Myth-Using lesser numbers or higher numbers of Anchor Bolts….Lesser known facts.

“Lesser Known facts. Client/Layman recommends to use higher numbers of Anchor Bolts if span is large and eve height is more but designer should be careful about using higher number of bolts”

Hey there, i am sharing my experience so that you would be careful.

Recently have designed a pre-engineered building shed of span 28m & clear eve height of 10m. In my design, i have taken pinned connection and calculated anchor bolts came out to be four numbers. According to this pinned connection design concept, foundation drawings prepared.

Now, when i discussed about the numbers of bolt, he told me to use 8 numbers of bolt. He told me that using 8 numbers of bolt will give higher strength.

If 8 numbers of bolts are arranged, then system changes from pinned connection to fixed connection. As in earlier case, all 4 numbers of bolts were placed inside two flange of built-up section. But when 8-numbers of bolts are placed, then we have to use 4-extra bolts beyond the flange of built-up section. With this arrangement, system become a fixed connection.

In case of pinned connection, 4-numbers of 25dia of anchor bots were safe but with 8-numbers of bolts, even 32dia of anchor bots were failing.

How does it happen. I would say, when we use bolts inside the two flange, then we make base plate flexible. In design, all moments are released in pinned connection and lower sections are achieved at baseplate level and also, lower number of bolts are required. Also, stiffener plates are not required in pinned connection.

On the other hand, when we use some bolts inside two flange and some bots beyond flange, then it make base plate rigid and system become a fixed connection. With this arrangement, fixed connection are generated and it create moments at the base plate levels. We need stiffener plates in that case. Also, structural steel section at the baseplate level comes out to be more and even if more numbers of bolts of same diameter as used in the condition of pinned connection may failed in fixed connection. Therefore, my beloved reader, be careful when someone who is not an expert in structural designing and tell you to increase number of bolts with keeping diameter same, then never ever blindly increase the number of anchor bolts. If you do this, in be disastrous. It could lead to the failure of anchor bolts, base plates and even footing.

I hope you understood the concept of pinned connection and fixed connection.


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