A couple of weeks ago I had two posts about carriage bolts and deck attachments. I had written Simpson Strong Tie with some questions as some information I received in a seminar was not completely understood.
In the seminar they had suggested that carriage bolts should not be used in deck attachments, particularly guardrail posts. The reason, as I understood, was that the connection can weaken overtime as the wood shrinks, and when tightened they can be drawn into the wood, further weakening the attachment.
Yesterday I received a very kind, and long, phone call from a Simpson engineer. He went over my questions individually and we had a very nice discussion. These were my questions, as written, to Simpson:
1. My deck is 13 1/2 years old and carriage bolts were used everywhere. I do tighten them periodically and the heads have been sucked into the wood somewhat. None is weak or wobbly. At what point do these bolts become an issue?
2. How much of an issue is it really, unless the posts, et al, get loose? I don't know what to tell my clients!
3. Should they be replaced with a proper through bolt and washer arrangement?
4. Will codes begin to reflect this information?
This is a carriage bolt and the thing in question. It comes in various lengths and is characterized by its round head and square shank under the head. These bolts have been used for years to attach deck guardrail posts to decks, and decks to houses! These bolts are used all over the country.
The engineer, David F., began by saying that Simpson Strong Tie has a general concern, which they mention in their education classes. They are about designing and testing attachments and producing holders and an array of other things mentioned in my previous post.
They do not produce nuts and bolts and washers, so it isn't that they are bashing one attachment, like carriage bolts, in favor of one of their products.
David further said that the actual EFFECT of using carriage bolts, as opposed to washers and through bolts, is UNKNOWN. They are devising tests now to determine that EFFECT. As engineers, they are interested in DATA!
The American Wood Council puts out what is essentially a code book for engineers and wood designs. The information is called the National Design Standard. In that standard it does not mention carriage bolts as meeting the standards of the American Society of Mechanical Engineers (ASME). The NDS is the information from which local codes are developed and implemented.
Here is the issue. The through bolt and carriage bolt are essentially the same steel "dowel." They have the same strength.
The problem comes with the head. The 1/2" carriage bolt head has 35% less BEARING AREA than the standard 1 3/8" washer under a 1/2" through bolt's ending nut.
The head of the carriage bolt is what takes the force if a guardrail post is pushed out. It is the head, or the washer with a through bolt, that takes the brunt of the force. Having less surface area A CARRIAGE BOLT HEAD IS SIMPLY ABLE TO WITHSTAND LESS FORCE.
How much less force? That is what they are trying to determine! Until that is determined ASME cannot cover carriage bolts in any standard. Nor can a new product be designed.
Always thinking I asked why they don't just design a carriage bolt with more surface area. That leads to the second concern.
A carriage bolt can only be tightened so much. Eventually the head will spin due to tension and it cannot be tightened any more. There is nothing to grip on both ends, so friction takes over. A through bolt with its washer, as shown above, can be held with a wrench on both ends and can always be tightened further.
Again, always thinking, I suggested that they design a carriage bolt with a larger head with a square or Allen-wrench hole in it so it can be held in place and tightened without spinning! I think they should design it and call it the Jay Bolt! I'm wondering if right now he is thinking, "Hmmmm!?"
I appreciated David's willingness to call me and speak with me about this. I think he said they have never gotten a response or a question from a home inspector before! He was glad to do it.
My recommendation: when you don't know something, try to find out! You can't always get what you want, but if you try, sometimes, you just might find, you get what you need! So go, and do that voodoo that you do so well...