1+1+1+1+1 ≠ 5 !?

Hold-down brackets (preventing up lift of walls) and square steel plates (preventing lateral translation of walls) are used to transfer lateral loads acting on walls. Hold-down brackets are positioned as close to the edge of the wall as possible to maximize the lever arm and thus obtain the maximum capacity. The square steel plates are positioned at regular intervals along the lower side of the wall.

Image is from “The Carpenter’s book”

It is important to note that hold-down brackets provide no shear resistance and thus should only be used in areas where up lift forces are involved. Appropriate sizing of ground connections is the basis of a successful project as it is one of the most critical element of any structure.

“Chain” concrete steel wood

Steel-timber connections are like a chain, the weakest link determines the strength of the connection.

We must therefore investigate the tension failure mechanisms:

• Timber failure
• Steel failure
• Cone failure of concrete
• Tear out failure
• Splitting failure

and shear failure mechanisms:

• Timber shear failure
• Pure shear failure of steel
• Shear failure of steel with bending
• Edge rupture failure
• Pry out failure

All fixing systems have an area of influence surrounding it which is used to transfer load from the structure to ground.

To achieve full capacity of an anchorage system the following conditions must be met:

• adequate distance from the edges of the base;
• adequate distance from other anchors;
• sufficient base depth (concrete);
• driving the anchor rod to the calculated depth

It should also be noted that the capacity of a steel-timber connection depends on the thickness of the steel plates, which must be verified. The failure load of a cylindrical shank connection can be determined by the plastic theory developed by Johansen. Characteristic bearing capacity for nails, bolts, pins and screws, for single shear plane and single fastener shall be taken as the minimum value determined by the expressions defined in paragraph 8.2.3 of the EC5.

Rupture modes for steel-wood connections (source: EC5, paragraph 8.2.3, figure 8.3)

By using ETAG 001 (Edition 1997) compliant software, we wanted to compare the resistance values of a single-to-five single-hold berth system side by side.

Case 1: single hold-down with 45 anker nails 4.0 × 60, C25/30 concrete with M16 5.8 threaded anchor rod and vinyl ester resin.

Case 2: 5 hold-downs with 45 nails 4.0 × 60, C25/30 concrete with M16 5.8 threaded anchor rods and vinyl ester resin. Distance between the bars are 10cm and minimum edge distance is maintained.

A single hold-down has R_d,concrete = 48,48kN while with five have R_3,5 = 102,35kN: proportionally the group has lower capacity than the single hold-down because there is the interference that the bars create between themselves. If we assume a linear relationship between R_d,concrete and the number of hold-downs present, its value would be equal to

R_3,5 = 48,48 * 5 = 242,40 kN

Comparing this with the R_3,5 value calculated using software, you get

102,35/242,40 = 0,42                  so the tear out resistance of the concrete is about 60% less

It is therefore clear that proportionally a single anchor has greater capacity than 5 anchors system:

102,35/5 = 20,47 kN/cad          always 60% less than the capacity of a single connector.

It is interesting to note that the use of only 3 hold-downs at 20 cm distance leads to the same result. In fact, not only is the centre of gravity of the connection the same but the concrete cone rupture occurs at 102,35 kN, exactly the same value obtained with 5 threaded rods. This implies that 2 of the 5 hold-downs do not contribute to increasing the capacity of the connection, thus rendering them obsolete.

Considering the previously discussed failure modes of concrete (adequate distance from the edges of the base and other anchor rods) and the results of the concrete strength calculation, the following configuration is not recommended. The lever arm reduction due to the spacing of the anchors also affects the overturning strength of the wall.

In conclusion, building the perfect structure does not necessarily mean excessive use of anchoring systems: this solution, in addition to being very costly to the customer, will also result in an extremely hard work on site.