Myth-Using lesser numbers or higher numbers of Anchor Bolts….Lesser known facts.
“Lesser Known facts. Client/Layman recomends 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 pineed connection and caluclated anchor bolts came out to be four numbers. According to this pinned conncetion 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 arrangment, 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 flexbile. In design, all moments are releasd 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 craete momments at the base plate levels. We need stiffener plates in that case. Also, structural eteel section at the baseplate level comes out to be more and even if more numbers of botlts of same dia as used in the condition of pinned connction may failed in fixed connction. 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 disasterrous.It could lead to the fauure of anchor bolts, base plates and even footing.
I hope you understood the concept of pinned connection and fixed conncetion.