer loads to pilasters (designed out-of plane horizontally pin-pin). Which worked out to be #4 @16" o.c. And this building will be all fully grouted.

However, should I introduce either bond beams or tie beams at the mid-height and the top of the wall. What is the difference between a bond beam and tie beam anyway? That way the reinforcement can fit in the bond beams. So the new load path in the out-of-plane direction would be vertical span of the wall to the tie beams to the masonry pilasters as point loads. But what about the in-plane direction?

Have we completely eliminated the shearwalls all together? Or do they still resist some of the load. Are we officially counting on the pilaster to take all the load? So, Fixed at the bottom and free at the top? Or with the flexible diaphragm fixed at the bottom-pin at the top.

What I am seeing is the tie beams connected to the pilasters acting as a moment frame not shearwalls. The walls in between are strictly for components and cladding. Therefore, in seismic design what is the resisting system? Are we still looking at special sherawalls or is it now a moment frame system.

Let me know what you guys think, thanks I appreciate the help.

-Jake

 

[BAretired]

Depends on the location of wall openings.

BA

 

[jtwright4216]

Attached is a Wall Elevations showing the opening locations.

 

[hokie66]

Fully grouted, just design the walls between the pilasters as slabs spanning horizontally, with some openings which need to be trimmed around. It sounds like that is what you have with #4 @ 16". Don't worry about the terminology of bond beams, tie beams, etc....the wall acts as a slab.

In the plane of the wall, the wall acts as a shear wall. The connection of the roof "diaphragm" to the pilasters and/or walls has to transmit the forces.

How are the loads at the roof level resisted? What do you mean by "flexible diaphragm" in this case?

 

[BAretired]

The lower half of the South Elevation needs to be designed on the basis of element stiffness. The upper half and the North Elevation should act more or less like a shearwall.

Continuous bond beams above and below windows would seem to be a good idea.

BA

 

[jtwright4216]

The roof will be typical plywood sheathed with rafters running perp. over intermediate glulams. Eveything will be broken into sub-diaphragms and the force from the roof will be transmitted into the masonry pilasters. And connected to resist the out of plane controlling force.

So, that makes sense, basically it is acting like a slab. And I designed for a one-foot strip horizontally.

I think the in-plane direction confuses me a bit because without the pilasters you really cannot have a shearwall 30'-0" tall in this case because it is too slender to span the entire height. Therefore, we needed to introduce pilasters.

So, what your saying is the wall, in-plane, will act as a shearwall even with the pilasters. But based off stiffness won't the pilasters see a majority of the load? And also if you were to look at typical uplift forces at the ends of the shearwalls, in this case, would it be between the pilasters or the entire wall? Just Moment/width of wall. Basically, would you distribute the shear force evenly to the pilasters or design the whole wall for the entire wall for the total load?

Hopefully, I am not confusing you guys. I appreciate your help.

 

[hokie66]

In plane, the stiffness of the wall will be much greater than the stiffness of the individual pilasters. The pilasters just act to stiffen the wall out of plane...similar to the way your rafters will stiffen the roof diaphragm out of plane.

 

[jtwright4216]

So, basically (in-plane) is just one big masonry shearwall design. The pilaster main purpose was just for out-of plane forces. The shearwall will be designed as a 30'-0" tall wall with openings, where of course you need lintels, bond beams and jamb reinforcement as needed, Correct?

 

[hokie66]

Yes, you have to reinforce around the openings. Try to forget that it is a masonry wall...just think of it as reinforced concrete, and do the same type reinforcement. Watch out for control joints...standard specifications often require them at certain spacings. Just don't design the wall one way and find out that the mason has placed control joints where you don't want them.

 

[jtwright4216]

Thanks Hokie66, I think I understand everything now. I appreciate the help.

-Jake