Anchors into grouted CMU

[I-beam]

Help
I need to make a connection to a fully grouted CMU wall (8"t). I have a load of 5000# to support and am trying to use
the Hilti Profis software to design the anchors. I am hoping to fasten a L8x4x3/8 x 8" seat to the CMU with(4) 3/4" exp. or adhesive anchors placed on a 5.5" sq. pattern.
I am having great difficulty in getting the anchors to provide enough capacity due to anchor spacing. It doesn't make sense to me that a single 3/4" anchor can support over 5k in concrete,
but (4) 3/4" anchors into fully grouted CMU block can't.

This is for a steel framed stair support header.

Thanks in advance for any feed back

 

[sbisteel]

You need a bearing plate set in a grouted masonry pocket. Post installed anchors in a CMU wall don't have much capacity, before you even get into the issue of properly locating and spacing the bolts so they don't land on a joint.

 

[jayrod12]

Agreed with the above. Often post-installed connections into masonry require extremely large plates to get the bolt spacings to 8" square. Then at least you aren't penalized as much by the spacing factors, and it gives you a shot of actually lining them up with the cores.

 

[JoshPlumSE]

Have you thought about through bolting it? That's the solution I've used in the past when faced with similar limitations.

 

[I-beam]

Thank you for the feed back. Does anyone no why there are such stringent requirements
on bolt spacing and load capacity limitations, even when using epoxy anchors, only loaded in shear, and yet those limitations
would largely be ignored if using a thru-bolt ?

 

[RabitPete]

It is how it is. CMU is weaker than concrete, lower compressive strength etc. Mortar adhesion is marginal to non existent, so joints are much weaker in tension vs concrete. Basically you have edge effect around every block. Even when fully grouted, only cores are grouted, and not the spacing between the blocks. Many times there are chunks of mortar inside which prevent proper grouting, unless contactor cares to remove (have not seen one doing it yet). That's why max lift height for grouting is restricted and higher lifts require cleanups at the bottom. And most contractors tend not to follow unless strictly enforced with inspections.

 

[I-beam]

Thank you RabitPete - that makes a lot of sense

 

[dold]

Id agree with all of the above. One way to squeeze every bit of capacity out of it is to make the top 2 holes a vertical slot such that they are in pure tension (assuming you are also applying some amount of eccentricity to the fixture). Then the bottom 2 holes are taking the full shear load (idealized ever so slightly). I think profis actually has a template for this situation.