Technical>Static

Connection between platform frame walls and concrete using hold-downs

The correct design of a frame building involves a series of precautions necessary in order to guarantee the fulfillment of the static requirements in compliance with the regulations. In this article we will analyze an aspect related to the correct design of the ground connection detail.
In the case of solid wood walls (CLT) the structural element does not involve particular problems on the connection between timber and concrete but in the case of platformframe structures there are some aspects that need to be assessed both in the design phase and in the assembly.

For economic reasons, the section of the posts within a frame wall is reduced to a minimum, but often this is done neglecting side effects. In the following case, for example, there is a frame structure (post 60x120mm) braced on both sides by 15 mm thick OSB panels. As for the solid walls, even for the frame walls, the equilibrium must be guaranteed by preparing two types of connection with the foundation: shear and hold-down brackets. If for the shear connection the presence of a lower beam does not involve particular types of problems *, the connection in tension needs some extra attention.

Assuming to use a 60mm wide hold-down bracket and to fix it to the wall using 4.0 mm nails (anker), it is immediately clear that the distance between the nail axis and the edge of the post is very small. In the specific case, the distance measures 10mm (or 2.5d), a measure that does not respect the limits imposed by the legislation for nails with or without pre-hole.

You also have to be aware that the assembler, not seeing the wooden post which is placed inside the structure of the wall, can hardly manage to lay the bracket perfectly aligned as shown in the drawing. In addition, we must consider that the bracket, due to the intersection between the anchor and the steel bars of the foundation, can be moved (by a few mm or even a few cm).

Having said that, in this configuration, assuming to have hold-down and post in axis, there is a reduction in the resistance of the connection caused by the fact that the most external nails are too close to the edge of the post.

A similar situation will occur if the hold-down is moved by 20mm due to interference with the steel bars as shown below: also in this case there will be a considerable reduction in resistance.

It is therefore recommended to increase the post width from 60mm to 80mm so as to ensure the respect of the edge distances.

Attention: with an 80mm post, however, there is a critical situation because only one millimeter is enough to reduce the nail-edge distance to less than 5d corresponding to 20mm; it is therefore recommended to maintain a certain margin by using 100mm posts where the hold-downs are.

N.b.: Enlarging only the outermost posts does not always avoid the above-mentioned problems because of the possible problems that arise from the fixing on the foundation. In some extreme cases, you may have to move the hold-down on the near post, which is likely to be about 60cm to the left or to the right. This corresponds to a reduction of the lever arm of 60cm which could create static (an increase of the tension force on the connector).

The same goes for the use of hold-downs with greater width recently proposed by some companies on the market and with a significant load-bearing capacity.

*the lower beam must be appropriately sized in order to allow the shear brackets to be fixed respecting the distances imposed by the national standard

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