Top course cmu hold down

[asixth]

Hi guys

Is there any way to hold down the top course of a cmu wall. I was anticipating anchoring into the top course as shown.

If you see the attached sketch, I was hoping to provide a U-bar which loops thru the H-block recess of the top course and anchors down into the third course with an plate welded to the end of the reinforcement. Of course, this would create an issue with cover to the U-bar and also grout-filling the cmu would be difficult because of the plate welded to the end of the reinforcement.

My other option would be to provide a raked course to the underside of the roof sheeting so horizontal reinforcement for the second course could be placed with sufficient cover.

 

[concretemasonry]

I am a little at a disadvantage without any dimensions on the drawing, so have to rely on the apparent dimensions.

Why is the beam on the left of the wall supported differently than the right side. Eliminating the "purlin cleat" would make the detail cleaner and provide symetrical loading. There may also be enough room to add a course of block and make that a bond beam and reduce the potential bending problem of an angle supported at the very bottom and resisting an eccentric load.

In the left side, the extra anchored steel member seems to be supporting only a suspended ceiling and could be eliminated if the portion was hung from the left beams.

You have relatively close spacing of vertical reinforcement, but the height and loading may require something like that.

If you use the correct shape masonry units it is only necessary to grout the cells with reinforcement since the extraneous or extra grout does really not provide a benefit and may be detrimental in many physical and cost situations.

Dick

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

 

[msquared48]

There should be a couple of #4 horizontal bars in the top boond beam. The verticals could hook over these bars and tie to the courses below. OR...

You could periodically use #4 or 5 vertical threaded rebar with couplers that could extend down as far as you need, still with a hook at the top to catch the horizontals in the top bond beam.

Is it all along the top of the wall that you need the vertical holddowns, or at selected locations?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[hokie66]

asixth,

Is there a reason for not taking the purlins over the wall? The roof sheeting needs support in that 200 dimension.

I like to use threaded reinforcing bars or all-thread rods, developed into say the third course down. These tie down a continuous angle with the purlin cleats. If you are required to take the block all the way up to the sheeting for fire purposes, something like your detail works best.

 

[asixth]

concretemasonry,

The beam on the left is different to the beam on the right because it needs to cantilever out from the wall. That is the reason that I have gone with a channel opposed to an angle. It also has a lower ceiling height so I can anchor that beam into the second course from the top. Is the top course generally a bond beam. If the top course is a bond beam block then I can pass horizontal reinforcement above the anchor which should fix my hold down problems.

In the left side, the extra anchored steel member seems to be supporting only a suspended ceiling and could be eliminated if the portion was hung from the left beams.

Not sure what you mean by this.

Thanks for your input on grouting cells. Generally I have seen all vertical cells grouted so I will raise this issue with my colleagues to see if we can eliminate unnecessary grout filling of unreinforced vertical cells.

msquared48,

thanks for the thoughts, I will consider hooking the vertical reinforcement over the horizontal reinforcement in the top course (bond beam). I will need to check the minimum radius for a #4 to see whether it can fit in the cell of a bond beam.

The hold down is required at every purlin location (2'/600mm) because the roof is located in close proximity to a windward wall.

hokie66,

I have not considered whether the block needs to be taken to the underside of the roof sheet for fire reasons but this would definitely make sense if it does. If this is the reason, then I should probably have another raked course on top of the top course currently shown. How do I determine whether the wall needs to be taken to the underside of the sheet for fire purposes. The architect on this job is a junior and has no idea.

Are you saying that you provide the purlin cleat of the top of the block walls like I have shown in the attached (without the continuous angle or purlin cleats).

Thanks for the replies.

 

[rowingengineer]

Asixth,
The BCA takes care of this; it states somewhere that a fire wall must extend to the underside of a non-combustible roof sheeting.

I would run with an idea similar to hokies, bolting down a 250pfc at 1200 centers, if the wall is not a fire wall.


An expert is a man who has made all the mistakes which can be made in a very narrow field

 

[OzEng80]

Asixth

Further to RE's comment (ie not fire rated wall) the cantilever beam could be placed on top of the wall (HD bolts through bottom flange) eliminating the angle, very long purlin cleats?, & the masonry anchors. Pretty common detail for avoiding exposed beams to eaves but given your lower external ceiling profile this may not be an advantage. This also concentrates the load into specific cores (HD bolt locations) allowing you to provide additional vertical reinforcing at these points if required (no bending in bondbeam).

 

[hokie66]

asixth,

Yes, that is correct, on top of the wall. RE and OzEng's approach will work as well, provided you can deal with the roof slope. As to the architect's involvement, it is his job to determine fire rating requirements, so insist he do it.

In this case, I would stick with grouting all cores. Australian builders expect to use H-blocks or knockout blocks, allowing grout to flow freely. I know this is not US practice, but we do a lot of things differently.

 

[dik]

Can you not just bear the channels on the masonry wall? or weld an angle with the leg away from the channel. It might be a lot easier.

There was a building in Lindsay on the top of a hill, that the wind blew off the roof several times before they secured the top coarse to the rest of the wall... took them three times before they got it right.

Dik

 

[hokie66]

dik,

Don't know where Lindsay is, but that is what asixth is trying to prevent. And yes, the suggestions are that he put a channel on top of the wall, fire provisions permitting.

 

[dik]

Easy to use firestopping... I fixed the building the last time by grouting anchors 4' long into grout... they had to replace the top bond course, anyway and it was easy stuffing 'bag' fragments into the voids and rodding them down to the 4' level.

Dik

 

[asixth]

The problem that I have is that I am trying to ensure the blockwall is tied down at the position of the anchor. If I specify that the anchor for the purlins is 300mm (12") from the top course. This will only develop 200mm max. of the No.4 (12mm dia) vertical bar. Would this be enough bond strength to assume that the vertical reinforcement can act as "tie-down" for the blockwall. I do not need the vertical reinforcement to develop full tensile strength to resist uplift, only 30MPa (4.4ksi) so the minimum lap length of 150mm (6") will be sufficient to develop this stress in the reinforcement.

I also noticed something on-site today, they where laying the H-blocks upside down as I got shown in the original sketch. This meant that the recess for the horizontal reinforcement is above the bed-joint. Is this normal construction practice?

Dik,

Where is Lindsay? I entered it into google maps and a search result came up south of Oklahoma City so I wouldn't be surprised if the roof kept blowing off.

 

[hokie66]

I assume you are referring to your original sketch. I think the left side is fine, but the right side lacks purchase.

It doesn't matter whether the bars are below or above the joint.

 

[asixth]

I don't think the original sketch has enough purchase on the right hand side either, I am going to make sure all anchors have a minimum 300mm (12") from the top of wall.

 

[dik]

Lindsay is north of Oshawa, Ontario... Building was wide open and on the top of a hill...

You should be able to anchor the bearing into the top one or two courses and design the bond beam to accommodate 10M/15M (#4/#5) as required.

Dik

 

[hokie66]

asixth,
That looks good to me. But does that sheeting rib over the wall have to be screwed down? The other issue with this type detail is what happens with roof bracing at this point, if there is any.

 

[OzEng80]

asixth

It is conventional to hook the wall reinforcing over the bondbeam which will reduce development lengths. Make sure the masonry anchors are specified to miss each other and the wall reinforcing.

 

[asixth]

Ozeng,

Do you have a typical detail of this that you can send through.