Shtraba or sandwich: tying a block wall into a concrete column, and how it goes into the BOQ
A shtraba (שטראבה, also spelled שטרבה) is the Israeli site term for block-to-column toothing: a new block wall is tied into a concrete column or an existing wall with steel dowels and a strip of concrete cast for the full height of the wall. Dowels are drilled into the column, a gap of about ten centimetres is left between the column and the blocks, and concrete is cast into it, so that the column, the strip and the wall work as one unit. I have experience in quantity takeoff for structural frame works and blockwork, and connections like these are among the items most easily forgotten in a bill of quantities (BOQ).
Why block-to-column toothing is used
Concrete and blocks are different materials: they shrink, expand and move in different ways. When the blocks are only butted against the column, the joint between them is a weak line, and over time cracks tend to appear in the plaster along it. The concrete strip with the dowels ties the materials together and reduces the risk of a crack at the joint. The same method is also used to close an opening in an existing wall, for example a door that is being removed.
In a typical residential building these connections appear wherever a block wall meets a column, a concrete stair-core wall or the wall of a protected space (the reinforced safe rooms of Israeli buildings). In extensions and renovations they also appear where a new wall meets the old building. So it pays to count them at the BOQ stage, from the floor plan, rather than discover them on site.
Shtraba against the sandwich method and simple butting
Not every contractor works with a shtraba. On site you meet several solutions, and their names are not consistent:
| Method | What is done | What to ask for in the BOQ |
|---|---|---|
| Shtraba | Dowels drilled into the column, a gap of about ten centimetres, a vertical pour | Dowel diameter, spacing, strip width and concrete grade |
| Butting with mortar | The blocks are butted against the column with mortar, with no strip | Whether it is allowed, and in which walls |
| Bars and blocks | Steel bars between the block courses, anchored to the column | Vertical spacing of the bars, per the structural engineer's detail |
| Sandwich method | The name is used on different sites for different solutions | A written detail and a drawing, not just the name |
Because the names change from site to site, a word is not enough in the BOQ. Write the detail itself, or refer to the detail in the structural drawings, so that all the bidders price the same work.
How block-to-column toothing is measured in the BOQ
In many BOQs it is a separate item, measured per linear metre of connection, that is, by the height of the connection, and sometimes by number. In others it is included in the price of the block wall, with a sentence such as "the price includes connection to columns and existing walls". Both ways are legitimate. What does not work is a BOQ that says nothing, and then the connection comes back as a claim for an extra.
When the item is separate, the description should include everything that affects the price: the diameter and type of the dowels, the anchorage depth in the column, the vertical spacing, the strip width, the concrete grade, whether the formwork is included, and whether mesh over the joint is included or measured separately. A description like this lets you compare quotes on the same basis.
An example for a typical floor, for illustration: 24 connection points between block walls and columns, each 2.8 m high.
| Item | Calculation | Quantity per floor |
|---|---|---|
| Connection length | 24 times 2.8 | 67.2 m |
| Dowels every 40 cm up the height | 2.8 divided by 0.4 is 7 per connection, times 24 | 168 dowels |
| Concrete in a 10 by 20 cm strip | 0.1 times 0.2 times 2.8 is 0.056 m³, times 24 | about 1.34 m³ |
| In a 6-storey building | times 6 | about 403 m, 1,008 dowels, about 8 m³ |
In short: column → drilled dowels → a concrete strip about ten centimetres wide → the block wall → plaster, and sometimes mesh over the joint, as specified. The amount of concrete is small, but there is drilling, fixing the dowels with adhesive, narrow formwork and pouring by hand, so the price per metre is high relative to the volume.
What to check on site before the pour
- Dowels: diameter, drilling depth and vertical spacing as in the detail, and clean holes before bonding.
- An even gap: the strip width stays the same over the full height, with no narrow points the concrete will not get into.
- Formwork: closed and stable, so the concrete does not escape between the block courses.
- Pouring in stages: over a large height the pour is done in several stages, with vibration or rodding, so that no voids remain.
- Records: a photo of the dowels before closing and an entry in the site diary, because after the pour they can no longer be seen, and they come up when the work is measured for the interim payment application.
A one-minute check: count on the plan the points where blocks meet concrete, and check that they have an item or a "the price includes" sentence. In a building of several storeys this comes to hundreds of metres, and the gap usually surfaces in the middle of construction.
With connection details like these, a quantity surveyor checks both the plan and the detail, because quite often the detail appears only on the structural sheet.
Frequently asked questions
What is the difference between שטראבה and שטרבה?
It is the same word, spelled two ways in Hebrew. On site you hear both, and it matters more that the detail is written down. More Hebrew spelling variants are on the Hebrew construction terms page.
Does block-to-column toothing prevent cracks completely?
It reduces the risk of a crack at the joint, but the quality of workmanship, the curing and the plaster matter just as much. No solution rules out cracks completely.
Is such a connection needed in an internal wall too?
It depends on the designer's detail. For short partitions some designers settle for a different anchorage, and for external walls and long walls the connection is usually detailed more fully. What counts is what is written in the drawings and in the BOQ.
How is its price broken down?
Into labour hours for drilling, bonding, formwork and pouring, and into materials: dowels, adhesive and concrete. Breaking a price down into labour hours and materials is covered on the construction inputs page.
Who makes sure it appears in the BOQ?
A quantity surveyor for construction projects who goes through the drawings and the details. For the finishes that come afterwards, such as plaster, see finishes quantity takeoff.
Have a project? Tell me briefly what the project is and what already exists. When the project is a good fit, I usually reply with focused questions and a written quote.
Contact me for a quote* Created with AI for marketing, for illustration only. Not reviewed by Yaakov Avital and not professional advice. Every project must be checked on its own by a professional quantity surveyor and an engineer.