Withdrawal Capacity of Threaded Rod in Tapped Steel Plate
Withdrawal Capacity of Threaded Rod in Tapped Steel Plate
(OP)
Hi,
I would like to calculate the withdrawal capacity of a 1" threaded rod, 36 ksi, threaded into a 1" thick, 36 ksi, steel plate which has been tapped on site to receive the threaded rod.
Thanks in advance for any guidance in this topic.
I would like to calculate the withdrawal capacity of a 1" threaded rod, 36 ksi, threaded into a 1" thick, 36 ksi, steel plate which has been tapped on site to receive the threaded rod.
Thanks in advance for any guidance in this topic.






RE: Withdrawal Capacity of Threaded Rod in Tapped Steel Plate
The effective cross section area, or tensile area, of the external thread.
The shear area of the external thread which depends upon minor diameter of the tapped hole
The shear area of the internal thread which depends on the major diameter of the external thread
Root Area of Thread (in2) = Pi*r**2 = .605 in**2 (assume 8 threads/inch)
Rod fails before threads fails
Most common Specification for the rods are :A36, ASTM A193 (125ksi), and ASTM A307 (60ksi)
RE: Withdrawal Capacity of Threaded Rod in Tapped Steel Plate
A single (non-vibrating, non-rotating) steady pull directly perpendicular to the surface plane of the plate with no bending or sideways forces?
Also, remember not to use the full length of the tapped hole: you need to reject (not use) the first thread on the near side (because it has only half-developed thread walls) of the tapped hole and the last thread on the far side (because it too has only half-developed thread wall sides). But a common 1 inch dia nut is thinner than a 1 inch thick plate, so the pull-out theory above remains true. The male threads (the rod) strips before the female does.
RE: Withdrawal Capacity of Threaded Rod in Tapped Steel Plate
Out of curiosity, what would be the minimum length of thread engagement for the rod to fail before the threads fail? Is there a standard engagement length that I could use as a rule of thumb?
Thanks
RE: Withdrawal Capacity of Threaded Rod in Tapped Steel Plate
RE: Withdrawal Capacity of Threaded Rod in Tapped Steel Plate
RE: Withdrawal Capacity of Threaded Rod in Tapped Steel Plate
RE: Withdrawal Capacity of Threaded Rod in Tapped Steel Plate
RE: Withdrawal Capacity of Threaded Rod in Tapped Steel Plate
RE: Withdrawal Capacity of Threaded Rod in Tapped Steel Plate
RE: Withdrawal Capacity of Threaded Rod in Tapped Steel Plate
Rod fails before threads fails"
maybe i'm especially (or perhaps just normally) stupid today, but i don't see this, at least not as presented.
ok, so the rod has 0.6in2 working in tension
and the plate has something like pi*d*t = about 3in2 working in shear
same material rod and plate ...
so the rod can support 0.6Ftu
and the plate 3*0.57 = 1.7Ftu
so, yes, the rod is more critical than the plate
Quando Omni Flunkus Moritati
RE: Withdrawal Capacity of Threaded Rod in Tapped Steel Plate
RE: Withdrawal Capacity of Threaded Rod in Tapped Steel Plate
Quando Omni Flunkus Moritati
RE: Withdrawal Capacity of Threaded Rod in Tapped Steel Plate
I'll bet some will say that two structural plates riveted together develop their usable shear resistance by the cross section area of the rivets. Wrong. Its the friction between the plates. Probably applies to bolted joints also. Not my theory: Prof George Winter Cornell U 1950 structural Dept Head.
RE: Withdrawal Capacity of Threaded Rod in Tapped Steel Plate
In my mind this would achieve the same intent as the usual high strength nut provided the bolt is threaded right through the plate. I.E. that the bolt yields first.
RE: Withdrawal Capacity of Threaded Rod in Tapped Steel Plate
I randomly came across this today which is relevant to the discussion, there are some equations for the length of engagement. Hopefully it helps.
RE: Withdrawal Capacity of Threaded Rod in Tapped Steel Plate
But the reason that happens - that pattern of stripping out threads in the threaded rod that you describe - is NOT because the formula assuming equal root lengths and equal material strengths is correct, but because the threads are NOT "perfect 100% engagement-100% cut to diameter" threads. For tapping, the tapped female threads are first drilled over-sized so the diameter of the drilled hole is greater than the theoretical diameter of the minor diameter of the thread. (As I recall, 68% - 75% thread engagement.) This is to allow room while tapping for the shavings to get removed back into the "curl" of the tap. If it is not done, the tap tends to break off in the drilled hole.
Thus, the "real world" thread in 98% of the tapped holes is "loose" and only the tip of the male threads on the threaded rod or bolt actually engage the female threads. Since the male bolt threads are pointed, this means that the root of the male threads where they are actually engaged is significantly less than the root of of the female threads in the tapped hole.
So, combine a smaller male thread diameter with a smaller root length on the male threads means that the force created inside the bolted joint is divided equally in half: half into the small cross-section in the male threads, and half in the larger cross-section area of the female threads. Obviously, the stress in the male threads are closer to the material yield point stress, and the male threads will always strip first.
But, they will strip the at the "tips" of the male thread, not across the (unengaged!) root of the male threads.
Now, if you deliberately call for a designed joint with a "perfect" very-closely-fitting thread specification, you are increasing costs significantly (remember that greatly increased probability the tap will break off?) and slow productivity down by requiring much higher tool costs, but you are also increasing the chances that the stripped threads and galled threads on disassembbly will be in the very expensive female threads!